**⚠️ 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.

[[Category:Puzzles]] [[Category:Games]] {{draft task}}

'''21''' is a two player game, the game is played by choosing a number ('''1''', '''2''', or '''3''') to be added to the ''running total''.

The game is won by the player whose chosen number causes the ''running total'' to reach ''exactly'' '''21'''.

The ''running total'' starts at zero.

One player will be the computer. Players alternate supplying a number to be added to the ''running total''.

;Task: Write a computer program that will: ::* do the prompting (or provide a button menu), ::* check for errors and display appropriate error messages, ::* do the additions (add a chosen number to the ''running total''), ::* display the ''running total'', ::* provide a mechanism for the player to quit/exit/halt/stop/close the program, ::* issue a notification when there is a winner, and ::* determine who goes first (possibly a random choice, or it can be specified when the game begins).

**TOC**

## Go

To give the human player a reasonable chance whoever goes first, the computer always chooses randomly when the running total is below 18 but otherwise chooses the exact number needed to win the game.

package main import ( "bufio" "fmt" "log" "math/rand" "os" "strconv" "time" ) var scanner = bufio.NewScanner(os.Stdin) var ( total = 0 quit = false ) func itob(i int) bool { if i == 0 { return false } return true } func getChoice() { for { fmt.Print("Your choice 1 to 3 : ") scanner.Scan() if scerr := scanner.Err(); scerr != nil { log.Fatalln(scerr, "when choosing number") } text := scanner.Text() if text == "q" || text == "Q" { quit = true return } input, err := strconv.Atoi(text) if err != nil { fmt.Println("Invalid number, try again") continue } newTotal := total + input switch { case input < 1 || input > 3: fmt.Println("Out of range, try again") case newTotal > 21: fmt.Println("Too big, try again") default: total = newTotal fmt.Println("Running total is now", total) return } } } func main() { rand.Seed(time.Now().UnixNano()) computer := itob(rand.Intn(2)) fmt.Println("Enter q to quit at any time\n") if computer { fmt.Println("The computer will choose first") } else { fmt.Println("You will choose first") } fmt.Println("\nRunning total is now 0\n") var choice int for round := 1; ; round++ { fmt.Printf("ROUND %d:\n\n", round) for i := 0; i < 2; i++ { if computer { if total < 18 { choice = 1 + rand.Intn(3) } else { choice = 21 - total } total += choice fmt.Println("The computer chooses", choice) fmt.Println("Running total is now", total) if total == 21 { fmt.Println("\nSo, commiserations, the computer has won!") return } } else { getChoice() if quit { fmt.Println("OK, quitting the game") return } if total == 21 { fmt.Println("\nSo, congratulations, you've won!") return } } fmt.Println() computer = !computer } } }

{{out}} A sample game where the human player manages to win even though the computer (chosen randomly) goes first.

```
Enter q to quit at any time
The computer will choose first
Running total is now 0
ROUND 1:
The computer chooses 1
Running total is now 1
Your choice 1 to 3 : 3
Running total is now 4
ROUND 2:
The computer chooses 2
Running total is now 6
Your choice 1 to 3 : 4
Out of range, try again
Your choice 1 to 3 : 3
Running total is now 9
ROUND 3:
The computer chooses 2
Running total is now 11
Your choice 1 to 3 : 2
Running total is now 13
ROUND 4:
The computer chooses 2
Running total is now 15
Your choice 1 to 3 : 2
Running total is now 17
ROUND 5:
The computer chooses 1
Running total is now 18
Your choice 1 to 3 : 3
Running total is now 21
So, congratulations, you've won!
```

A sample game where the human player plays sensibly but still loses when the computer goes first.

```
Enter q to quit at any time
The computer will choose first
Running total is now 0
ROUND 1:
The computer chooses 3
Running total is now 3
Your choice 1 to 3 : 3
Running total is now 6
ROUND 2:
The computer chooses 3
Running total is now 9
Your choice 1 to 3 : 3
Running total is now 12
ROUND 3:
The computer chooses 1
Running total is now 13
Your choice 1 to 3 : 3
Running total is now 16
ROUND 4:
The computer chooses 1
Running total is now 17
Your choice 1 to 3 : 1
Running total is now 18
ROUND 5:
The computer chooses 3
Running total is now 21
So, commiserations, the computer has won!
```

