**⚠️ Warning: This is a draft ⚠️**

This means it might contain formatting issues, incorrect code, conceptual problems, or other severe issues.

If you want to help to improve and eventually enable this page, please fork RosettaGit's repository and open a merge request on GitHub.

{{task}}

For this task, the Stern-Brocot sequence is to be generated by an algorithm similar to that employed in generating the [[Fibonacci sequence]].

# The first and second members of the sequence are both 1:

#* 1, 1

# Start by considering the second member of the sequence

# Sum the considered member of the sequence and its precedent, (1 + 1) = 2, and append it to the end of the sequence:

#* 1, 1, 2

# Append the considered member of the sequence to the end of the sequence:

#* 1, 1, 2, 1

# Consider the next member of the series, (the third member i.e. 2)

# GOTO 3

#* #* ─── Expanding another loop we get: ─── #*

# Sum the considered member of the sequence and its precedent, (2 + 1) = 3, and append it to the end of the sequence:

#* 1, 1, 2, 1, 3

# Append the considered member of the sequence to the end of the sequence:

#* 1, 1, 2, 1, 3, 2

# Consider the next member of the series, (the fourth member i.e. 1)

;The task is to:

- Create a function/method/subroutine/procedure/... to generate the Stern-Brocot sequence of integers using the method outlined above.
- Show the first fifteen members of the sequence. (This should be: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4)
- Show the (1-based) index of where the numbers 1-to-10 first appears in the sequence.
- Show the (1-based) index of where the number 100 first appears in the sequence.
- Check that the greatest common divisor of all the two consecutive members of the series up to the 1000
^{th}member, is always one.

Show your output on this page.

;Related tasks: :* [[Fusc sequence]]. :* [[Continued fraction/Arithmetic]]

;Ref:

- [https://www.youtube.com/watch?v=DpwUVExX27E Infinite Fractions - Numberphile] (Video).
- [http://www.ams.org/samplings/feature-column/fcarc-stern-brocot Trees, Teeth, and Time: The mathematics of clock making].
- [https://oeis.org/A002487 A002487] The On-Line Encyclopedia of Integer Sequences.

## 360 Assembly

{{trans|Fortran}}

```
* Stern-Brocot sequence - 12/03/2019
STERNBR CSECT
USING STERNBR,R13 base register
B 72(R15) skip savearea
DC 17F'0' savearea
SAVE (14,12) save previous context
ST R13,4(R15) link backward
ST R15,8(R13) link forward
LR R13,R15 set addressability
LA R4,SB+2 k=2; @sb(k)
LA R2,SB+2 i=1; @sb(k-i)
LA R3,SB+0 j=2; @sb(k-j)
LA R1,NN/2 loop counter
LOOP LA R4,2(R4) @sb(k)++
LH R0,0(R2) sb(k-i)
AH R0,0(R3) sb(k-i)+sb(k-j)
STH R0,0(R4) sb(k)=sb(k-i)+sb(k-j)
LA R3,2(R3) @sb(k-j)++
LA R4,2(R4) @sb(k)++
LH R0,0(R3) sb(k-j)
STH R0,0(R4) sb(k)=sb(k-j)
LA R2,2(R2) @sb(k-i)++
BCT R1,LOOP end loop
LA R9,15 n=15
MVC PG(5),=CL80'FIRST'
XDECO R9,XDEC edit n
MVC PG+5(3),XDEC+9 output n
XPRNT PG,L'PG print buffer
LA R10,PG @pg
LA R6,1 i=1
DO WHILE=(CR,R6,LE,R9) do i=1 to n
LR R1,R6 i
SLA R1,1 ~
LH R2,SB-2(R1) sb(i)
XDECO R2,XDEC edit sb(i)
MVC 0(4,R10),XDEC+8 output sb(i)
LA R10,4(R10) @pg+=4
LA R6,1(R6) i++
ENDDO , enddo i
XPRNT PG,L'PG print buffer
LA R7,1 j=1
DO WHILE=(C,R7,LE,=A(11)) do j=1 to 11
IF C,R7,EQ,=F'11' THEN if j=11 then
LA R7,100 j=100
ENDIF , endif
LA R6,1 i=1
DO WHILE=(C,R6,LE,=A(NN)) do i=1 to nn
LR R1,R6 i
SLA R1,1 ~
LH R2,SB-2(R1) sb(i)
CR R2,R7 if sb(i)=j
BE EXITI then leave i
LA R6,1(R6) i++
ENDDO , enddo i
EXITI MVC PG,=CL80'FIRST INSTANCE OF'
XDECO R7,XDEC edit j
MVC PG+17(4),XDEC+8 output j
MVC PG+21(7),=C' IS AT '
XDECO R6,XDEC edit i
MVC PG+28(4),XDEC+8 output i
XPRNT PG,L'PG print buffer
LA R7,1(R7) j++
ENDDO , enddo j
L R13,4(0,R13) restore previous savearea pointer
RETURN (14,12),RC=0 restore registers from calling sav
LTORG
NN EQU 2400 nn
PG DC CL80' ' buffer
XDEC DS CL12 temp for xdeco
SB DC (NN)H'1' sb(nn)
REGEQU
END STERNBR
```

{{out}}

```
FIRST 15
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
FIRST INSTANCE OF 1 IS AT 1
FIRST INSTANCE OF 2 IS AT 3
FIRST INSTANCE OF 3 IS AT 5
FIRST INSTANCE OF 4 IS AT 9
FIRST INSTANCE OF 5 IS AT 11
FIRST INSTANCE OF 6 IS AT 33
FIRST INSTANCE OF 7 IS AT 19
FIRST INSTANCE OF 8 IS AT 21
FIRST INSTANCE OF 9 IS AT 35
FIRST INSTANCE OF 10 IS AT 39
FIRST INSTANCE OF 100 IS AT 1179
```

The nice part is the coding of the sequense:

k=2; i=1; j=2; while(k<nn); k++; sb[k]=sb[k-i]+sb[k-j]; k++; sb[k]=sb[k-j]; i++; j++; }

Only five registers are used. No Horner's rule to access sequence items.

```
LA R4,SB+2 k=2; @sb(k)
LA R2,SB+2 i=1; @sb(k-i)
LA R3,SB+0 j=2; @sb(k-j)
LA R1,NN/2 k=nn/2 'loop counter
LOOP LA R4,2(R4) @sb(k)++
LH R0,0(R2) sb(k-i)
AH R0,0(R3) sb(k-i)+sb(k-j)
STH R0,0(R4) sb(k)=sb(k-i)+sb(k-j)
LA R3,2(R3) @sb(k-j)++
LA R4,2(R4) @sb(k)++
LH R0,0(R3) sb(k-j)
STH R0,0(R4) sb(k)=sb(k-j)
LA R2,2(R2) @sb(k-i)++
BCT R1,LOOP k--; if k>0 then goto loop
```

## Ada

```
with Ada.Text_IO, Ada.Containers.Vectors;
procedure Sequence is
package Vectors is new
Ada.Containers.Vectors(Index_Type => Positive, Element_Type => Positive);
use type Vectors.Vector;
type Sequence is record
List: Vectors.Vector;
Index: Positive;
-- This implements some form of "lazy evaluation":
-- + List holds the elements computed, so far, it is extended
-- if the user tries to "Get" an element not yet computed;
-- + Index is the location of the next element under consideration
end record;
function Initialize return Sequence is
(List => (Vectors.Empty_Vector & 1 & 1), Index => 2);
function Get(Seq: in out Sequence; Location: Positive) return Positive is
-- returns the Location'th element of the sequence
-- extends Seq.List (and then increases Seq.Index) if neccessary
That: Positive := Seq.List.Element(Seq.Index);
This: Positive := That + Seq.List.Element(Seq.Index-1);
begin
while Seq.List.Last_Index < Location loop
Seq.List := Seq.List & This & That;
Seq.Index := Seq.Index + 1;
end loop;
return Seq.List.Element(Location);
end Get;
S: Sequence := Initialize;
J: Positive;
use Ada.Text_IO;
begin
-- show first fifteen members
Put("First 15:");
for I in 1 .. 15 loop
Put(Integer'Image(Get(S, I)));
end loop;
New_Line;
-- show the index where 1, 2, 3, ... first appear in the sequence
for I in 1 .. 10 loop
J := 1;
while Get(S, J) /= I loop
J := J + 1;
end loop;
Put("First" & Integer'Image(I) & " at" & Integer'Image(J) & "; ");
if I mod 4 = 0 then
New_Line; -- otherwise, the output gets a bit too ugly
end if;
end loop;
-- show the index where 100 first appears in the sequence
J := 1;
while Get(S, J) /= 100 loop
J := J + 1;
end loop;
Put_Line("First 100 at" & Integer'Image(J) & ".");
-- check GCDs
declare
function GCD (A, B : Integer) return Integer is
M : Integer := A;
N : Integer := B;
T : Integer;
begin
while N /= 0 loop
T := M;
M := N;
N := T mod N;
end loop;
return M;
end GCD;
A, B: Positive;
begin
for I in 1 .. 999 loop
A := Get(S, I);
B := Get(S, I+1);
if GCD(A, B) /= 1 then
raise Constraint_Error;
end if;
end loop;
Put_Line("Correct: The first 999 consecutive pairs are relative prime!");
exception
when Constraint_Error => Put_Line("Some GCD > 1; this is wrong!!!") ;
end;
end Sequence;
```

{{out}}

```
First 15: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
First 1 at 1; First 2 at 3; First 3 at 5; First 4 at 9;
First 5 at 11; First 6 at 33; First 7 at 19; First 8 at 21;
First 9 at 35; First 10 at 39; First 100 at 1179.
Correct: The first 999 consecutive pairs are relative prime!
```

