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[[Category:Games]]

There are many techniques that people use to shuffle [[Playing cards|cards]] for card games. Some are more effective than others.

;Task Implement the (seemingly) more common techniques of the riffle shuffle and overhand shuffle for ''n'' iterations.

Implementing playing cards is not necessary if it would be easier to implement these shuffling methods for generic collections.

Where possible, compare this to a standard/built-in shuffling procedure.

One iteration of the riffle shuffle is defined as:

# The merged deck is now the new "shuffled" deck

One iteration of the overhand shuffle is defined as:

# The second pile is now the new "shuffled" deck

;Bonus Implement other methods described in the Wikipedia article: [https://en.wikipedia.org/wiki/Shuffling#Shuffling_techniques card shuffling].

Allow for "human errors" of imperfect cutting and interleaving.

## C

{{trans|Modula-2}}

```#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

void init() {
srand((unsigned int)time(NULL));
}

int random(int low, int high) {
int diff, val;

diff = high - low;
if (diff == 0) {
return low;
}

val = rand() % diff;
return val + low;
}

void initDeck(int *deck, const int size) {
int i;
for (i = 0; i < size; ++i) {
*deck++ = i + 1;
}
}

void writeDeck(const int *deck, const int size) {
int i;

printf("[");
if (size > 0) {
printf("%d", *deck++);
}
for (i = 1; i < size; ++i) {
printf(", %d", *deck++);
}
printf("]");
}

void riffleShuffle(int * const deck, const int size, int flips) {
int n, cutPoint, nlp, lp, rp, bound;
int *nl;

nl = (int *)malloc(size * sizeof(int));

for (n = 0; n < flips; ++n) {
cutPoint = size / 2;
if (random(0, 2) > 0) {
cutPoint = cutPoint + random(0, size / 10);
} else {
cutPoint = cutPoint - random(0, size / 10);
}

nlp = 0;
lp = 0;
rp = cutPoint;

while (lp < cutPoint && rp < size) {
/* Allow for an imperfect riffling so that more than one card can come from the same side in a row
biased towards the side with more cards. Remove the IF statement for perfect riffling. */
bound = (cutPoint - lp) * 50 / (size - rp);
if (random(0, 50) >= bound) {
nl[nlp++] = deck[rp++];
} else {
nl[nlp++] = deck[lp++];
}
}
while (lp < cutPoint) {
nl[nlp++] = deck[lp++];
}
while (rp < size) {
nl[nlp++] = deck[rp++];
}

memcpy(deck, nl, size * sizeof(int));
}

free(nl);
}

void overhandShuffle(int * const mainHand, const int size, int passes) {
int n, cutSize, mp, op, tp, i;
int *otherHand, *temp;

otherHand = (int *)malloc(size * sizeof(int));
temp = (int *)malloc(size * sizeof(int));

for (n = 0; n < passes; ++n) {
mp = 0;
op = 0;
tp = 0;

while (mp < size) {
cutSize = random(0, size / 5) + 1;

/* grab the next cut up to the end of the cards left in the main hand */
for (i = 0; i < cutSize && mp < size; ++i) {
temp[tp++] = mainHand[mp++];
}

/* add them to the cards in the other hand, sometimes to the fron sometimes to the back */
if (random(0, 10) >= 1) {
/* front most of the time */

/* move the elements of other hand forward to make room for temp */
for (i = op - 1; i >= 0; --i) {
otherHand[i + tp] = otherHand[i];
}

/* copy temp to the front of other hand */
memcpy(otherHand, temp, tp * sizeof(int));
op += tp;
tp = 0;
} else {
/* end sometimes */
for (i = 0; i < tp; ++i, ++op) {
otherHand[op] = temp[i];
}
tp = 0;
}
}

/* move the cards back to the main hand */
memcpy(mainHand, otherHand, size * sizeof(int));
}

free(otherHand);
free(temp);
}

#define SIZE 20
int main() {
int deck[SIZE];

init();

printf("Riffle shuffle\n");
initDeck(deck, SIZE);
writeDeck(deck, SIZE);
printf("\n");
riffleShuffle(deck, SIZE, 10);
writeDeck(deck, SIZE);
printf("\n\n");

printf("Riffle shuffle\n");
initDeck(deck, SIZE);
writeDeck(deck, SIZE);
printf("\n");
riffleShuffle(deck, SIZE, 1);
writeDeck(deck, SIZE);
printf("\n\n");

printf("Overhand shuffle\n");
initDeck(deck, SIZE);
writeDeck(deck, SIZE);
printf("\n");
overhandShuffle(deck, SIZE, 10);
writeDeck(deck, SIZE);
printf("\n\n");

printf("Overhand shuffle\n");
initDeck(deck, SIZE);
writeDeck(deck, SIZE);
printf("\n");
overhandShuffle(deck, SIZE, 1);
writeDeck(deck, SIZE);
printf("\n\n");

return 0;
}
```