## Julia

The computer or player can always win if they go first and choose a number that brings the total to one of the following: 1, 5, 9, 13, or 17. To make things vary more, there is a choice of computer play level at the start. The computer will randomly respond with level 1 and choose a random response 1/4 of the time at level 2.

function trytowin(n) if 21 - n < 4 println("Computer chooses $(21 - n) and wins. GG!") exit(0) end end function choosewisely(n) trytowin(n) targets = [1, 5, 9, 13, 17, 21] pos = findfirst(x -> x > n, targets) bestmove = targets[pos] - n if bestmove > 3 println("Looks like I could lose. Choosing a 1, total now $(n + 1).") return n + 1 end println("On a roll, choosing a $bestmove, total now $(n + bestmove).") n + bestmove end function choosefoolishly(n) trytowin(n) move = rand([1, 2, 3]) println("Here goes, choosing $move, total now $(n + move).") n + move end function choosesemiwisely(n) trytowin(n) if rand() > 0.75 choosefoolishly(n) else choosewisely(n) end end prompt(s) = (println(s, ": => "); return readline()) function playermove(n) rang = (n > 19) ? "1 is all" : ((n > 18) ? "1 or 2" : "1, 2 or 3") choice = 0 while true nstr = prompt("Your choice ($rang), 0 to exit") if nstr == "0" exit(0) elseif nstr == "1" return n + 1 elseif nstr == "2" && n < 20 return n + 2 elseif nstr == "3" && n < 19 return n + 3 end end end function play21game() n = 0 level = prompt("Level of play (1=dumb, 3=smart)") algo = choosewisely if level == "1" algo = choosefoolishly elseif level == "2" algo = choosesemiwisely elseif level != "3" println("Bad choice syntax--default to smart choice") end whofirst = prompt("Does computer go first? (y or n)") if whofirst[1] == 'y' || whofirst[1] == 'Y' n = algo(n) end while n < 21 n = playermove(n) if n == 21 println("Player wins! Game over, gg!") break end n = algo(n) end end play21game()

## Perl

{{trans|Perl 6}}

print <<'HERE'; The 21 game. Each player chooses to add 1, 2, or 3 to a running total. The player whose turn it is when the total reaches 21 wins. Enter q to quit. HERE my $total = 0; while () { print "Running total is: $total\n"; my ($me,$comp); while () { print 'What number do you play> '; $me = <>; chomp $me; last if $me =~ /^[123]$/; insult($me); } $total += $me; win('Human') if $total >= 21; print "Computer plays: " . ($comp = 1+int(rand(3))) . "\n"; $total += $comp; win('Computer') if $total >= 21; } sub win { my($player) = @_; print "$player wins.\n"; exit; } sub insult { my($g) = @_; exit if $g =~ /q/i; my @insults = ('Yo mama', 'Jeez', 'Ummmm', 'Grow up'); my $i = $insults[1+int rand($#insults)]; print "$i, $g is not an integer between 1 and 3...\n" }

{{out}}

```
The 21 game. Each player chooses to add 1, 2, or 3 to a running total.
The player whose turn it is when the total reaches 21 wins. Enter q to quit.
Running total is: 0
What number do you play> 3
Computer plays: 3
Running total is: 6
What number do you play> 3
Computer plays: 3
Running total is: 12
What number do you play> 3
Computer plays: 2
Running total is: 17
What number do you play> 1
Computer plays: 1
Running total is: 19
What number do you play> 2
Human wins.
```