## AppleScript

use AppleScript version "2.4" use framework "Foundation" use scripting additions -- sternBrocot :: Generator [Int] on sternBrocot() script go on |λ|(xs) set x to snd(xs) tail(xs) & {fst(xs) + x, x} end |λ| end script fmapGen(my head, iterate(go, {1, 1})) end sternBrocot -- TEST ------------------------------------------------------------------ on run set sbs to take(1200, sternBrocot()) set ixSB to zip(sbs, enumFrom(1)) script low on |λ|(x) 12 ≠ fst(x) end |λ| end script script sameFst on |λ|(a, b) fst(a) = fst(b) end |λ| end script script asList on |λ|(x) {fst(x), snd(x)} end |λ| end script script below100 on |λ|(x) 100 ≠ fst(x) end |λ| end script script fullyReduced on |λ|(ab) 1 = gcd(|1| of ab, |2| of ab) end |λ| end script unlines(map(showJSON, ¬ {take(15, sbs), ¬ take(10, map(asList, ¬ nubBy(sameFst, ¬ sortBy(comparing(fst), ¬ takeWhile(low, ixSB))))), ¬ asList's |λ|(fst(take(1, dropWhile(below100, ixSB)))), ¬ all(fullyReduced, take(1000, zip(sbs, tail(sbs))))})) end run --> [1,1,2,1,3,2,3,1,4,3,5,2,5,3,4] --> [[1,32],[2,24],[3,40],[4,36],[5,44],[6,33],[7,38],[8,42],[9,35],[10,39]] --> [100,1179] --> true -- GENERIC ABSTRACTIONS ------------------------------------------------------- -- Absolute value. -- abs :: Num -> Num on abs(x) if 0 > x then -x else x end if end abs -- Applied to a predicate and a list, `all` determines if all elements -- of the list satisfy the predicate. -- all :: (a -> Bool) -> [a] -> Bool on all(p, xs) tell mReturn(p) set lng to length of xs repeat with i from 1 to lng if not |λ|(item i of xs, i, xs) then return false end repeat true end tell end all -- comparing :: (a -> b) -> (a -> a -> Ordering) on comparing(f) script on |λ|(a, b) tell mReturn(f) set fa to |λ|(a) set fb to |λ|(b) if fa < fb then -1 else if fa > fb then 1 else 0 end if end tell end |λ| end script end comparing -- drop :: Int -> [a] -> [a] -- drop :: Int -> String -> String on drop(n, xs) set c to class of xs if c is not script then if c is not string then if n < length of xs then items (1 + n) thru -1 of xs else {} end if else if n < length of xs then text (1 + n) thru -1 of xs else "" end if end if else take(n, xs) -- consumed return xs end if end drop -- dropWhile :: (a -> Bool) -> [a] -> [a] -- dropWhile :: (Char -> Bool) -> String -> String on dropWhile(p, xs) set lng to length of xs set i to 1 tell mReturn(p) repeat while i ≤ lng and |λ|(item i of xs) set i to i + 1 end repeat end tell drop(i - 1, xs) end dropWhile -- enumFrom :: a -> [a] on enumFrom(x) script property v : missing value property blnNum : class of x is not text on |λ|() if missing value is not v then if blnNum then set v to 1 + v else set v to succ(v) end if else set v to x end if return v end |λ| end script end enumFrom -- filter :: (a -> Bool) -> [a] -> [a] on filter(f, xs) tell mReturn(f) set lst to {} set lng to length of xs repeat with i from 1 to lng set v to item i of xs if |λ|(v, i, xs) then set end of lst to v end repeat return lst end tell end filter -- fmapGen <$> :: (a -> b) -> Gen [a] -> Gen [b] on fmapGen(f, gen) script property g : gen property mf : mReturn(f)'s |λ| on |λ|() set v to g's |λ|() if v is missing value then v else mf(v) end if end |λ| end script end fmapGen -- fst :: (a, b) -> a on fst(tpl) if class of tpl is record then |1| of tpl else item 1 of tpl end if end fst -- gcd :: Int -> Int -> Int on gcd(a, b) set x to abs(a) set y to abs(b) repeat until y = 0 if x > y then set x to x - y else set y to y - x end if end repeat return x end gcd -- head :: [a] -> a on head(xs) if xs = {} then missing value else item 1 of xs end if end head -- iterate :: (a -> a) -> a -> Gen [a] on iterate(f, x) script property v : missing value property g : mReturn(f)'s |λ| on |λ|() if missing value is v then set v to x else set v to g(v) end if return v end |λ| end script end iterate -- length :: [a] -> Int on |length|(xs) set c to class of xs if list is c or string is c then length of xs else (2 ^ 29 - 1) -- (maxInt - simple proxy for non-finite) end if end |length| -- map :: (a -> b) -> [a] -> [b] on map(f, xs) tell mReturn(f) set lng to length of xs set lst to {} repeat with i from 1 to lng set end of lst to |λ|(item i of xs, i, xs) end repeat return lst end tell end map -- min :: Ord a => a -> a -> a on min(x, y) if y < x then y else x end if end min -- Lift 2nd class handler function into 1st class script wrapper -- mReturn :: First-class m => (a -> b) -> m (a -> b) on mReturn(f) if class of f is script then f else script property |λ| : f end script end if end mReturn -- nubBy :: (a -> a -> Bool) -> [a] -> [a] on nubBy(f, xs) set g to mReturn(f)'s |λ| script notEq property fEq : g on |λ|(a) script on |λ|(b) not fEq(a, b) end |λ| end script end |λ| end script script go on |λ|(xs) if (length of xs) > 1 then set x to item 1 of xs {x} & go's |λ|(filter(notEq's |λ|(x), items 2 thru -1 of xs)) else xs end if end |λ| end script go's |λ|(xs) end nubBy -- partition :: predicate -> List -> (Matches, nonMatches) -- partition :: (a -> Bool) -> [a] -> ([a], [a]) on partition(f, xs) tell mReturn(f) set ys to {} set zs to {} repeat with x in xs set v to contents of x if |λ|(v) then set end of ys to v else set end of zs to v end if end repeat end tell Tuple(ys, zs) end partition -- showJSON :: a -> String on showJSON(x) set c to class of x if (c is list) or (c is record) then set ca to current application set {json, e} to ca's NSJSONSerialization's dataWithJSONObject:x options:0 |error|:(reference) if json is missing value then e's localizedDescription() as text else (ca's NSString's alloc()'s initWithData:json encoding:(ca's NSUTF8StringEncoding)) as text end if else if c is date then "\"" & ((x - (time to GMT)) as «class isot» as string) & ".000Z" & "\"" else if c is text then "\"" & x & "\"" else if (c is integer or c is real) then x as text else if c is class then "null" else try x as text on error ("«" & c as text) & "»" end try end if end showJSON -- snd :: (a, b) -> b on snd(tpl) if class of tpl is record then |2| of tpl else item 2 of tpl end if end snd -- Enough for small scale sorts. -- Use instead sortOn :: Ord b => (a -> b) -> [a] -> [a] -- which is equivalent to the more flexible sortBy(comparing(f), xs) -- and uses a much faster ObjC NSArray sort method -- sortBy :: (a -> a -> Ordering) -> [a] -> [a] on sortBy(f, xs) if length of xs > 1 then set h to item 1 of xs set f to mReturn(f) script on |λ|(x) f's |λ|(x, h) ≤ 0 end |λ| end script set lessMore to partition(result, rest of xs) sortBy(f, |1| of lessMore) & {h} & ¬ sortBy(f, |2| of lessMore) else xs end if end sortBy -- tail :: [a] -> [a] on tail(xs) set blnText to text is class of xs if blnText then set unit to "" else set unit to {} end if set lng to length of xs if 1 > lng then missing value else if 2 > lng then unit else if blnText then text 2 thru -1 of xs else rest of xs end if end if end tail -- take :: Int -> [a] -> [a] -- take :: Int -> String -> String on take(n, xs) set c to class of xs if list is c then if 0 < n then items 1 thru min(n, length of xs) of xs else {} end if else if string is c then if 0 < n then text 1 thru min(n, length of xs) of xs else "" end if else if script is c then set ys to {} repeat with i from 1 to n set v to xs's |λ|() if missing value is v then return ys else set end of ys to v end if end repeat return ys else missing value end if end take -- takeWhile :: (a -> Bool) -> [a] -> [a] -- takeWhile :: (Char -> Bool) -> String -> String on takeWhile(p, xs) if script is class of xs then takeWhileGen(p, xs) else tell mReturn(p) repeat with i from 1 to length of xs if not |λ|(item i of xs) then ¬ return take(i - 1, xs) end repeat end tell return xs end if end takeWhile -- takeWhileGen :: (a -> Bool) -> Gen [a] -> [a] on takeWhileGen(p, xs) set ys to {} set v to |λ|() of xs tell mReturn(p) repeat while (|λ|(v)) set end of ys to v set v to xs's |λ|() end repeat end tell return ys end takeWhileGen -- Tuple (,) :: a -> b -> (a, b) on Tuple(a, b) {type:"Tuple", |1|:a, |2|:b, length:2} end Tuple -- unlines :: [String] -> String on unlines(xs) set {dlm, my text item delimiters} to ¬ {my text item delimiters, linefeed} set str to xs as text set my text item delimiters to dlm str end unlines -- zip :: [a] -> [b] -> [(a, b)] on zip(xs, ys) zipWith(Tuple, xs, ys) end zip -- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] on zipWith(f, xs, ys) set lng to min(|length|(xs), |length|(ys)) if 1 > lng then return {} set xs_ to take(lng, xs) -- Allow for non-finite set ys_ to take(lng, ys) -- generators like cycle etc set lst to {} tell mReturn(f) repeat with i from 1 to lng set end of lst to |λ|(item i of xs_, item i of ys_) end repeat return lst end tell end zipWith

{{Out}}

```
[1,1,2,1,3,2,3,1,4,3,5,2,5,3,4]
[[1,32],[2,24],[3,40],[4,36],[5,44],[6,33],[7,38],[8,42],[9,35],[10,39]]
[100,1179]
true
```

## AutoHotkey

```
Found := FindOneToX(100), FoundList := ""
Loop, 10
FoundList .= "First " A_Index " found at " Found[A_Index] "`n"
MsgBox, 64, Stern-Brocot Sequence
, % "First 15: " FirstX(15) "`n"
. FoundList
. "First 100 found at " Found[100] "`n"
. "GCDs of all two consecutive members are " (GCDsUpToXAreOne(1000) ? "" : "not ") "one."
return
class SternBrocot
{
__New()
{
this[1] := 1
this[2] := 1
this.Consider := 2
}
InsertPair()
{
n := this.Consider
this.Push(this[n] + this[n - 1], this[n])
this.Consider++
}
}
; Show the first fifteen members of the sequence. (This should be: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3,
; 5, 2, 5, 3, 4)
FirstX(x)
{
SB := new SternBrocot()
while SB.MaxIndex() < x
SB.InsertPair()
Loop, % x
Out .= SB[A_Index] ", "
return RTrim(Out, " ,")
}
; Show the (1-based) index of where the numbers 1-to-10 first appears in the sequence.
; Show the (1-based) index of where the number 100 first appears in the sequence.
FindOneToX(x)
{
SB := new SternBrocot(), xRequired := x, Found := []
while xRequired > 0 ; While the count of numbers yet to be found is > 0.
{
Loop, 2 ; Consider the second last member and then the last member.
{
n := SB[i := SB.MaxIndex() - 2 + A_Index]
; If number (n) has not been found yet, and it is less than the maximum number to
; find (x), record the index (i) and decrement the count of numbers yet to be found.
if (Found[n] = "" && n <= x)
Found[n] := i, xRequired--
}
SB.InsertPair() ; Insert the two members that will be checked next.
}
return Found
}
; Check that the greatest common divisor of all the two consecutive members of the series up to
; the 1000th member, is always one.
GCDsUpToXAreOne(x)
{
SB := new SternBrocot()
while SB.MaxIndex() < x
SB.InsertPair()
Loop, % x - 1
if GCD(SB[A_Index], SB[A_Index + 1]) > 1
return 0
return 1
}
GCD(a, b) {
while b
b := Mod(a | 0x0, a := b)
return a
}
```

{{out}}

```
First 15: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4
First 1 found at 1
First 2 found at 3
First 3 found at 5
First 4 found at 9
First 5 found at 11
First 6 found at 33
First 7 found at 19
First 8 found at 21
First 9 found at 35
First 10 found at 39
First 100 found at 1179
GCDs of all two consecutive members are one.
```

## C

Recursive function.

#include <stdio.h> typedef unsigned int uint; /* the sequence, 0-th member is 0 */ uint f(uint n) { return n < 2 ? n : (n&1) ? f(n/2) + f(n/2 + 1) : f(n/2); } uint gcd(uint a, uint b) { return a ? a < b ? gcd(b%a, a) : gcd(a%b, b) : b; } void find(uint from, uint to) { do { uint n; for (n = 1; f(n) != from ; n++); printf("%3u at Stern #%u.\n", from, n); } while (++from <= to); } int main(void) { uint n; for (n = 1; n < 16; n++) printf("%u ", f(n)); puts("are the first fifteen."); find(1, 10); find(100, 0); for (n = 1; n < 1000 && gcd(f(n), f(n+1)) == 1; n++); printf(n == 1000 ? "All GCDs are 1.\n" : "GCD of #%d and #%d is not 1", n, n+1); return 0; }

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4 are the first fifteen.
1 at Stern #1.
2 at Stern #3.
3 at Stern #5.
4 at Stern #9.
5 at Stern #11.
6 at Stern #33.
7 at Stern #19.
8 at Stern #21.
9 at Stern #35.
10 at Stern #39.
100 at Stern #1179.
All GCDs are 1.
```

The above `f()`

can be replaced by the following, which is much faster but probably less obvious as to how it arrives from the recurrence relations.

uint f(uint n) { uint a = 1, b = 0; while (n) { if (n&1) b += a; else a += b; n >>= 1; } return b; }

## C++

#include <iostream> #include <iomanip> #include <algorithm> #include <vector> unsigned gcd( unsigned i, unsigned j ) { return i ? i < j ? gcd( j % i, i ) : gcd( i % j, j ) : j; } void createSequence( std::vector<unsigned>& seq, int c ) { if( 1500 == seq.size() ) return; unsigned t = seq.at( c ) + seq.at( c + 1 ); seq.push_back( t ); seq.push_back( seq.at( c + 1 ) ); createSequence( seq, c + 1 ); } int main( int argc, char* argv[] ) { std::vector<unsigned> seq( 2, 1 ); createSequence( seq, 0 ); std::cout << "First fifteen members of the sequence:\n "; for( unsigned x = 0; x < 15; x++ ) { std::cout << seq[x] << " "; } std::cout << "\n\n"; for( unsigned x = 1; x < 11; x++ ) { std::vector<unsigned>::iterator i = std::find( seq.begin(), seq.end(), x ); if( i != seq.end() ) { std::cout << std::setw( 3 ) << x << " is at pos. #" << 1 + distance( seq.begin(), i ) << "\n"; } } std::cout << "\n"; std::vector<unsigned>::iterator i = std::find( seq.begin(), seq.end(), 100 ); if( i != seq.end() ) { std::cout << 100 << " is at pos. #" << 1 + distance( seq.begin(), i ) << "\n"; } std::cout << "\n"; unsigned g; bool f = false; for( int x = 0, y = 1; x < 1000; x++, y++ ) { g = gcd( seq[x], seq[y] ); if( g != 1 ) f = true; std::cout << std::setw( 4 ) << x + 1 << ": GCD (" << seq[x] << ", " << seq[y] << ") = " << g << ( g != 1 ? " <-- ERROR\n" : "\n" ); } std::cout << "\n" << ( f ? "THERE WERE ERRORS --- NOT ALL GCDs ARE '1'!" : "CORRECT: ALL GCDs ARE '1'!" ) << "\n\n"; return 0; }

{{out}}

```
First fifteen members of the sequence:
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
1 is at pos. #1
2 is at pos. #3
3 is at pos. #5
4 is at pos. #9
5 is at pos. #11
6 is at pos. #33
7 is at pos. #19
8 is at pos. #21
9 is at pos. #35
10 is at pos. #39
100 is at pos. #1179
1: GCD (1, 1) = 1
2: GCD (1, 2) = 1
3: GCD (2, 1) = 1
4: GCD (1, 3) = 1
5: GCD (3, 2) = 1
6: GCD (2, 3) = 1
7: GCD (3, 1) = 1
8: GCD (1, 4) = 1
[...]
993: GCD (26, 21) = 1
994: GCD (21, 37) = 1
995: GCD (37, 16) = 1
996: GCD (16, 43) = 1
997: GCD (43, 27) = 1
998: GCD (27, 38) = 1
999: GCD (38, 11) = 1
1000: GCD (11, 39) = 1
CORRECT: ALL GCDs ARE '1'!
```

## C#

using System; using System.Collections.Generic; using System.Linq; static class Program { static List<int> l = new List<int>() { 1, 1 }; static int gcd(int a, int b) { return a > 0 ? a < b ? gcd(b % a, a) : gcd(a % b, b) : b; } static void Main(string[] args) { int max = 1000; int take = 15; int i = 1; int[] selection = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100 }; do { l.AddRange(new List<int>() { l[i] + l[i - 1], l[i] }); i += 1; } while (l.Count < max || l[l.Count - 2] != selection.Last()); Console.Write("The first {0} items In the Stern-Brocot sequence: ", take); Console.WriteLine("{0}\n", string.Join(", ", l.Take(take))); Console.WriteLine("The locations of where the selected numbers (1-to-10, & 100) first appear:"); foreach (int ii in selection) { int j = l.FindIndex(x => x == ii) + 1; Console.WriteLine("{0,3}: {1:n0}", ii, j); } Console.WriteLine(); bool good = true; for (i = 1; i <= max; i++) { if (gcd(l[i], l[i - 1]) != 1) { good = false; break; } } Console.WriteLine("The greatest common divisor of all the two consecutive items of the" + " series up to the {0}th item is {1}always one.", max, good ? "" : "not "); } }

{{out}}