{{out}}

```Riffle shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[1, 15, 6, 2, 11, 18, 9, 5, 3, 4, 7, 16, 13, 8, 10, 14, 19, 12, 17, 20]

Riffle shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[1, 2, 11, 3, 4, 5, 12, 13, 6, 7, 14, 8, 15, 16, 9, 17, 10, 18, 19, 20]

Overhand shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 19, 4, 10, 11, 8, 12, 7, 6, 3, 16, 14, 18, 1, 5, 13, 9, 15, 17, 20]

Overhand shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[20, 17, 18, 19, 14, 15, 16, 10, 11, 12, 13, 9, 8, 7, 5, 6, 1, 2, 3, 4]
```

## C++

```
#include <time.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <deque>

class riffle
{
public:
void shuffle( std::deque<int>* v, int tm )
{
std::deque<int> tmp;
bool fl;
size_t len;
std::deque<int>::iterator it;

copyTo( v, &tmp );

for( int t = 0; t < tm; t++ )
{
std::deque<int> lHand( rand() % ( tmp.size() / 3 ) + ( tmp.size() >> 1 ) ), rHand( tmp.size() - lHand.size() );

std::copy( tmp.begin(), tmp.begin() + lHand.size(), lHand.begin() );
std::copy( tmp.begin() + lHand.size(), tmp.end(), rHand.begin() );
tmp.clear();

while( lHand.size() && rHand.size() )
{
fl = rand() % 10 < 5;
if( fl )
len = 1 + lHand.size() > 3 ? rand() % 3 + 1 : rand() % ( lHand.size() ) + 1;
else
len = 1 + rHand.size() > 3 ? rand() % 3 + 1 : rand() % ( rHand.size() ) + 1;

while( len )
{
if( fl )
{
tmp.push_front( *lHand.begin() );
lHand.erase( lHand.begin() );
}
else
{
tmp.push_front( *rHand.begin() );
rHand.erase( rHand.begin() );
}
len--;
}
}

if( lHand.size() < 1 )
{
for( std::deque<int>::iterator x = rHand.begin(); x != rHand.end(); x++ )
tmp.push_front( *x );
}
if( rHand.size() < 1 )
{
for( std::deque<int>::iterator x = lHand.begin(); x != lHand.end(); x++ )
tmp.push_front( *x );
}
}
copyTo( &tmp, v );
}
private:
void copyTo( std::deque<int>* a, std::deque<int>* b )
{
for( std::deque<int>::iterator x = a->begin(); x != a->end(); x++ )
b->push_back( *x );
a->clear();
}
};

class overhand
{
public:
void shuffle( std::deque<int>* v, int tm )
{
std::deque<int> tmp;
bool top;
for( int t = 0; t < tm; t++ )
{
while( v->size() )
{
size_t len = rand() % ( v->size() ) + 1;
top = rand() % 10 < 5;
while( len )
{
if( top ) tmp.push_back( *v->begin() );
else tmp.push_front( *v->begin() );
v->erase( v->begin() );
len--;
}
}
for( std::deque<int>::iterator x = tmp.begin(); x != tmp.end(); x++ )
v->push_back( *x );

tmp.clear();
}
}
};

// global - just to make things simpler ---------------------------------------------------
std::deque<int> cards;

void fill()
{
cards.clear();
for( int x = 0; x < 20; x++ )
cards.push_back( x + 1 );
}

void display( std::string t )
{
std::cout << t << "\n";
for( std::deque<int>::iterator x = cards.begin(); x != cards.end(); x++ )
std::cout << *x << " ";
std::cout << "\n\n";
}

int main( int argc, char* argv[] )
{
srand( static_cast<unsigned>( time( NULL ) ) );
riffle r; overhand o;

fill(); r.shuffle( &cards, 10 ); display( "RIFFLE" );
fill(); o.shuffle( &cards, 10 ); display( "OVERHAND" );
fill(); std::random_shuffle( cards.begin(), cards.end() ); display( "STD SHUFFLE" );

return 0;
}