## Perl 6

{{works with|Rakudo|2018.09}} Since there is no requirement that the computer play sensibly, it always plays a random guess so the player has some chance to win.

```
say qq :to 'HERE';
The 21 game. Each player chooses to add 1, 2, or 3 to a running total.
The player whose turn it is when the total reaches 21 wins. Enter q to quit.
HERE
my $total = 0;
loop {
say "Running total is: $total";
my ($me,$comp);
loop {
$me = prompt 'What number do you play> ';
last if $me ~~ /^<[123]>$/;
insult $me;
}
$total += $me;
win('Human') if $total >= 21;
say "Computer plays: { $comp = (1,2,3).roll }\n";
$total += $comp;
win('Computer') if $total >= 21;
}
sub win ($player) {
say "$player wins.";
exit;
}
sub insult ($g) {
exit if $g eq 'q';
print ('Yo mama,', 'Jeez,', 'Ummmm,', 'Grow up,', 'Did you even READ the instructions?').roll;
say " $g is not an integer between 1 & 3..."
}
```

{{out|Sample game}}

```
The 21 game. Each player chooses to add 1, 2, or 3 to a running total.
The player whose turn it is when the total reaches 21 wins. Enter q to quit.
Running total is: 0
What number do you play> 5
Did you even READ the instructions? 5 is not an integer between 1 & 3...
What number do you play> g
Yo mama, g is not an integer between 1 & 3...
What number do you play> 12
Jeez, 12 is not an integer between 1 & 3...
What number do you play> 3
Computer plays: 2
Running total is: 5
What number do you play> 3
Computer plays: 3
Running total is: 11
What number do you play> 1
Computer plays: 1
Running total is: 13
What number do you play> 3
Computer plays: 2
Running total is: 18
What number do you play> 3
Human wins.
```

## Phix

If the computer goes first you cannot win.

Once the computer stops displaying "no clear strategy" you cannot win.

The computer_first flag only applies to the first game. After winning, losing, or conceding, you go first.

```
bool computer_first = false
bool show_spoiler = false
integer total = 0
procedure computer_play()
integer move = 0
for i=1 to 3 do
if mod(total+i,4)=1 then
move = i
exit
end if
end for
if move=0 then
puts(1,"no clear strategy\n")
move = rand(min(3,21-total))
end if
printf(1,"Total is %d. I play %d.\n",{total,move})
total += move
if total=21 then
puts(1,"21! I win!\n")
end if
end procedure
puts(1,"\n21 game\n\n")
puts(1,"Press escape or q to quit the game, c to concede and start a new game from 0\n\n")
if computer_first then
printf(1,"Total is %d.\n",{total})
computer_play()
elsif show_spoiler then
-- The secret to winning!
puts(1,sq_sub("Uif!pomz!xbz!up!xjo!jt!qmbz!2!gjstu-!uifo!5.=dpnqvufs!npwf?!fwfsz!ujnf",1)&"\n\n")
end if
while 1 do
printf(1,"Total is %d. enter 1, 2, or 3: ",{total})
integer ch = wait_key()
puts(1,iff(ch=#1B?"esc":ch)&"\n")
if ch>='1' and ch<='3' then
ch -= '0'
if total+ch>21 then
puts(1,"Too big\n")
else
total += ch
if total=21 then
puts(1,"21! You win!\n")
else
computer_play()
end if
end if
elsif ch=#1B or lower(ch)='q' then
puts(1,"Quitting\n")
exit
end if
if lower(ch)='c' or total=21 then
total = 0
end if
end while
```

{{out}}

```
21 game
Press escape or q to quit the game, c to concede and start a new game from 0
Total is 0. enter 1, 2, or 3: 1
no clear strategy
Total is 1. I play 3.
Total is 4. enter 1, 2, or 3: 1
no clear strategy
Total is 5. I play 1.
Total is 6. enter 1, 2, or 3: 3
no clear strategy
Total is 9. I play 1.
Total is 10. enter 1, 2, or 3: 1
Total is 11. I play 2.
Total is 13. enter 1, 2, or 3: 3
Total is 16. I play 1.
Total is 17. enter 1, 2, or 3: 2
Total is 19. I play 2.
21! I win!
Total is 0. enter 1, 2, or 3: q
Quitting
```