```
The first 15 items In the Stern-Brocot sequence: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4
The locations of where the selected numbers (1-to-10, & 100) first appear:
1: 1
2: 3
3: 5
4: 9
5: 11
6: 33
7: 19
8: 21
9: 35
10: 39
100: 1,179
The greatest common divisor of all the two consecutive items of the series up to the 1000th item is always one.
```

## Clojure

;; each step adds two items (defn sb-step [v] (let [i (quot (count v) 2)] (conj v (+ (v (dec i)) (v i)) (v i)))) ;; A lazy, infinite sequence -- `take` what you want. (def all-sbs (sequence (map peek) (iterate sb-step [1 1]))) ;; zero-based (defn first-appearance [n] (first (keep-indexed (fn [i x] (when (= x n) i)) all-sbs))) ;; inlined abs; rem is slightly faster than mod, and the same result for positive values (defn gcd [a b] (loop [a (if (neg? a) (- a) a) b (if (neg? b) (- b) b)] (if (zero? b) a (recur b (rem a b))))) (defn check-pairwise-gcd [cnt] (let [sbs (take (inc cnt) all-sbs)] (every? #(= 1 %) (map gcd sbs (rest sbs))))) ;; one-based index required by problem statement (defn report-sb [] (println "First 15 Stern-Brocot members:" (take 15 all-sbs)) (println "First appearance of N at 1-based index:") (doseq [n [1 2 3 4 5 6 7 8 9 10 100]] (println " first" n "at" (inc (first-appearance n)))) (println "Check pairwise GCDs = 1 ..." (check-pairwise-gcd 1000)) true) (report-sb)

{{Output}}

```
First 15 Stern-Brocot members: (1 1 2 1 3 2 3 1 4 3 5 2 5 3 4)
First appearance of N at 1-based index:
first 1 at 1
first 2 at 3
first 3 at 5
first 4 at 9
first 5 at 11
first 6 at 33
first 7 at 19
first 8 at 21
first 9 at 35
first 10 at 39
first 100 at 1179
Check pairwise GCDs = 1 ... true
true
```

### Clojure: Using Lazy Sequences

(ns test-p.core) (defn gcd "(gcd a b) computes the greatest common divisor of a and b." [a b] (if (zero? b) a (recur b (mod a b)))) (defn stern-brocat-next [p] " p is the block of the sequence we are using to compute the next block This routine computes the next block " (into [] (concat (rest p) [(+ (first p) (second p))] [(second p)]))) (defn seq-stern-brocat ([] (seq-stern-brocat [1 1])) ([p] (lazy-seq (cons (first p) (seq-stern-brocat (stern-brocat-next p)))))) ; First 15 elements (println (take 15 (seq-stern-brocat))) ; Where numbers 1 to 10 first appear (doseq [n (concat (range 1 11) [100])] (println "The first appearnce of" n "is at index" (some (fn [[i k]] (when (= k n) (inc i))) (map-indexed vector (seq-stern-brocat))))) ;; Check that gcd between 1st 1000 consecutive elements equals 1 ; Create cosecutive pairs of 1st 1000 elements (def one-thousand-pairs (take 1000 (partition 2 1 (seq-stern-brocat)))) ; Check every pair has a gcd = 1 (println (every? (fn [[ith ith-plus-1]] (= (gcd ith ith-plus-1) 1)) one-thousand-pairs))

{{Output}}

```
(1 1 2 1 3 2 3 1 4 3 5 2 5 3 4)
The first appearnce of 1 is at index 1
The first appearnce of 2 is at index 3
The first appearnce of 3 is at index 5
The first appearnce of 4 is at index 9
The first appearnce of 5 is at index 11
The first appearnce of 6 is at index 33
The first appearnce of 7 is at index 19
The first appearnce of 8 is at index 21
The first appearnce of 9 is at index 35
The first appearnce of 10 is at index 39
The first appearnce of 100 is at index 1179
true
```

## Common Lisp

(defun stern-brocot (numbers) (declare ((or null (vector integer)) numbers)) (cond ((null numbers) (setf numbers (make-array 2 :element-type 'integer :adjustable t :fill-pointer t :initial-element 1))) ((zerop (length numbers)) (vector-push-extend 1 numbers) (vector-push-extend 1 numbers)) (t (assert (evenp (length numbers))) (let* ((considered-index (/ (length numbers) 2)) (considered (aref numbers considered-index)) (precedent (aref numbers (1- considered-index)))) (vector-push-extend (+ considered precedent) numbers) (vector-push-extend considered numbers)))) numbers) (defun first-15 () (loop for input = nil then seq for seq = (stern-brocot input) while (< (length seq) 15) finally (format t "First 15: ~{~A~^ ~}~%" (coerce (subseq seq 0 15) 'list)))) (defun first-1-to-10 () (loop with seq = (stern-brocot nil) for i from 1 to 10 for index = (loop with start = 0 for pos = (position i seq :start start) until pos do (setf start (length seq) seq (stern-brocot seq)) finally (return (1+ pos))) do (format t "First ~D at ~D~%" i index))) (defun first-100 () (loop for input = nil then seq for start = (length input) for seq = (stern-brocot input) for pos = (position 100 seq :start start) until pos finally (format t "First 100 at ~D~%" (1+ pos)))) (defun check-gcd () (loop for input = nil then seq for seq = (stern-brocot input) while (< (length seq) 1000) finally (if (loop for i from 0 below 999 always (= 1 (gcd (aref seq i) (aref seq (1+ i))))) (write-line "Correct. The GCDs of all the two consecutive numbers are 1.") (write-line "Wrong.")))) (defun main () (first-15) (first-1-to-10) (first-100) (check-gcd))

{{out}}

```
First 15: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
First 1 at 1
First 2 at 3
First 3 at 5
First 4 at 9
First 5 at 11
First 6 at 33
First 7 at 19
First 8 at 21
First 9 at 35
First 10 at 39
First 100 at 1179
Correct. The GCDs of all the two consecutive numbers are 1.
```

## D

{{trans|Python}}

import std.stdio, std.numeric, std.range, std.algorithm; /// Generates members of the stern-brocot series, in order, /// returning them when the predicate becomes false. uint[] sternBrocot(bool delegate(in uint[]) pure nothrow @safe @nogc pred=seq => seq.length < 20) pure nothrow @safe { typeof(return) sb = [1, 1]; size_t i = 0; while (pred(sb)) { sb ~= [sb[i .. i + 2].sum, sb[i + 1]]; i++; } return sb; } void main() { enum nFirst = 15; writefln("The first %d values:\n%s\n", nFirst, sternBrocot(seq => seq.length < nFirst).take(nFirst)); foreach (immutable nOccur; iota(1, 10 + 1).chain(100.only)) writefln("1-based index of the first occurrence of %3d in the series: %d", nOccur, sternBrocot(seq => nOccur != seq[$ - 2]).length - 1); enum nGcd = 1_000; auto s = sternBrocot(seq => seq.length < nGcd).take(nGcd); assert(zip(s, s.dropOne).all!(ss => ss[].gcd == 1), "A fraction from adjacent terms is reducible."); }

{{out}}

```
The first 15 values:
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
1-based index of the first occurrence of 1 in the series: 1
1-based index of the first occurrence of 2 in the series: 3
1-based index of the first occurrence of 3 in the series: 5
1-based index of the first occurrence of 4 in the series: 9
1-based index of the first occurrence of 5 in the series: 11
1-based index of the first occurrence of 6 in the series: 33
1-based index of the first occurrence of 7 in the series: 19
1-based index of the first occurrence of 8 in the series: 21
1-based index of the first occurrence of 9 in the series: 35
1-based index of the first occurrence of 10 in the series: 39
1-based index of the first occurrence of 100 in the series: 1179
```

This uses a queue from the Queue/usage Task:

import std.stdio, std.algorithm, std.range, std.numeric, queue_usage2; struct SternBrocot { private auto sb = GrowableCircularQueue!uint(1, 1); enum empty = false; @property uint front() pure nothrow @safe @nogc { return sb.front; } uint popFront() pure nothrow @safe { sb.push(sb.front + sb[1]); sb.push(sb[1]); return sb.pop; } } void main() { SternBrocot().drop(50_000_000).front.writeln; }

{{out}}

```
7004
```

**Direct Version:**
{{trans|C}}

void main() { import std.stdio, std.numeric, std.range, std.algorithm, std.bigint, std.conv; /// Stern-Brocot sequence, 0-th member is 0. T sternBrocot(T)(T n) pure nothrow /*safe*/ { T a = 1, b = 0; while (n) { if (n & 1) b += a; else a += b; n >>= 1; } return b; } alias sb = sternBrocot!uint; enum nFirst = 15; writefln("The first %d values:\n%s\n", nFirst, iota(1, nFirst + 1).map!sb); foreach (immutable nOccur; iota(1, 10 + 1).chain(100.only)) writefln("1-based index of the first occurrence of %3d in the series: %d", nOccur, sequence!q{n}.until!(n => sb(n) == nOccur).walkLength); auto s = iota(1, 1_001).map!sb; assert(s.zip(s.dropOne).all!(ss => ss[].gcd == 1), "A fraction from adjacent terms is reducible."); sternBrocot(10.BigInt ^^ 20_000).text.length.writeln; }

{{out}}

```
The first 15 values:
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
1-based index of the first occurrence of 1 in the series: 1
1-based index of the first occurrence of 2 in the series: 3
1-based index of the first occurrence of 3 in the series: 5
1-based index of the first occurrence of 4 in the series: 9
1-based index of the first occurrence of 5 in the series: 11
1-based index of the first occurrence of 6 in the series: 33
1-based index of the first occurrence of 7 in the series: 19
1-based index of the first occurrence of 8 in the series: 21
1-based index of the first occurrence of 9 in the series: 35
1-based index of the first occurrence of 10 in the series: 39
1-based index of the first occurrence of 100 in the series: 1179
7984
```

## EchoLisp

### Function

;; stern (2n ) = stern (n) ;; stern(2n+1) = stern(n) + stern(n+1) (define (stern n) (cond (( < n 3) 1) ((even? n) (stern (/ n 2))) (else (let ((m (/ (1- n) 2))) (+ (stern m) (stern (1+ m))))))) (remember 'stern)

{{out}}

; generate the sequence and check GCD (for ((n 10000)) (unless (= (gcd (stern n) (stern (1+ n))) 1) (error "BAD GCD" n))) → #t ;; first items (define sterns (cache 'stern)) (subvector sterns 1 16) → #( 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4) ;; first occurences index (for ((i (in-range 1 11))) (write (vector-index i sterns))) → 0 3 5 9 11 33 19 21 35 39 ;; 100 (writeln (vector-index 100 sterns)) → 1179 (stern 900000) → 446 (stern 900001) → 2479

### Stream

From [https://oeis.org/A002487 A002487], if we group the elements by two, we get (uniquely) all the rationals. Another way to generate the rationals, hence the stern sequence, is to iterate the function f(x) = floor(x) + 1 - fract(x).

;; grouping (for ((i (in-range 2 40 2))) (write (/ (stern i)(stern (1+ i))))) → 1/2 1/3 2/3 1/4 3/5 2/5 3/4 1/5 4/7 3/8 5/7 2/7 5/8 3/7 4/5 1/6 5/9 4/11 7/10 ;; computing f(1), f(f(1)), etc. (define (f x) (let [(a (/ (- (floor x) -1 (fract x))))] (if (> a 1) (f a) (cons a a)))) (define T (make-stream f 1)) (for((i 19)) (write (stream-iterate T))) → 1/2 1/3 2/3 1/4 3/5 2/5 3/4 1/5 4/7 3/8 5/7 2/7 5/8 3/7 4/5 1/6 5/9 4/11 7/10

## Elixir

defmodule SternBrocot do def sequence do Stream.unfold({0,{1,1}}, fn {i,acc} -> a = elem(acc, i) b = elem(acc, i+1) {a, {i+1, Tuple.append(acc, a+b) |> Tuple.append(b)}} end) end def task do IO.write "First fifteen members of the sequence:\n " IO.inspect Enum.take(sequence, 15) Enum.each(Enum.concat(1..10, [100]), fn n -> i = Enum.find_index(sequence, &(&1==n)) + 1 IO.puts "#{n} first appears at #{i}" end) Enum.take(sequence, 1000) |> Enum.chunk(2,1) |> Enum.all?(fn [a,b] -> gcd(a,b) == 1 end) |> if(do: "All GCD's are 1", else: "Whoops, not all GCD's are 1!") |> IO.puts end defp gcd(a,0), do: abs(a) defp gcd(a,b), do: gcd(b, rem(a,b)) end SternBrocot.task

{{out}}

```
First fifteen members of the sequence:
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
1 first appears at 1
2 first appears at 3
3 first appears at 5
4 first appears at 9
5 first appears at 11
6 first appears at 33
7 first appears at 19
8 first appears at 21
9 first appears at 35
10 first appears at 39
100 first appears at 1179
All GCD's are 1
```

=={{header|F_Sharp|F#}}==

### The function

// Generate Stern-Brocot Sequence. Nigel Galloway: October 11th., 2018 let sb=Seq.unfold(fun (n::g::t)->Some(n,[g]@t@[n+g;g]))[1;1]

### The Task

Uses [[Greatest_common_divisor#F.23]]

sb |> Seq.take 15 |> Seq.iter(printf "%d ");printfn "" [1..10] |> List.map(fun n->(n,(sb|>Seq.findIndex(fun g->g=n))+1)) |> List.iter(printf "%A ");printfn "" printfn "%d" ((sb|>Seq.findIndex(fun g->g=100))+1) printfn "There are %d consecutive members, of the first thousand members, with GCD <> 1" (sb |> Seq.take 1000 |>Seq.pairwise |> Seq.filter(fun(n,g)->gcd n g <> 1) |> Seq.length)

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
(1, 1) (2, 3) (3, 5) (4, 9) (5, 11) (6, 33) (7, 19) (8, 21) (9, 35) (10, 39)
1179
There are 0 consecutive members, of the first thousand members, with GCD <> 1
```

## Factor

Using the alternative function given in the C example for computing the Stern-Brocot sequence.

```
USING: formatting io kernel lists lists.lazy locals math
math.ranges prettyprint sequences ;
IN: rosetta-code.stern-brocot
: fn ( n -- m )
[ 1 0 ] dip
[ dup zero? ] [
dup 1 bitand zero?
[ dupd [ + ] 2dip ]
[ [ dup ] [ + ] [ ] tri* ] if
-1 shift
] until drop nip ;
:: search ( n -- m )
1 0 lfrom [ fn n = ] lfilter ltake list>array first ;
: first15 ( -- )
15 [1,b] [ fn pprint bl ] each
"are the first fifteen." print ;
: first-appearances ( -- )
10 [1,b] 100 suffix
[ dup search "First %3u at Stern #%u.\n" printf ] each ;
: gcd-test ( -- )
1,000 [1,b] [ dup 1 + [ fn ] bi@ gcd nip 1 = not ] filter
empty? "" " not" ? "All GCDs are%s 1.\n" printf ;
: main ( -- ) first15 first-appearances gcd-test ;
MAIN: main
```

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4 are the first fifteen.
First 1 at Stern #1.
First 2 at Stern #3.
First 3 at Stern #5.
First 4 at Stern #9.
First 5 at Stern #11.
First 6 at Stern #33.
First 7 at Stern #19.
First 8 at Stern #21.
First 9 at Stern #35.
First 10 at Stern #39.
First 100 at Stern #1179.
All GCDs are 1.
```

## Fortran

{{trans|VBScript}}

### Fortran IV

```
* STERN-BROCOT SEQUENCE - FORTRAN IV
DIMENSION ISB(2400)
NN=2400
ISB(1)=1
ISB(2)=1
I=1
J=2
K=2
1 IF(K.GE.NN) GOTO 2
K=K+1
ISB(K)=ISB(K-I)+ISB(K-J)
K=K+1
ISB(K)=ISB(K-J)
I=I+1
J=J+1
GOTO 1
2 N=15
WRITE(*,101) N
101 FORMAT(1X,'FIRST',I4)
WRITE(*,102) (ISB(I),I=1,15)
102 FORMAT(15I4)
DO 5 J=1,11
JJ=J
IF(J.EQ.11) JJ=100
DO 3 I=1,K
IF(ISB(I).EQ.JJ) GOTO 4
3 CONTINUE
4 WRITE(*,103) JJ,I
103 FORMAT(1X,'FIRST',I4,' AT ',I4)
5 CONTINUE
END
```

{{out}}

```
FIRST 15
FIRST 15
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
FIRST 1 AT 1
FIRST 2 AT 3
FIRST 3 AT 5
FIRST 4 AT 9
FIRST 5 AT 11
FIRST 6 AT 33
FIRST 7 AT 19
FIRST 8 AT 21
FIRST 9 AT 35
FIRST 10 AT 39
FIRST 100 AT 1179
```

### Fortran 90

```
! Stern-Brocot sequence - Fortran 90
parameter (nn=2400)
dimension isb(nn)
isb(1)=1; isb(2)=1
i=1; j=2; k=2
do while(k.lt.nn)
k=k+1; isb(k)=isb(k-i)+isb(k-j)
k=k+1; isb(k)=isb(k-j)
i=i+1; j=j+1
end do
n=15
write(*,"(1x,'First',i4)") n
write(*,"(15i4)") (isb(i),i=1,15)
do j=1,11
jj=j
if(j==11) jj=100
do i=1,k
if(isb(i)==jj) exit
end do
write(*,"(1x,'First',i4,' at ',i4)") jj,i
end do
end
```

{{out}}

```
First 15
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
First 1 at 1
First 2 at 3
First 3 at 5
First 4 at 9
First 5 at 11
First 6 at 33
First 7 at 19
First 8 at 21
First 9 at 35
First 10 at 39
First 100 at 1179
```

## FreeBASIC

```
' version 02-03-2019
' compile with: fbc -s console
#Define max 2000
Dim Shared As UInteger stern(max +2)
Sub stern_brocot
stern(1) = 1
stern(2) = 1
Dim As UInteger i = 2 , n = 2, ub = UBound(stern)
Do While i < ub
i += 1
stern(i) = stern(n) + stern(n -1)
i += 1
stern(i) = stern(n)
n += 1
Loop
End Sub
Function gcd(x As UInteger, y As UInteger) As UInteger
Dim As UInteger t
While y
t = y
y = x Mod y
x = t
Wend
Return x
End Function
' ------=< MAIN >=------
Dim As UInteger i
stern_brocot
Print "The first 15 are: " ;
For i = 1 To 15
Print stern(i); " ";
Next
Print : Print
Print " Index First nr."
Dim As UInteger d = 1
For i = 1 To max
If stern(i) = d Then
Print Using " ######"; i; stern(i)
d += 1
If d = 11 Then d = 100
If d = 101 Then Exit For
i = 0
End If
Next
Print : Print
d = 0
For i = 1 To 1000
If gcd(stern(i), stern(i +1)) <> 1 Then
d = gcd(stern(i), stern(i +1))
Exit For
End If
Next
If d = 0 Then
Print "GCD of two consecutive members of the series up to the 1000th member is 1"
Else
Print "The GCD for index "; i; " and "; i +1; " = "; d
End If
' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End
```

{{out}}

```
The first 15 are: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
Index First nr.
1 1
3 2
5 3
9 4
11 5
33 6
19 7
21 8
35 9
39 10
1179 100
GCD of two consecutive members of the series up to the 1000th member is 1
```

## Go

package main import ( "fmt" "sternbrocot" ) func main() { // Task 1, using the conventional sort of generator that generates // terms endlessly. g := sb.Generator() // Task 2, demonstrating the generator. fmt.Println("First 15:") for i := 1; i <= 15; i++ { fmt.Printf("%2d: %d\n", i, g()) } // Task 2 again, showing a simpler technique that might or might not be // considered to "generate" terms. s := sb.New() fmt.Println("First 15:", s.FirstN(15)) // Tasks 3 and 4. for _, x := range []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100} { fmt.Printf("%3d at 1-based index %d\n", x, 1+s.Find(x)) } // Task 5. fmt.Println("1-based indexes: gcd") for n, f := range s.FirstN(1000)[:999] { g := gcd(f, (*s)[n+1]) fmt.Printf("%d,%d: gcd(%d, %d) = %d\n", n+1, n+2, f, (*s)[n+1], g) if g != 1 { panic("oh no!") return } } } // gcd copied from greatest common divisor task func gcd(x, y int) int { for y != 0 { x, y = y, x%y } return x }

// SB implements the Stern-Brocot sequence. // // Generator() satisfies RC Task 1. For remaining tasks, Generator could be // used but FirstN(), and Find() are simpler methods for specific stopping // criteria. FirstN and Find might also be considered to satisfy Task 1, // in which case Generator would not really be needed. Anyway, there it is. package sb // Seq represents an even number of terms of a Stern-Brocot sequence. // // Terms are stored in a slice. Terms start with 1. // (Specifically, the zeroth term, 0, given in OEIS A002487 is not represented.) // Term 1 (== 1) is stored at slice index 0. // // Methods on Seq rely on Seq always containing an even number of terms. type Seq []int // New returns a Seq with the two base terms. func New() *Seq { return &Seq{1, 1} // Step 1 of the RC task. } // TwoMore appends two more terms to p. // It's the body of the loop in the RC algorithm. // Generate(), FirstN(), and Find() wrap this body in different ways. func (p *Seq) TwoMore() { s := *p n := len(s) / 2 // Steps 2 and 5 of the RC task. c := s[n] *p = append(s, c+s[n-1], c) // Steps 3 and 4 of the RC task. } // Generator returns a generator function that returns successive terms // (until overflow.) func Generator() func() int { n := 0 p := New() return func() int { if len(*p) == n { p.TwoMore() } t := (*p)[n] n++ return t } } // FirstN lazily extends p as needed so that it has at least n terms. // FirstN then returns a list of the first n terms. func (p *Seq) FirstN(n int) []int { for len(*p) < n { p.TwoMore() } return []int((*p)[:n]) } // Find lazily extends p as needed until it contains the value x // Find then returns the slice index of x in p. func (p *Seq) Find(x int) int { for n, f := range *p { if f == x { return n } } for { p.TwoMore() switch x { case (*p)[len(*p)-2]: return len(*p) - 2 case (*p)[len(*p)-1]: return len(*p) - 1 } } }

{{out}}

```
First 15:
1: 1
2: 1
3: 2
4: 1
5: 3
6: 2
7: 3
8: 1
9: 4
10: 3
11: 5
12: 2
13: 5
14: 3
15: 4
First 15: [1 1 2 1 3 2 3 1 4 3 5 2 5 3 4]
1 at 1-based index 1
2 at 1-based index 3
3 at 1-based index 5
4 at 1-based index 9
5 at 1-based index 11
6 at 1-based index 33
7 at 1-based index 19
8 at 1-based index 21
9 at 1-based index 35
10 at 1-based index 39
100 at 1-based index 1179
1-based indexes: gcd
1,2: gcd(1, 1) = 1
2,3: gcd(1, 2) = 1
3,4: gcd(2, 1) = 1
4,5: gcd(1, 3) = 1
...
998,999: gcd(27, 38) = 1
999,1000: gcd(38, 11) = 1
```

## Haskell

import Data.List (elemIndex) sb :: [Int] sb = 1 : 1 : f (tail sb) sb where f (a:aa) (b:bb) = a + b : a : f aa bb main :: IO () main = do print $ take 15 sb print [ (i, 1 + (\(Just i) -> i) (elemIndex i sb)) | i <- [1 .. 10] ++ [100] ] print $ all (\(a, b) -> 1 == gcd a b) $ take 1000 $ zip sb (tail sb)

{{out}}

```
[1,1,2,1,3,2,3,1,4,3,5,2,5,3,4]
[(1,1),(2,3),(3,5),(4,9),(5,11),(6,33),(7,19),(8,21),(9,35),(10,39),(100,1179)]
True
```

Or, expressed in terms of iterate:

import Data.List (nubBy, sortBy) import Data.Ord (comparing) import Data.Monoid ((<>)) import Data.Function (on) sternBrocot :: [Int] sternBrocot = let go (a:b:xs) = (b : xs) <> [a + b, b] in head <$> iterate go [1, 1] -- TEST ------------------------------------------------------------- main :: IO () main = do print $ take 15 sternBrocot print $ take 10 $ nubBy (on (==) fst) $ sortBy (comparing fst) $ takeWhile ((110 >=) . fst) $ zip sternBrocot [1 ..] print $ take 1 $ dropWhile ((100 /=) . fst) $ zip sternBrocot [1 ..] print $ (all ((1 ==) . uncurry gcd) . (zip <*> tail)) $ take 1000 sternBrocot

{{Out}}