## Racket

```
#lang racket
(define limit 21)
(define max-resp 3)
(define (get-resp)
(let loop ()
(match (read-line)
[(app (conjoin string? string->number) n)
#:when (and (exact-integer? n) (<= 1 n max-resp))
n]
["q" (exit)]
[n (printf "~a is not in range 1 and ~a\n" n max-resp)
(loop)])))
(define (win human?) (printf "~a wins\n" (if human? "Human" "Computer")))
(printf "The ~a game. Each player chooses to add a number
in range 1 and ~a to a running total.
The player whose turn it is when the total reaches exactly ~a wins.
Enter q to quit.\n\n" limit max-resp limit)
(let loop ([total 0] [human-turn? (= 0 (random 2))])
(define new-total
(+ total
(cond
[human-turn? (printf "Running total is: ~a\n" total)
(printf "Your turn:\n")
(get-resp)]
[else (define resp (random 1 (add1 max-resp)))
(printf "Computer plays: ~a\n" resp)
resp])))
(cond
[(= new-total limit) (win human-turn?)]
[(> new-total limit) (win (not human-turn?))]
[else (loop new-total (not human-turn?))]))
```

{{out}}

```
The 21 game. Each player chooses to add an integer
in range 1 and 3 to a running total.
The player whose turn it is when the total reaches exactly 21 wins.
Enter q to quit.
Running total is: 0
Your turn:
1
Computer plays: 3
Running total is: 4
Your turn:
foo
foo is not an integer in range 1 and 3
Your turn:
bar
bar is not an integer in range 1 and 3
Your turn:
3
Computer plays: 1
Running total is: 8
Your turn:
1
Computer plays: 1
Running total is: 10
Your turn:
1
Computer plays: 2
Running total is: 13
Your turn:
1
Computer plays: 3
Running total is: 17
Your turn:
1
Computer plays: 2
Running total is: 20
Your turn:
1
Human wins
```

## REXX

Around half of the REXX program deals with incorrect or missing input (of the required integer).

```
/*REXX program plays the 21 game with a human, each player chooses 1, 2, or 3 which */
/*──────────── is added to the current sum, the first player to reach 21 exactly wins.*/
sep= copies('─', 8); sep2= " "copies('═', 8)" " /*construct an eye─catching msg fences.*/
say sep 'Playing the 21 game.' /*tell what's happening here at the zoo*/
high= 3 /*the highest integer that can be used.*/
game= 21 /*the game target (winning total/sum).*/
$= 0 /*the sum [or running total] (so far).*/
do j=1 while $<game; bad= 0 /*obtain optional move from ARG or ask.*/
low = (j \== 1) /*allow user to have computer go first.*/
say; say sep 'Please enter a number from ' low " ───► 3 (or Quit):"
if j==1 then say sep '[A value of 0 (zero) means you want the computer to go first.]'
pull x _ . 1 ax /*obtain an uppercased answer & others.*/
if x='' then call ser "Nothing entered."
if _\=='' then call ser "Too many arguments entered: " ax
if abbrev('QUIT', x, 1) then do; say; say sep "quitting."; exit 1
end
if \datatype(x, 'N') then call ser "Argument isn't numeric: " x
if \datatype(x, 'W') then call ser "Number isn't an integer: " x
if x<0 then call ser "Number can't be negative: " x
if x=0 & j>1 then call ser "Number can't be zero: " x
if x>high then call ser "Number is too large (>"high'): ' x
if bad then iterate /*Had an error? Then get another number*/
x= x / 1 /*normalize the X number; +2 001 3. */
if $+x > game then call ser "Number will cause the sum to exceed " g': ' x
if x\==0 then call tot x, 1 /*Not 0? The user wants to go first. */
if $==game then leave /*the user won the game with the last #*/
y=. /*indicate that no choice has been made*/
do c=1 for high until y\==. /*find a # that yields the best choice.*/
if (c+$) // (1+high) == 1 then y= c /*a bit of a highfalutin calculation. */
end /*c*/
if y==. then y= random(1, high) /*get random choice (maybe a lost game)*/
say sep 'The computer chooses ' y " as it's choice." /*inform player.*/
call tot y, 0 /*call subroutine to show the total. */
end /*j*/
say
if who then say sep 'Congratulations! You have won the 21 game.'
else say sep 'The computer has won the 21 game.'
exit 0 /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
ser: if bad then return; bad=1; say; say; say sep '***error***' arg(1); say; return
tot: arg q,who; $=$+q; say sep 'The game total is now' sep2 $ sep2; return /*add; show $*/
```