```
[1,1,2,1,3,2,3,1,4,3,5,2,5,3,4]
[(1,1),(2,3),(3,5),(4,9),(5,11),(6,33),(7,19),(8,21),(9,35),(10,39)]
[(100,1179)]
True
```

## JavaScript

(() => { 'use strict'; const main = () => { // sternBrocot :: Generator [Int] const sternBrocot = () => { const go = xs => { const x = snd(xs); return tail(append(xs, [fst(xs) + x, x])); }; return fmapGen(head, iterate(go, [1, 1])); }; // TESTS ------------------------------------------ const sbs = take(1200, sternBrocot()), ixSB = zip(sbs, enumFrom(1)); return unlines(map( JSON.stringify, [ take(15, sbs), take(10, map(listFromTuple, nubBy( on(eq, fst), sortBy( comparing(fst), takeWhile(x => 12 !== fst(x), ixSB) ) ) ) ), listFromTuple( take(1, dropWhile(x => 100 !== fst(x), ixSB))[0] ), all(tpl => 1 === gcd(fst(tpl), snd(tpl)), take(1000, zip(sbs, tail(sbs))) ) ] )); }; // GENERIC ABSTRACTIONS ------------------------------- // Just :: a -> Maybe a const Just = x => ({ type: 'Maybe', Nothing: false, Just: x }); // Nothing :: Maybe a const Nothing = () => ({ type: 'Maybe', Nothing: true, }); // Tuple (,) :: a -> b -> (a, b) const Tuple = (a, b) => ({ type: 'Tuple', '0': a, '1': b, length: 2 }); // | Absolute value. // abs :: Num -> Num const abs = Math.abs; // Determines whether all elements of the structure // satisfy the predicate. // all :: (a -> Bool) -> [a] -> Bool const all = (p, xs) => xs.every(p); // append (++) :: [a] -> [a] -> [a] // append (++) :: String -> String -> String const append = (xs, ys) => xs.concat(ys); // chr :: Int -> Char const chr = String.fromCodePoint; // comparing :: (a -> b) -> (a -> a -> Ordering) const comparing = f => (x, y) => { const a = f(x), b = f(y); return a < b ? -1 : (a > b ? 1 : 0); }; // dropWhile :: (a -> Bool) -> [a] -> [a] // dropWhile :: (Char -> Bool) -> String -> String const dropWhile = (p, xs) => { const lng = xs.length; return 0 < lng ? xs.slice( until( i => i === lng || !p(xs[i]), i => 1 + i, 0 ) ) : []; }; // enumFrom :: a -> [a] function* enumFrom(x) { let v = x; while (true) { yield v; v = succ(v); } } // eq (==) :: Eq a => a -> a -> Bool const eq = (a, b) => { const t = typeof a; return t !== typeof b ? ( false ) : 'object' !== t ? ( 'function' !== t ? ( a === b ) : a.toString() === b.toString() ) : (() => { const aks = Object.keys(a); return aks.length !== Object.keys(b).length ? ( false ) : aks.every(k => eq(a[k], b[k])); })(); }; // fmapGen <$> :: (a -> b) -> Gen [a] -> Gen [b] function* fmapGen(f, gen) { const g = gen; let v = take(1, g)[0]; while (0 < v.length) { yield(f(v)) v = take(1, g)[0] } } // fst :: (a, b) -> a const fst = tpl => tpl[0]; // gcd :: Int -> Int -> Int const gcd = (x, y) => { const _gcd = (a, b) => (0 === b ? a : _gcd(b, a % b)), abs = Math.abs; return _gcd(abs(x), abs(y)); }; // head :: [a] -> a const head = xs => xs.length ? xs[0] : undefined; // isChar :: a -> Bool const isChar = x => ('string' === typeof x) && (1 === x.length); // iterate :: (a -> a) -> a -> Gen [a] function* iterate(f, x) { let v = x; while (true) { yield(v); v = f(v); } } // Returns Infinity over objects without finite length // this enables zip and zipWith to choose the shorter // argument when one is non-finite, like cycle, repeat etc // length :: [a] -> Int const length = xs => xs.length || Infinity; // listFromTuple :: (a, a ...) -> [a] const listFromTuple = tpl => Array.from(tpl); // map :: (a -> b) -> [a] -> [b] const map = (f, xs) => xs.map(f); // nubBy :: (a -> a -> Bool) -> [a] -> [a] const nubBy = (p, xs) => { const go = xs => 0 < xs.length ? (() => { const x = xs[0]; return [x].concat( go(xs.slice(1) .filter(y => !p(x, y)) ) ) })() : []; return go(xs); }; // e.g. sortBy(on(compare,length), xs) // on :: (b -> b -> c) -> (a -> b) -> a -> a -> c const on = (f, g) => (a, b) => f(g(a), g(b)); // ord :: Char -> Int const ord = c => c.codePointAt(0); // snd :: (a, b) -> b const snd = tpl => tpl[1]; // sortBy :: (a -> a -> Ordering) -> [a] -> [a] const sortBy = (f, xs) => xs.slice() .sort(f); // succ :: Enum a => a -> a const succ = x => isChar(x) ? ( chr(1 + ord(x)) ) : isNaN(x) ? ( undefined ) : 1 + x; // tail :: [a] -> [a] const tail = xs => 0 < xs.length ? xs.slice(1) : []; // take :: Int -> [a] -> [a] // take :: Int -> String -> String const take = (n, xs) => xs.constructor.constructor.name !== 'GeneratorFunction' ? ( xs.slice(0, n) ) : [].concat.apply([], Array.from({ length: n }, () => { const x = xs.next(); return x.done ? [] : [x.value]; })); // takeWhile :: (a -> Bool) -> [a] -> [a] // takeWhile :: (Char -> Bool) -> String -> String const takeWhile = (p, xs) => xs.constructor.constructor.name !== 'GeneratorFunction' ? (() => { const lng = xs.length; return 0 < lng ? xs.slice( 0, until( i => lng === i || !p(xs[i]), i => 1 + i, 0 ) ) : []; })() : takeWhileGen(p, xs); // takeWhileGen :: (a -> Bool) -> Gen [a] -> [a] const takeWhileGen = (p, xs) => { const ys = []; let nxt = xs.next(), v = nxt.value; while (!nxt.done && p(v)) { ys.push(v); nxt = xs.next(); v = nxt.value } return ys; }; // uncons :: [a] -> Maybe (a, [a]) const uncons = xs => { const lng = length(xs); return (0 < lng) ? ( lng < Infinity ? ( Just(Tuple(xs[0], xs.slice(1))) // Finite list ) : (() => { const nxt = take(1, xs); return 0 < nxt.length ? ( Just(Tuple(nxt[0], xs)) ) : Nothing(); })() // Lazy generator ) : Nothing(); }; // unlines :: [String] -> String const unlines = xs => xs.join('\n'); // until :: (a -> Bool) -> (a -> a) -> a -> a const until = (p, f, x) => { let v = x; while (!p(v)) v = f(v); return v; }; // Use of `take` and `length` here allows for zipping with non-finite // lists - i.e. generators like cycle, repeat, iterate. // zip :: [a] -> [b] -> [(a, b)] const zip = (xs, ys) => { const lng = Math.min(length(xs), length(ys)); return Infinity !== lng ? (() => { const bs = take(lng, ys); return take(lng, xs).map((x, i) => Tuple(x, bs[i])); })() : zipGen(xs, ys); }; // zipGen :: Gen [a] -> Gen [b] -> Gen [(a, b)] const zipGen = (ga, gb) => { function* go(ma, mb) { let a = ma, b = mb; while (!a.Nothing && !b.Nothing) { let ta = a.Just, tb = b.Just yield(Tuple(fst(ta), fst(tb))); a = uncons(snd(ta)); b = uncons(snd(tb)); } } return go(uncons(ga), uncons(gb)); }; // MAIN --- return main(); })();

{{Out}}

```
[1,1,2,1,3,2,3,1,4,3,5,2,5,3,4]
[[1,1],[2,3],[3,5],[4,9],[5,11],[6,33],[7,19],[8,21],[9,35],[10,39]]
[100,1179]
true
```

## J

We have two different kinds of list specifications here (length of the sequence, and the presence of certain values in the sequence). Also the underlying list generation mechanism is somewhat arbitrary. So let's generate the sequence iteratively and provide a truth valued function of the intermediate sequences to determine when we have generated one which is adequately long:

```
sternbrocot=:1 :0
ind=. 0
seq=. 1 1
while. -. u seq do.
ind=. ind+1
seq=. seq, +/\. seq {~ _1 0 +ind
end.
)
```

(Grammatical aside: this is an adverb which generates a noun without taking any x/y arguments. So usage is: `u sternbrocot`

. J does have precedence rules, just not very many of them. Users of other languages can get a rough idea of the grammatical terms like this: adverb is approximately like a macro, verb approximately like a function and noun is approximately like a number. Also x and y are J's names for left and right noun arguments, and u and v are J's names for left and right verb arguments. An adverb has a left verb argument. There are some other important constraints but that's probably more than enough detail for this task.)

First fifteen members of the sequence:

```
15{.(15<:#) sternbrocot
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
```

One based indices of where numbers 1-10 first appear in the sequence:

```
1+(10 e. ]) sternbrocot i.1+i.10
1 3 5 9 11 33 19 21 35 39
```

One based index of where the number 100 first appears:

```
1+(100 e. ]) sternbrocot i. 100
1179
```

List of distinct greatest common divisors of adjacent number pairs from a sternbrocot sequence which includes the first 1000 elements:

```
~.2 +./\ (1000<:#) sternbrocot
1
```

## Java

{{works with|Java|1.5+}}
This example generates the first 1200 members of the sequence since that is enough to cover all of the tests in the description. It borrows the `gcd`

method from `BigInteger`

rather than using its own.

```
import java.math.BigInteger;
import java.util.LinkedList;
public class SternBrocot {
static LinkedList<Integer> sequence = new LinkedList<Integer>(){{
add(1); add(1);
}};
private static void genSeq(int n){
for(int conIdx = 1; sequence.size() < n; conIdx++){
int consider = sequence.get(conIdx);
int pre = sequence.get(conIdx - 1);
sequence.add(consider + pre);
sequence.add(consider);
}
}
public static void main(String[] args){
genSeq(1200);
System.out.println("The first 15 elements are: " + sequence.subList(0, 15));
for(int i = 1; i <= 10; i++){
System.out.println("First occurrence of " + i + " is at " + (sequence.indexOf(i) + 1));
}
System.out.println("First occurrence of 100 is at " + (sequence.indexOf(100) + 1));
boolean failure = false;
for(int i = 0; i < 999; i++){
failure |= !BigInteger.valueOf(sequence.get(i)).gcd(BigInteger.valueOf(sequence.get(i + 1))).equals(BigInteger.ONE);
}
System.out.println("All GCDs are" + (failure ? " not" : "") + " 1");
}
}
```

{{out}}

```
The first 15 elements are: [1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
First occurrence of 1 is at 1
First occurrence of 2 is at 3
First occurrence of 3 is at 5
First occurrence of 4 is at 9
First occurrence of 5 is at 11
First occurrence of 6 is at 33
First occurrence of 7 is at 19
First occurrence of 8 is at 21
First occurrence of 9 is at 35
First occurrence of 10 is at 39
First occurrence of 100 is at 1179
All GCDs are 1
```

=== Stern-Brocot Tree === {{works with|Java|8}}

import java.awt.*; import javax.swing.*; public class SternBrocot extends JPanel { public SternBrocot() { setPreferredSize(new Dimension(800, 500)); setFont(new Font("Arial", Font.PLAIN, 18)); setBackground(Color.white); } private void drawTree(int n1, int d1, int n2, int d2, int x, int y, int gap, int lvl, Graphics2D g) { if (lvl == 0) return; // mediant int numer = n1 + n2; int denom = d1 + d2; if (lvl > 1) { g.drawLine(x + 5, y + 4, x - gap + 5, y + 124); g.drawLine(x + 5, y + 4, x + gap + 5, y + 124); } g.setColor(getBackground()); g.fillRect(x - 10, y - 15, 35, 40); g.setColor(getForeground()); g.drawString(String.valueOf(numer), x, y); g.drawString("_", x, y + 2); g.drawString(String.valueOf(denom), x, y + 22); drawTree(n1, d1, numer, denom, x - gap, y + 120, gap / 2, lvl - 1, g); drawTree(numer, denom, n2, d2, x + gap, y + 120, gap / 2, lvl - 1, g); } @Override public void paintComponent(Graphics gg) { super.paintComponent(gg); Graphics2D g = (Graphics2D) gg; g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON); int w = getWidth(); drawTree(0, 1, 1, 0, w / 2, 50, w / 4, 4, g); } public static void main(String[] args) { SwingUtilities.invokeLater(() -> { JFrame f = new JFrame(); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setTitle("Stern-Brocot Tree"); f.setResizable(false); f.add(new SternBrocot(), BorderLayout.CENTER); f.pack(); f.setLocationRelativeTo(null); f.setVisible(true); }); } }

[[File:stern_brocot_tree_java.gif]]

## jq

{{works with|jq|1.4}} In jq 1.4, there is no equivalent of "yield" for unbounded streams, and so the following uses "until".

'''Foundations:'''

```
def until(cond; update):
def _until:
if cond then . else (update | _until) end;
try _until catch if .== "break" then empty else . end ;
def gcd(a; b):
# subfunction expects [a,b] as input
# i.e. a ~ .[0] and b ~ .[1]
def rgcd: if .[1] == 0 then .[0]
else [.[1], .[0] % .[1]] | rgcd
end;
[a,b] | rgcd ;
```

'''The A002487 integer sequence:'''

The following definition is in strict accordance with https://oeis.org/A002487:
i.e. a(0) = 0, a(1) = 1; for n > 0: a(2*n) = a(n), a(2*n+1) = a(n) + a(n+1).
The n-th element of the Rosetta Code sequence (counting from 1)
is thus a[n], which accords with the fact that jq arrays have an index origin of 0.

```
# If n is non-negative, then A002487(n)
# generates an array with at least n elements of
# the A002487 sequence;
# if n is negative, elements are added until (-n)
# is found.
def A002487(n):
[0,1]
| until(
length as $l
| if n >= 0 then $l >= n
else .[$l-1] == -n
end;
length as $l
| ($l / 2) as $n
| .[$l] = .[$n]
| if (.[$l-2] == -n) then .
else .[$l + 1] = .[$n] + .[$n+1]
end ) ;
```

'''The tasks:'''

```
# Generate a stream of n integers beginning with 1,1...
def stern_brocot(n): A002487(n+1) | .[1:n+1][];
# Return the index (counting from 1) of n in the
# sequence starting with 1,1,...
def stern_brocot_index(n):
A002487(-n) | length -1 ;
def index_task:
(range(1;11), 100) as $i
| "index of \($i) is \(stern_brocot_index($i))" ;
def gcd_task:
A002487(1000)
| . as $A
| reduce range(0; length-1) as $i
( [];
gcd( $A[$i]; $A[$i+1] ) as $gcd
| if $gcd == 1 then . else . + [$gcd] end)
| if length == 0 then "GCDs are all 1"
else "GCDs include \(.)" end ;
"First 15 integers of the Stern-Brocot sequence",
"as defined in the task description are:",
stern_brocot(15),
"",
"Using an index origin of 1:",
index_task,
"",
gcd_task
```

{{out}}

$ jq -r -n -f stern_brocot.jq First 15 integers of the Stern-Brocot sequence as defined in the task description are: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4 Using an index origin of 1: index of 1 is 1 index of 2 is 3 index of 3 is 5 index of 4 is 9 index of 5 is 11 index of 6 is 33 index of 7 is 19 index of 8 is 21 index of 9 is 35 index of 10 is 39 index of 100 is 1179 GCDs are all 1

## Julia

{{trans|Python}}

function sternbrocot(f::Function=(x) -> length(x) ≥ 20)::Vector{Int} rst = Int[1, 1] i = 3 while !f(rst) append!(rst, Int[rst[i-1] + rst[i-2], rst[i-2]]) i += 1 end return rst end println("First 15 elements of Stern-Brocot series:\n", sternbrocot(x -> length(x) ≥ 15)[1:15], "\n") for i in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100) occurr = findfirst(x -> x == i, sternbrocot(x -> i ∈ x)) @printf("Index of first occurrence of %3i in the series: %4i\n", i, occurr) end print("\nAssertion: the greatest common divisor of all the two\nconsecutive members of the series up to the 1000th member, is always one: ") sb = sternbrocot(x -> length(x) > 1000) if all(gcd(prev, this) == 1 for (prev, this) in zip(sb[1:1000], sb[2:1000])) println("Confirmed.") else println("Rejected.") end

{{out}}

```
First 15 elements of Stern-Brocot series:
[1, 1, 2, 1, 3, 1, 3, 2, 4, 1, 4, 3, 4, 1, 5]
Index of first occurrence of 1 in the series: 1
Index of first occurrence of 2 in the series: 3
Index of first occurrence of 3 in the series: 5
Index of first occurrence of 4 in the series: 9
Index of first occurrence of 5 in the series: 15
Index of first occurrence of 6 in the series: 17
Index of first occurrence of 7 in the series: 23
Index of first occurrence of 8 in the series: 31
Index of first occurrence of 9 in the series: 33
Index of first occurrence of 10 in the series: 51
Index of first occurrence of 100 in the series: 1855
Assertion: the greatest common divisor of all the two
consecutive members of the series up to the 1000th member, is always one: Rejected.
```

## Kotlin

// version 1.1.0 val sbs = mutableListOf(1, 1) fun sternBrocot(n: Int, fromStart: Boolean = true) { if (n < 4 || (n % 2 != 0)) throw IllegalArgumentException("n must be >= 4 and even") var consider = if (fromStart) 1 else n / 2 - 1 while (true) { val sum = sbs[consider] + sbs[consider - 1] sbs.add(sum) sbs.add(sbs[consider]) if (sbs.size == n) break consider++ } } fun gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b) fun main(args: Array<String>) { var n = 16 // needs to be even to ensure 'considered' number is added println("First 15 members of the Stern-Brocot sequence") sternBrocot(n) println(sbs.take(15)) val firstFind = IntArray(11) // all zero by default firstFind[0] = -1 // needs to be non-zero for subsequent test for ((i, v) in sbs.withIndex()) if (v <= 10 && firstFind[v] == 0) firstFind[v] = i + 1 loop@ while (true) { n += 2 sternBrocot(n, false) val vv = sbs.takeLast(2) var m = n - 1 for (v in vv) { if (v <= 10 && firstFind[v] == 0) firstFind[v] = m if (firstFind.all { it != 0 }) break@loop m++ } } println("\nThe numbers 1 to 10 first appear at the following indices:") for (i in 1..10) println("${"%2d".format(i)} -> ${firstFind[i]}") print("\n100 first appears at index ") while (true) { n += 2 sternBrocot(n, false) val vv = sbs.takeLast(2) if (vv[0] == 100) { println(n - 1); break } if (vv[1] == 100) { println(n); break } } print("\nThe GCDs of each pair of the series up to the 1000th member are ") for (p in 0..998 step 2) { if (gcd(sbs[p], sbs[p + 1]) != 1) { println("not all one") return } } println("all one") }

{{out}}

```
First 15 members of the Stern-Brocot sequence
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
The numbers 1 to 10 first appear at the following indices:
1 -> 1
2 -> 3
3 -> 5
4 -> 9
5 -> 11
6 -> 33
7 -> 19
8 -> 21
9 -> 35
10 -> 39
100 first appears at index 1179
The GCDs of each pair of the series up to the 1000th member are all one
```

## Lua

-- Task 1 function sternBrocot (n) local sbList, pos, c = {1, 1}, 2 repeat c = sbList[pos] table.insert(sbList, c + sbList[pos - 1]) table.insert(sbList, c) pos = pos + 1 until #sbList >= n return sbList end -- Return index in table 't' of first value matching 'v' function findFirst (t, v) for key, value in pairs(t) do if v then if value == v then return key end else if value ~= 0 then return key end end end return nil end -- Return greatest common divisor of 'x' and 'y' function gcd (x, y) if y == 0 then return math.abs(x) else return gcd(y, x % y) end end -- Check GCD of adjacent values in 't' up to 1000 is always 1 function task5 (t) for pos = 1, 1000 do if gcd(t[pos], t[pos + 1]) ~= 1 then return "FAIL" end end return "PASS" end -- Main procedure local sb = sternBrocot(10000) io.write("Task 2: ") for n = 1, 15 do io.write(sb[n] .. " ") end print("\n\nTask 3:") for i = 1, 10 do print("\t" .. i, findFirst(sb, i)) end print("\nTask 4: " .. findFirst(sb, 100)) print("\nTask 5: " .. task5(sb))

{{out}}

```
Task 2: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
Task 3:
1 1
2 3
3 5
4 9
5 11
6 33
7 19
8 21
9 35
10 39
Task 4: 1179
Task 5: PASS
```

## Oforth

```
: stern(n)
| l i |
ListBuffer new dup add(1) dup add(1) dup ->l
n 1- 2 / loop: i [ l at(i) l at(i 1+) tuck + l add l add ]
n 2 mod ifFalse: [ dup removeLast drop ] dup freeze ;
stern(10000) Constant new: Sterns
```

{{out}}

```
>Sterns left(15) .
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4] ok
>10 seq apply(#[ dup . Sterns indexOf . printcr ])
1 1
2 3
3 5
4 9
5 11
6 33
7 19
8 21
9 35
10 39
ok
>Sterns indexOf(100) .
1179 ok
>999 seq map(#[ dup Sterns at swap 1 + Sterns at gcd ]) conform(#[ 1 == ]) .
1 ok
>
```

## PARI/GP

{{Works with|PARI/GP|2.7.4 and above}}

```
\\ Stern-Brocot sequence
\\ 5/27/16 aev
SternBrocot(n)={
my(L=List([1,1]),k=2); if(n<3,return(L));
for(i=2,n, listput(L,L[i]+L[i-1]); if(k++>=n, break); listput(L,L[i]);if(k++>=n, break));
return(Vec(L));
}
\\ Find the first item in any list starting with sind index (return 0 or index).
\\ 9/11/2015 aev
findinlist(list, item, sind=1)={
my(idx=0, ln=#list); if(ln==0 || sind<1 || sind>ln, return(0));
for(i=sind, ln, if(list[i]==item, idx=i; break;)); return(idx);
}
{
\\ Required tests:
my(v,j);
v=SternBrocot(15);
print1("The first 15: "); print(v);
v=SternBrocot(1200);
print1("The first i@n: "); \\print(v);
for(i=1,10, if(j=findinlist(v,i), print1(i,"@",j,", ")));
if(j=findinlist(v,100), print(100,"@",j));
v=SternBrocot(10000);
print1("All GCDs=1?: ");
j=1; for(i=2,10000, j*=gcd(v[i-1],v[i]));
if(j==1, print("Yes"), print("No"));
}
```

{{Output}}

```
The first 15: [1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
The first i@n: 1@1, 2@3, 3@5, 4@9, 5@11, 6@33, 7@19, 8@21, 9@35, 10@39, 100@1179
All GCDs=1?: Yes
```

## Pascal

{{works with|Free Pascal}}

program StrnBrCt; {$IFDEF FPC} {$MODE DELPHI} {$ENDIF} const MaxCnt = 10835282;{ seq[i] < 65536 = high(Word) } //MaxCnt = 500*1000*1000;{ 2Gbyte -> real 0.85 s user 0.31 } type tSeqdata = word;//cardinal LongWord pSeqdata = pWord;//pcardinal pLongWord tseq = array of tSeqdata; function SternBrocotCreate(size:NativeInt):tseq; var pSeq,pIns : pSeqdata; PosIns : NativeInt; sum : tSeqdata; Begin setlength(result,Size+1); dec(Size); //== High(result) pIns := @result[size];// set at end PosIns := -size+2; // negative index campare to 0 pSeq := @result[0]; sum := 1; pSeq[0]:= sum;pSeq[1]:= sum; repeat pIns[PosIns+1] := sum;//append copy of considered inc(sum,pSeq[0]); pIns[PosIns ] := sum; inc(pSeq); inc(PosIns,2);sum := pSeq[1];//aka considered until PosIns>= 0; setlength(result,length(result)-1); end; function FindIndex(const s:tSeq;value:tSeqdata):NativeInt; Begin result := 0; while result <= High(s) do Begin if s[result] = value then EXIT(result+1); inc(result); end; end; function gcd_iterative(u, v: NativeInt): NativeInt; //http://rosettacode.org/wiki/Greatest_common_divisor#Pascal_.2F_Delphi_.2F_Free_Pascal var t: NativeInt; begin while v <> 0 do begin t := u;u := v;v := t mod v; end; gcd_iterative := abs(u); end; var seq : tSeq; i : nativeInt; Begin seq:= SternBrocotCreate(MaxCnt); // Show the first fifteen members of the sequence. For i := 0 to 13 do write(seq[i],',');writeln(seq[14]); //Show the (1-based) index of where the numbers 1-to-10 first appears in the For i := 1 to 10 do write(i,' @ ',FindIndex(seq,i),','); writeln(#8#32); //Show the (1-based) index of where the number 100 first appears in the sequence. writeln(100,' @ ',FindIndex(seq,100)); //Check that the greatest common divisor of all the two consecutive members of the series up to the 1000th member, is always one. i := 999; if i > High(seq) then i := High(seq); Repeat IF gcd_iterative(seq[i],seq[i+1]) <>1 then Begin writeln(' failure at ',i+1,' ',seq[i],' ',seq[i+1]); BREAK; end; dec(i); until i <0; IF i< 0 then writeln('GCD-test is O.K.'); setlength(seq,0); end.

{{Out}}

```
1,1,2,1,3,2,3,1,4,3,5,2,5,3,4
1 @ 1,2 @ 3,3 @ 5,4 @ 9,5 @ 11,6 @ 33,7 @ 38,8 @ 42,9 @ 47,10 @ 57
100 @ 1179
GCD-test is O.K.
```

## Perl

use strict; use warnings; sub stern_brocot { my @list = (1, 1); sub { push @list, $list[0] + $list[1], $list[1]; shift @list; } } { my $generator = stern_brocot; print join ' ', map &$generator, 1 .. 15; print "\n"; } for (1 .. 10, 100) { my $index = 1; my $generator = stern_brocot; $index++ until $generator->() == $_; print "first occurrence of $_ is at index $index\n"; } { sub gcd { my ($u, $v) = @_; $v ? gcd($v, $u % $v) : abs($u); } my $generator = stern_brocot; my ($a, $b) = ($generator->(), $generator->()); for (1 .. 1000) { die "unexpected GCD for $a and $b" unless gcd($a, $b) == 1; ($a, $b) = ($b, $generator->()); } }

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
first occurrence of 1 is at index 1
first occurrence of 2 is at index 3
first occurrence of 3 is at index 5
first occurrence of 4 is at index 9
first occurrence of 5 is at index 11
first occurrence of 6 is at index 33
first occurrence of 7 is at index 19
first occurrence of 8 is at index 21
first occurrence of 9 is at index 35
first occurrence of 10 is at index 39
first occurrence of 100 is at index 1179
```

A slightly different method:{{libheader|ntheory}}

use ntheory qw/gcd vecsum vecfirst/; sub stern_diatomic { my ($p,$q,$i) = (0,1,shift); while ($i) { if ($i & 1) { $p += $q; } else { $q += $p; } $i >>= 1; } $p; } my @s = map { stern_diatomic($_) } 1..15; print "First fifteen: [@s]\n"; @s = map { my $n=$_; vecfirst { stern_diatomic($_) == $n } 1..10000 } 1..10; print "Index of 1-10 first occurrence: [@s]\n"; print "Index of 100 first occurrence: ", (vecfirst { stern_diatomic($_) == 100 } 1..10000), "\n"; print "The first 999 consecutive pairs are ", (vecsum( map { gcd(stern_diatomic($_),stern_diatomic($_+1)) } 1..999 ) == 999) ? "all coprime.\n" : "NOT all coprime!\n";

{{out}}

```
First fifteen: [1 1 2 1 3 2 3 1 4 3 5 2 5 3 4]
Index of 1-10 first occurrence: [1 3 5 9 11 33 19 21 35 39]
Index of 100 first occurrence: 1179
The first 999 consecutive pairs are all coprime.
```

## Perl 6

{{works with|rakudo|2017-03}}

```
constant @Stern-Brocot = 1, 1, {
|(@_[$_ - 1] + @_[$_], @_[$_]) given ++$
} ... *;
say @Stern-Brocot[^15];
for (flat 1..10, 100) -> $ix {
say "first occurrence of $ix is at index : ", 1 + @Stern-Brocot.first($ix, :k);
}
say so 1 == all map ^1000: { [gcd] @Stern-Brocot[$_, $_ + 1] }
```

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
first occurrence of 1 is at index : 1
first occurrence of 2 is at index : 3
first occurrence of 3 is at index : 5
first occurrence of 4 is at index : 9
first occurrence of 5 is at index : 11
first occurrence of 6 is at index : 33
first occurrence of 7 is at index : 19
first occurrence of 8 is at index : 21
first occurrence of 9 is at index : 35
first occurrence of 10 is at index : 39
first occurrence of 100 is at index : 1179
True
```

## Phix

```
sequence sb = {1,1}
integer c = 2
function stern_brocot(integer n)
while length(sb)<n do
sb &= sb[c]+sb[c-1] & sb[c]
c += 1
end while
return sb[1..n]
end function
sequence s = stern_brocot(15)
puts(1,"first 15:")
?s
integer n = 16, k
sequence idx = tagset(10)
for i=1 to length(idx) do
while 1 do
k = find(idx[i],s)
if k!=0 then exit end if
n *= 2
s = stern_brocot(n)
end while
idx[i] = k
end for
puts(1,"indexes of 1..10:")
?idx
puts(1,"index of 100:")
while 1 do
k = find(100,s)
if k!=0 then exit end if
n *= 2
s = stern_brocot(n)
end while
?k
s = stern_brocot(1000)
integer maxgcd = 1
for i=1 to 999 do
maxgcd = max(gcd(s[i],s[i+1]),maxgcd)
end for
printf(1,"max gcd:%d\n",{maxgcd})
```

{{Out}}

```
first 15:{1,1,2,1,3,2,3,1,4,3,5,2,5,3,4}
indexes of 1..10:{1,3,5,9,11,33,19,21,35,39}
index of 100:1179
max gcd:1
```

## PicoLisp

{{trans|C}}
Using the `gcd` function defined at ''[[Greatest_common_divisor#PicoLisp]]'':

```
(de nmbr (N)
(let (A 1 B 0)
(while (gt0 N)
(if (bit? 1 N)
(inc 'B A)
(inc 'A B) )
(setq N (>> 1 N)) )
B ) )
(let Lst (mapcar nmbr (range 1 2000))
(println 'First-15: (head 15 Lst))
(for N 10
(println 'First N 'found 'at: (index N Lst)) )
(println 'First 100 'found 'at: (index 100 Lst))
(for (L Lst (cdr L) (cddr L))
(test 1 (gcd (car L) (cadr L))) )
(prinl "All consecutive pairs are relative prime!") )
```

{{out}}