{{out|output}}

```
──────── Playing the 21 game.
──────── Please enter a number from 0 ───► 3 (or Quit):
──────── [A value of 0 (zero) means you want the computer to go first.]
3 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 3 ════════
──────── The computer chooses 1 as it's choice.
──────── The game total is now ════════ 4 ════════
──────── Please enter a number from 1 ───► 3 (or Quit):
3 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 7 ════════
──────── The computer chooses 1 as it's choice.
──────── The game total is now ════════ 8 ════════
──────── Please enter a number from 1 ───► 3 (or Quit):
3 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 11 ════════
──────── The computer chooses 1 as it's choice.
──────── The game total is now ════════ 12 ════════
──────── Please enter a number from 1 ───► 3 (or Quit):
3 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 15 ════════
──────── The computer chooses 1 as it's choice.
──────── The game total is now ════════ 16 ════════
──────── Please enter a number from 1 ───► 3 (or Quit):
1 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 17 ════════
──────── The computer chooses 1 as it's choice.
──────── The game total is now ════════ 18 ════════
──────── Please enter a number from 1 ───► 3 (or Quit):
3 ◄■■■■■■■■■■ user input
──────── The game total is now ════════ 21 ════════
──────── Congratulations! You have won the 21 game.
```

## Ring

```
# Project : 21 Game
load "guilib.ring"
limit = 21
posold = 0
button = list(limit)
mynum = list(3)
yournum = list(3)
new qapp
{
win1 = new qwidget() {
setwindowtitle("21 Game")
setgeometry(100,100,1000,600)
label1 = new qlabel(win1) {
setgeometry(10,10,1000,600)
settext("")
}
label2 = new qlabel(win1) {
setgeometry(240,50,120,40)
setAlignment(Qt_AlignHCenter)
setFont(new qFont("Verdana",12,100,0))
settext("my number:")
}
label3 = new qlabel(win1) {
setgeometry(640,50,120,40)
setAlignment(Qt_AlignHCenter)
setFont(new qFont("Verdana",12,100,0))
settext("your number:")
}
for p = 1 to 3
mynum[p] = new qpushbutton(win1) {
setgeometry(200+p*40,100,40,40)
setstylesheet("background-color:orange")
settext(string(p))
setclickevent("choose(" + string(p) + ",1)")
}
next
for p = 1 to 3
yournum[p] = new qpushbutton(win1) {
setgeometry(600+p*40,100,40,40)
setstylesheet("background-color:white")
settext(string(p))
setclickevent("choose(" + string(p) + ",2)")
}
next
for n = 1 to limit
button[n] = new qpushbutton(win1) {
setgeometry(40+n*40,190,40,40)
settext(string(n))
}
next
show()
}
exec()
}
func choose(ch,ym)
pos = posold + ch
if pos > limit
msg = "You must choose number from 1 to " + string(limit - posold)
msgBox(msg)
for n = 1 to 3
mynum[n].setenabled(false)
yournum[n].setenabled(false)
next
return
ok
for n = posold+1 to pos
if ym = 1
button[n] { setstylesheet("background-color:orange") }
else
button[n] { setstylesheet("background-color:white") }
ok
next
posold = pos
if ym = 1
for n = 1 to 3
mynum[n].setenabled(false)
yournum[n].setenabled(true)
next
else
for n = 1 to 3
mynum[n].setenabled(true)
yournum[n].setenabled(false)
next
ok
if pos = 21
if ym = 1
msgBox("I won!")
else
msgBox("You won!")
ok
ok
func msgBox(text) {
m = new qMessageBox(win1) {
setWindowTitle("21 Game")
setText(text)
show()
}
}
```

Output:

[http://kepkezelo.com/images/1hyutakip6vo1t1rpypy.jpg 21 Game]