```
First-15: (1 1 2 1 3 2 3 1 4 3 5 2 5 3 4)
First 1 found at: 1
First 2 found at: 3
First 3 found at: 5
First 4 found at: 9
First 5 found at: 11
First 6 found at: 33
First 7 found at: 19
First 8 found at: 21
First 9 found at: 35
First 10 found at: 39
First 100 found at: 1179
All consecutive pairs are relative prime!
```

## PowerShell

# An iterative approach function iter_sb($count = 2000) { # Taken from RosettaCode GCD challenge function Get-GCD ($x, $y) { if ($y -eq 0) { $x } else { Get-GCD $y ($x%$y) } } $answer = @(1,1) $index = 1 while ($answer.Length -le $count) { $answer += $answer[$index] + $answer[$index - 1] $answer += $answer[$index] $index++ } 0..14 | foreach {$answer[$_]} 1..10 | foreach {'Index of {0}: {1}' -f $_, ($answer.IndexOf($_) + 1)} 'Index of 100: {0}' -f ($answer.IndexOf(100) + 1) [bool] $gcd = $true 1..999 | foreach {$gcd = $gcd -and ((Get-GCD $answer[$_] $answer[$_ - 1]) -eq 1)} 'GCD = 1 for first 1000 members: {0}' -f $gcd }

{{out}}

```
PS C:\> iter_sb
1
1
2
1
3
2
3
1
4
3
5
2
5
3
4
Index of 1: 1
Index of 2: 3
Index of 3: 5
Index of 4: 9
Index of 5: 11
Index of 6: 33
Index of 7: 19
Index of 8: 21
Index of 9: 35
Index of 10: 39
Index of 100: 1179
GCD = 1 for first 1000 members: True
```

## Python

### Python: procedural

def stern_brocot(predicate=lambda series: len(series) < 20): """\ Generates members of the stern-brocot series, in order, returning them when the predicate becomes false >>> print('The first 10 values:', stern_brocot(lambda series: len(series) < 10)[:10]) The first 10 values: [1, 1, 2, 1, 3, 2, 3, 1, 4, 3] >>> """ sb, i = [1, 1], 0 while predicate(sb): sb += [sum(sb[i:i + 2]), sb[i + 1]] i += 1 return sb if __name__ == '__main__': from fractions import gcd n_first = 15 print('The first %i values:\n ' % n_first, stern_brocot(lambda series: len(series) < n_first)[:n_first]) print() n_max = 10 for n_occur in list(range(1, n_max + 1)) + [100]: print('1-based index of the first occurrence of %3i in the series:' % n_occur, stern_brocot(lambda series: n_occur not in series).index(n_occur) + 1) # The following would be much faster. Note that new values always occur at odd indices # len(stern_brocot(lambda series: n_occur != series[-2])) - 1) print() n_gcd = 1000 s = stern_brocot(lambda series: len(series) < n_gcd)[:n_gcd] assert all(gcd(prev, this) == 1 for prev, this in zip(s, s[1:])), 'A fraction from adjacent terms is reducible'

{{out}}

```
The first 15 values:
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
1-based index of the first occurrence of 1 in the series: 1
1-based index of the first occurrence of 2 in the series: 3
1-based index of the first occurrence of 3 in the series: 5
1-based index of the first occurrence of 4 in the series: 9
1-based index of the first occurrence of 5 in the series: 11
1-based index of the first occurrence of 6 in the series: 33
1-based index of the first occurrence of 7 in the series: 19
1-based index of the first occurrence of 8 in the series: 21
1-based index of the first occurrence of 9 in the series: 35
1-based index of the first occurrence of 10 in the series: 39
1-based index of the first occurrence of 100 in the series: 1179
```

### Python: More functional

An iterator is used to produce successive members of the sequence. (its sb variable stores less compared to the procedural version above by popping the last element every time around the while loop.

In checking the gcd's, two iterators are tee'd off from the one stream with the second advanced by one value with its call to next().

See the [[Talk:Stern-Brocot_sequence#deque_over_list.3F|talk page]] for how a deque was selected over the use of a straightforward list'

```
from itertools import takewhile, tee, islice
>>> from collections import deque
>>> from fractions import gcd
>>>
>>> def stern_brocot():
sb = deque([1, 1])
while True:
sb += [sb[0] + sb[1], sb[1]]
yield sb.popleft()
>>> [s for _, s in zip(range(15), stern_brocot())]
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
>>> [1 + sum(1 for i in takewhile(lambda x: x != occur, stern_brocot()))
for occur in (list(range(1, 11)) + [100])]
[1, 3, 5, 9, 11, 33, 19, 21, 35, 39, 1179]
>>> prev, this = tee(stern_brocot(), 2)
>>> next(this)
1
>>> all(gcd(p, t) == 1 for p, t in islice(zip(prev, this), 1000))
True
>>>
```

===Python: Composing pure (curried) functions===

Composing and testing a Stern-Brocot function by composition of generic and reusable functional abstractions (curried for more flexible nesting and rearrangement). {{Works with|Python|3.7}}

'''Stern-Brocot sequence''' from itertools import (count, dropwhile, islice, takewhile) import operator import math # sternBrocot :: Generator [Int] def sternBrocot(): '''Non-finite list of the Stern-Brocot sequence of integers.''' def go(xs): x = xs[1] return (tail(xs) + [x + head(xs), x]) return fmapGen(head)( iterate(go)([1, 1]) ) # TESTS --------------------------------------------------- # main :: IO () def main(): '''Various tests''' [eq, ne, gcd] = map( curry, [operator.eq, operator.ne, math.gcd] ) sbs = take(1200)(sternBrocot()) ixSB = zip(sbs, enumFrom(1)) print(unlines(map(str, [ # First 15 members of the sequence. take(15)(sbs), # Indices of where the numbers [1..10] first appear. take(10)( nubBy(on(eq)(fst))( sorted( takewhile( compose(ne(12))(fst), ixSB ), key=fst ) ) ), # Index of where the number 100 first appears. take(1)(dropwhile(compose(ne(100))(fst), ixSB)), # Is the gcd of any two consecutive members, # up to the 1000th member, always one ? every(compose(eq(1)(gcd)))( take(1000)(zip(sbs, tail(sbs))) ) ]))) # GENERIC ABSTRACTIONS ------------------------------------ # compose (<<<) :: (b -> c) -> (a -> b) -> a -> c def compose(g): '''Right to left function composition.''' return lambda f: lambda x: g(f(x)) # curry :: ((a, b) -> c) -> a -> b -> c def curry(f): '''A curried function derived from an uncurried function.''' return lambda a: lambda b: f(a, b) # enumFrom :: Enum a => a -> [a] def enumFrom(x): '''A non-finite stream of enumerable values, starting from the given value.''' return count(x) if isinstance(x, int) else ( map(chr, count(ord(x))) ) # every :: (a -> Bool) -> [a] -> Bool def every(p): '''True if p(x) holds for every x in xs''' return lambda xs: all(map(p, xs)) # fmapGen <$> :: (a -> b) -> Gen [a] -> Gen [b] def fmapGen(f): '''A function f mapped over a non finite stream of values.''' def go(g): while True: v = next(g, None) if None is not v: yield f(v) else: return return lambda gen: go(gen) # fst :: (a, b) -> a def fst(tpl): '''First member of a pair.''' return tpl[0] # head :: [a] -> a def head(xs): '''The first element of a non-empty list.''' return xs[0] # iterate :: (a -> a) -> a -> Gen [a] def iterate(f): '''An infinite list of repeated applications of f to x.''' def go(x): v = x while True: yield v v = f(v) return lambda x: go(x) # nubBy :: (a -> a -> Bool) -> [a] -> [a] def nubBy(p): '''A sublist of xs from which all duplicates, (as defined by the equality predicate p) are excluded.''' def go(xs): if not xs: return [] x = xs[0] return [x] + go( list(filter( lambda y: not p(x)(y), xs[1:] )) ) return lambda xs: go(xs) # on :: (b -> b -> c) -> (a -> b) -> a -> a -> c def on(f): '''A function returning the value of applying the binary f to g(a) g(b)''' return lambda g: lambda a: lambda b: f(g(a))(g(b)) # tail :: [a] -> [a] # tail :: Gen [a] -> [a] def tail(xs): '''The elements following the head of a (non-empty) list or generator stream.''' if isinstance(xs, list): return xs[1:] else: list(islice(xs, 1)) # First item dropped. return xs # take :: Int -> [a] -> [a] # take :: Int -> String -> String def take(n): '''The prefix of xs of length n, or xs itself if n > length xs.''' return lambda xs: ( xs[0:n] if isinstance(xs, list) else list(islice(xs, n)) ) # unlines :: [String] -> String def unlines(xs): '''A single string derived by the intercalation of a list of strings with the newline character.''' return '\n'.join(xs) # MAIN --- if __name__ == '__main__': main()

{{Out}}

```
[1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
[(1, 1), (2, 3), (3, 5), (4, 9), (5, 11), (6, 33), (7, 19), (8, 21), (9, 35), (10, 39)]
[(100, 1179)]
True
```

## R

{{trans|PARI/GP}} {{Works with|R|3.3.2 and above}}

## Stern-Brocot sequence ## 12/19/16 aev SternBrocot <- function(n){ V <- 1; k <- n/2; for (i in 1:k) { V[2*i] = V[i]; V[2*i+1] = V[i] + V[i+1];} return(V); } ## Required tests: require(pracma); { cat(" *** The first 15:",SternBrocot(15),"\n"); cat(" *** The first i@n:","\n"); V=SternBrocot(40); for (i in 1:10) {j=match(i,V); cat(i,"@",j,",")} V=SternBrocot(1200); i=100; j=match(i,V); cat(i,"@",j,"\n"); V=SternBrocot(1000); j=1; for (i in 2:1000) {j=j*gcd(V[i-1],V[i])} if(j==1) {cat(" *** All GCDs=1!\n")} else {cat(" *** All GCDs!=1??\n")} }

{{Output}}

```
> require(pracma)
Loading required package: pracma
*** The first 15: 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
*** The first i@n:
1 @ 1 ,2 @ 3 ,3 @ 5 ,4 @ 9 ,5 @ 11 ,6 @ 33 ,7 @ 19 ,8 @ 21 ,9 @ 35 ,10 @ 39 ,100 @ 1179
*** All GCDs=1!
>
```

## Racket

```
#lang racket
;; OEIS Definition
;; A002487
;; Stern's diatomic series
;; (or Stern-Brocot sequence):
;; a(0) = 0, a(1) = 1;
;; for n > 0:
;; a(2*n) = a(n),
;; a(2*n+1) = a(n) + a(n+1).
(define A002487
(let ((memo (make-hash '((0 . 0) (1 . 1)))))
(lambda (n)
(hash-ref! memo n
(lambda ()
(define n/2 (quotient n 2))
(+ (A002487 n/2) (if (even? n) 0 (A002487 (add1 n/2)))))))))
(define Stern-Brocot A002487)
(displayln "Show the first fifteen members of the sequence.
(This should be: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4)")
(for/list ((i (in-range 1 (add1 15)))) (Stern-Brocot i))
(displayln "Show the (1-based) index of where the numbers 1-to-10 first appears in the sequence.")
(for ((n (in-range 1 (add1 10))))
(for/first ((i (in-naturals 1))
#:when (= n (Stern-Brocot i)))
(printf "~a first found at a(~a)~%" n i)))
(displayln "Show the (1-based) index of where the number 100 first appears in the sequence.")
(for/first ((i (in-naturals 1)) #:when (= 100 (Stern-Brocot i))) i)
(displayln "Check that the greatest common divisor of all the two consecutive members of the
series up to the 1000th member, is always one.")
(unless
(for/first ((i (in-range 1 1000))
#:unless (= 1 (gcd (Stern-Brocot i) (Stern-Brocot (add1 i))))) #t)
(display "\tdidn't find gcd > (or otherwise ≠) 1"))
```

{{out}}

```
Show the first fifteen members of the sequence.
(This should be: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4)
(1 1 2 1 3 2 3 1 4 3 5 2 5 3 4)
Show the (1-based) index of where the numbers 1-to-10 first appears in the sequence.
1 first found at a(1)
2 first found at a(3)
3 first found at a(5)
4 first found at a(9)
5 first found at a(11)
6 first found at a(33)
7 first found at a(19)
8 first found at a(21)
9 first found at a(35)
10 first found at a(39)
Show the (1-based) index of where the number 100 first appears in the sequence.
1179
Check that the greatest common divisor of all the two consecutive members of the
series up to the 1000th member, is always one.
didn't find gcd > (or otherwise ≠) 1
```

## REXX

```
/*REXX program generates & displays a Stern─Brocot sequence; finds 1─based indices; GCDs*/
parse arg N idx fix chk . /*get optional arguments from the C.L. */
if N=='' | N=="," then N= 15 /*Not specified? Then use the default.*/
if idx=='' | idx=="," then idx= 10 /* " " " " " " */
if fix=='' | fix=="," then fix= 100 /* " " " " " " */
if chk=='' | chk=="," then chk=1000 /* " " " " " " */
say center('the first' N "numbers in the Stern─Brocot sequence", 70, '═')
a=Stern_Brocot(N) /*invoke function to generate sequence.*/
say a /*display the sequence to the terminal.*/
say
say center('the 1─based index for the first' idx "integers", 70, '═')
a=Stern_Brocot(-idx) /*invoke function to generate sequence.*/
w=length(idx); do i=1 for idx
say 'for ' right(i, w)", the index is: " wordpos(i, a)
end /*i*/
say
say center('the 1─based index for' fix, 70, "═")
a=Stern_Brocot(-fix) /*invoke function to generate sequence.*/
say 'for ' fix", the index is: " wordpos(fix, a)
say
say center('checking if all two consecutive members have a GCD=1', 70, '═')
a=Stern_Brocot(chk) /*invoke function to generate sequence.*/
do c=1 for chk-1; if gcd(subword(a, c, 2))==1 then iterate
say 'GCD check failed at index' c; exit 13
end /*c*/
say '───── All ' chk " two consecutive members have a GCD of unity."
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
gcd: procedure; $=; do i=1 for arg(); $=$ arg(i) /*get arg list. */
end /*i*/
parse var $ x z .; if x=0 then x=z /*is zero case? */
x=abs(x) /*use absolute x*/
do j=2 to words($); y=abs( word($, j) )
if y=0 then iterate /*ignore zeros. */
do until y==0; parse value x//y y with y x /* ◄──heavy work*/
end /*until*/
end /*j*/
return x /*return the GCD*/
/*──────────────────────────────────────────────────────────────────────────────────────*/
Stern_Brocot: parse arg h 1 f; $=1 1; if h<0 then h=1e9
else f=0
f=abs(f)
do k=2 until words($)>=h | wordpos(f, $)\==0; _=word($, k)
$=$ (_ + word($, k-1) ) _
end /*k*/
if f==0 then return subword($, 1, h)
return $
```

{{out|output|text= when using the default inputs:}}

```
══════════the first 15 numbers in the Stern─Brocot sequence═══════════
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
═════════════the 1-based index for the first 10 integers══════════════
for 1, the index is: 1
for 2, the index is: 3
for 3, the index is: 5
for 4, the index is: 9
for 5, the index is: 11
for 6, the index is: 33
for 7, the index is: 19
for 8, the index is: 21
for 9, the index is: 35
for 10, the index is: 39
══════════════════════the 1-based index for 100═══════════════════════
for 100, the index is: 1179
═════════checking if all two consecutive members have a GCD=1═════════
───── All 1000 two consecutive members have a GCD of unity.
```

## Ring

```
# Project : Stern-Brocot sequence
limit = 1200
item = list(limit+1)
item[1] = 1
item[2] = 1
nr = 2
gcd = 1
gcdall = 1
for num = 3 to limit
item[num] = item[nr] + item[nr-1]
item[num+1] = item[nr]
nr = nr + 1
num = num + 1
next
showarray(item,15)
for x = 1 to 100
if x < 11 or x = 100
totalitem(x)
ok
next
for n = 1 to len(item) - 1
if gcd(item[n],item[n+1]) != 1
gcdall = gcd
ok
next
if gcdall = 1
see "Correct: The first 999 consecutive pairs are relative prime!" + nl
ok
func totalitem(p)
pos = find(item, p)
see string(x) + " at Stern #" + pos + "." + nl
func showarray(vect,ln)
svect = ""
for n = 1 to ln
svect = svect + vect[n] + ", "
next
svect = left(svect, len(svect) - 2)
see svect
see nl
func gcd(gcd,b)
while b
c = gcd
gcd = b
b = c % b
end
return gcd
```

Output:

```
1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4
1 at Stern #1.
2 at Stern #3.
3 at Stern #5.
4 at Stern #9.
5 at Stern #11.
6 at Stern #33.
7 at Stern #19.
8 at Stern #21.
9 at Stern #35.
10 at Stern #39.
100 at Stern #1179.
Correct: The first 999 consecutive pairs are relative prime!
```

## Ruby

{{works with|Ruby|2.1}}

def sb return enum_for :sb unless block_given? a=[1,1] 0.step do |i| yield a[i] a << a[i]+a[i+1] << a[i+1] end end puts "First 15: #{sb.first(15)}" [*1..10,100].each do |n| puts "#{n} first appears at #{sb.find_index(n)+1}." end if sb.take(1000).each_cons(2).all? { |a,b| a.gcd(b) == 1 } puts "All GCD's are 1" else puts "Whoops, not all GCD's are 1!" end

{{out}}

```
First 15: [1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
1 first appears at 1.
2 first appears at 3.
3 first appears at 5.
4 first appears at 9.
5 first appears at 11.
6 first appears at 33.
7 first appears at 19.
8 first appears at 21.
9 first appears at 35.
10 first appears at 39.
100 first appears at 1179.
All GCD's are 1
```

## Scala

lazy val sbSeq: Stream[BigInt] = { BigInt("1") #:: BigInt("1") #:: (sbSeq zip sbSeq.tail zip sbSeq.tail). flatMap{ case ((a,b),c) => List(a+b,c) } } // Show the results { println( s"First 15 members: ${(for( n <- 0 until 15 ) yield sbSeq(n)) mkString( "," )}" ) println for( n <- 1 to 10; pos = sbSeq.indexOf(n) + 1 ) println( s"Position of first $n is at $pos" ) println println( s"Position of first 100 is at ${sbSeq.indexOf(100) + 1}" ) println println( s"Greatest Common Divisor for first 1000 members is 1: " + (sbSeq zip sbSeq.tail).take(1000).forall{ case (a,b) => a.gcd(b) == 1 } ) }

{{out}}

```
First 15 members: 1,1,2,1,3,2,3,1,4,3,5,2,5,3,4
Position of first 1 is at 1
Position of first 2 is at 3
Position of first 3 is at 5
Position of first 4 is at 9
Position of first 5 is at 11
Position of first 6 is at 33
Position of first 7 is at 19
Position of first 8 is at 21
Position of first 9 is at 35
Position of first 10 is at 39
Position of first 100 is at 1179
Greatest Common Divisor for first 1000 members is 1: true
```

## Sidef

{{trans|Perl}}

# Declare a function to generate the Stern-Brocot sequence func stern_brocot { var list = [1, 1] { list.append(list[0]+list[1], list[1]) list.shift } } # Show the first fifteen members of the sequence. say 15.of(stern_brocot()).join(' ') # Show the (1-based) index of where the numbers 1-to-10 first appears # in the sequence, and where the number 100 first appears in the sequence. for i (1..10, 100) { var index = 1 var generator = stern_brocot() while (generator() != i) { ++index } say "First occurrence of #{i} is at index #{index}" } # Check that the greatest common divisor of all the two consecutive # members of the series up to the 1000th member, is always one. var generator = stern_brocot() var (a, b) = (generator(), generator()) { assert_eq(gcd(a, b), 1) a = b b = generator() } * 1000 say "All GCD's are 1"

{{out}}

```
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
First occurrence of 1 is at index 1
First occurrence of 2 is at index 3
First occurrence of 3 is at index 5
First occurrence of 4 is at index 9
First occurrence of 5 is at index 11
First occurrence of 6 is at index 33
First occurrence of 7 is at index 19
First occurrence of 8 is at index 21
First occurrence of 9 is at index 35
First occurrence of 10 is at index 39
First occurrence of 100 is at index 1179
All GCD's are 1
```

## Swift

struct SternBrocot: Sequence, IteratorProtocol { private var seq = [1, 1] mutating func next() -> Int? { seq += [seq[0] + seq[1], seq[1]] return seq.removeFirst() } } func gcd<T: BinaryInteger>(_ a: T, _ b: T) -> T { guard a != 0 else { return b } return a < b ? gcd(b % a, a) : gcd(a % b, b) } print("First 15: \(Array(SternBrocot().prefix(15)))") var found = Set<Int>() for (i, val) in SternBrocot().enumerated() { switch val { case 1...10 where !found.contains(val), 100 where !found.contains(val): print("First \(val) at \(i + 1)") found.insert(val) case _: continue } if found.count == 11 { break } } let firstThousand = SternBrocot().prefix(1000) let gcdIsOne = zip(firstThousand, firstThousand.dropFirst()).allSatisfy({ gcd($0.0, $0.1) == 1 }) print("GCDs of all two consecutive members are \(gcdIsOne ? "" : "not")one")

{{out}}

```
First 15: [1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4]
First 1 at 1
First 2 at 3
First 3 at 5
First 4 at 9
First 5 at 11
First 7 at 19
First 8 at 21
First 6 at 33
First 9 at 35
First 10 at 39
First 100 at 1179
GCDs of all two consecutive members are one
```

## Tcl

#!/usr/bin/env tclsh # package require generator ;# from tcllib namespace eval stern-brocot { proc generate {{count 100}} { set seq {1 1} set n 0 while {[llength $seq] < $count} { lassign [lrange $seq $n $n+1] a b lappend seq [expr {$a + $b}] $b incr n } return $seq } proc genr {} { yield [info coroutine] set seq {1 1} while {1} { set seq [lassign $seq a] set b [lindex $seq 0] set c [expr {$a + $b}] lappend seq $c $b yield $a } } proc Step {a b args} { set c [expr {$a + $b}] list $a [list $b {*}$args $c $b] } generator define gen {} { set cmd [list 1 1] while {1} { lassign [Step {*}$cmd] a cmd generator yield $a } } namespace export {[a-z]*} namespace ensemble create } interp alias {} sb {} stern-brocot # a simple adaptation of gcd from http://wiki.tcl.tk/2891 proc coprime {a args} { set gcd $a foreach arg $args { while {$arg != 0} { set t $arg set arg [expr {$gcd % $arg}] set gcd $t if {$gcd == 1} {return true} } } return false } proc main {} { puts "#1. First 15 members of the Stern-Brocot sequence:" puts \t[generator to list [generator take 16 [sb gen]]] puts "#2. First occurrences of 1 through 10:" set first {} set got 0 set i 0 generator foreach x [sb gen] { incr i if {$x>10} continue if {[dict exists $first $x]} continue dict set first $x $i if {[incr got] >= 10} break } foreach {a b} [lsort -integer -stride 2 $first] { puts "\tFirst $a at $b" } puts "#3. First occurrence of 100:" set i 0 generator foreach x [sb gen] { incr i if {$x eq 100} break } puts "\tFirst $x at $i" puts "#4. Check first 1k elements for common divisors:" set prev [expr {2*3*5*7*11*13*17*19+1}] ;# a handy prime set i 0 generator foreach x [sb gen] { if {[incr i] >= 1000} break if {![coprime $x $prev]} { error "Element $i, $x is not coprime with $prev!" } set prev $x } puts "\tFirst $i elements are all pairwise coprime" } main

{{Out}}

```
#1. First 15 members of the Stern-Brocot sequence:
1 1 2 1 3 2 3 1 4 3 5 2 5 3 4
#2. First occurrences of 1 through 10:
First 1 at 1
First 2 at 3
First 3 at 5
First 4 at 9
First 5 at 11
First 6 at 33
First 7 at 19
First 8 at 21
First 9 at 35
First 10 at 39
#3. First occurrence of 100:
First 100 at 1179
#4. Check first 1k elements for common divisors:
First 1000 elements are all pairwise coprime
```

## VBScript

```
sb = Array(1,1)
i = 1 'considered
j = 2 'precedent
n = 0 'loop counter
Do
ReDim Preserve sb(UBound(sb) + 1)
sb(UBound(sb)) = sb(UBound(sb) - i) + sb(UBound(sb) - j)
ReDim Preserve sb(UBound(sb) + 1)
sb(UBound(sb)) = sb(UBound(sb) - j)
i = i + 1
j = j + 1
n = n + 1
Loop Until n = 2000
WScript.Echo "First 15: " & DisplayElements(15)
For k = 1 To 10
WScript.Echo "The first instance of " & k & " is in #" & ShowFirstInstance(k) & "."
Next
WScript.Echo "The first instance of " & 100 & " is in #" & ShowFirstInstance(100) & "."
Function DisplayElements(n)
For i = 0 To n - 1
If i < n - 1 Then
DisplayElements = DisplayElements & sb(i) & ", "
Else
DisplayElements = DisplayElements & sb(i)
End If
Next
End Function
Function ShowFirstInstance(n)
For i = 0 To UBound(sb)
If sb(i) = n Then
ShowFirstInstance = i + 1
Exit For
End If
Next
End Function
```

{{Out}}

```
First 15: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4
The first instance of 1 is in #1.
The first instance of 2 is in #3.
The first instance of 3 is in #5.
The first instance of 4 is in #9.
The first instance of 5 is in #11.
The first instance of 6 is in #33.
The first instance of 7 is in #19.
The first instance of 8 is in #21.
The first instance of 9 is in #35.
The first instance of 10 is in #39.
The first instance of 100 is in #1179.
```

## Visual Basic .NET

{{trans|C#}}

```
Imports System
Imports System.Collections.Generic
Imports System.Linq
Module Module1
Dim l As List(Of Integer) = {1, 1}.ToList()
Function gcd(ByVal a As Integer, ByVal b As Integer) As Integer
Return If(a > 0, If(a < b, gcd(b Mod a, a), gcd(a Mod b, b)), b)
End Function
Sub Main(ByVal args As String())
Dim max As Integer = 1000, take As Integer = 15, i As Integer = 1,
selection As Integer() = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100}
Do : l.AddRange({l(i) + l(i - 1), l(i)}.ToList) : i += 1
Loop While l.Count < max OrElse l(l.Count - 2) <> selection.Last()
Console.Write("The first {0} items In the Stern-Brocot sequence: ", take)
Console.WriteLine("{0}" & vbLf, String.Join(", ", l.Take(take)))
Console.WriteLine("The locations of where the selected numbers (1-to-10, & 100) first appear:")
For Each ii As Integer In selection
Dim j As Integer = l.FindIndex(Function(x) x = ii) + 1
Console.WriteLine("{0,3}: {1:n0}", ii, j)
Next : Console.WriteLine() : Dim good As Boolean = True : For i = 1 To max
If gcd(l(i), l(i - 1)) <> 1 Then good = False : Exit For
Next
Console.WriteLine("The greatest common divisor of all the two consecutive items of the" &
" series up to the {0}th item is {1}always one.", max, If(good, "", "not "))
End Sub
End Module
```

{{out}}

```
The first 15 items In the Stern-Brocot sequence: 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4
The locations of where the selected numbers (1-to-10, & 100) first appear:
1: 1
2: 3
3: 5
4: 9
5: 11
6: 33
7: 19
8: 21
9: 35
10: 39
100: 1,179
The greatest common divisor of all the two consecutive items of the series up to the 1000th item is always one.
```

## zkl

```
fcn SB // Stern-Brocot sequence factory --> Walker
{ Walker(fcn(sb,n){ a,b:=sb; sb.append(a+b,b); sb.del(0); a }.fp(L(1,1))) }
SB().walk(15).println();
[1..10].zipWith('wrap(n){ [1..].zip(SB())
.filter(1,fcn(n,sb){ n==sb[1] }.fp(n)) })
.walk().println();
[1..].zip(SB()).filter1(fcn(sb){ 100==sb[1] }).println();
sb:=SB(); do(500){ if(sb.next().gcd(sb.next())!=1) println("Oops") }
```

{{out}}

```
L(1,1,2,1,3,2,3,1,4,3,5,2,5,3,4)
L(L(L(1,1)),L(L(3,2)),L(L(5,3)),L(L(9,4)),L(L(11,5)),L(L(33,6)),L(L(19,7)),L(L(21,8)),L(L(35,9)),L(L(39,10)))
L(1179,100)
```