```

{{out}}

```
RIFFLE
18 9 17 20 3 4 16 8 7 10 5 14 12 1 13 19 2 11 15 6

OVERHAND
2 13 12 11 10 9 18 17 6 5 4 3 7 20 19 15 8 14 16 1

STD SHUFFLE
14 4 17 3 12 5 19 6 20 2 16 11 8 15 7 13 10 18 9 1

```

## C#

{{trans|Java}}

```using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CardShuffles {
public static class Helper {
public static string AsString<T>(this ICollection<T> c) {
StringBuilder sb = new StringBuilder("[");
sb.Append(string.Join(", ", c));
return sb.Append("]").ToString();
}
}

class Program {
private static Random rand = new Random();

public static List<T> riffleShuffle<T>(ICollection<T> list, int flips) {
List<T> newList = new List<T>(list);

for (int n = 0; n < flips; n++) {
//cut the deck at the middle +/- 10%, remove the second line of the formula for perfect cutting
int cutPoint = newList.Count / 2
+ (rand.Next(0, 2) == 0 ? -1 : 1) * rand.Next((int)(newList.Count * 0.1));

//split the deck
List<T> left = new List<T>(newList.Take(cutPoint));
List<T> right = new List<T>(newList.Skip(cutPoint));

newList.Clear();

while (left.Count > 0 && right.Count > 0) {
//allow for imperfect riffling so that more than one card can come form the same side in a row
//biased towards the side with more cards
//remove the if and else and brackets for perfect riffling
if (rand.NextDouble() >= ((double)left.Count / right.Count) / 2) {
right.RemoveAt(0);
}
else {
left.RemoveAt(0);
}
}

//if either hand is out of cards then flip all of the other hand to the shuffled deck
}

return newList;
}

public static List<T> overhandShuffle<T>(List<T> list, int passes) {
List<T> mainHand = new List<T>(list);

for (int n = 0; n < passes; n++) {
List<T> otherHand = new List<T>();

while (mainHand.Count>0) {
//cut at up to 20% of the way through the deck
int cutSize = rand.Next((int)(list.Count * 0.2)) + 1;

List<T> temp = new List<T>();

//grab the next cut up to the end of the cards left in the main hand
for (int i = 0; i < cutSize && mainHand.Count > 0; i++) {
mainHand.RemoveAt(0);
}

//add them to the cards in the other hand, sometimes to the front sometimes to the back
if (rand.NextDouble()>=0.1) {
//front most of the time
otherHand = temp;
}
else {
//end sometimes
}
}

//move the cards back to the main hand
mainHand = otherHand;
}

return mainHand;
}

static void Main(string[] args) {
List<int> list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
Console.WriteLine(list.AsString());
list = riffleShuffle(list, 10);
Console.WriteLine(list.AsString());
Console.WriteLine();

list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
Console.WriteLine(list.AsString());
list = riffleShuffle(list, 1);
Console.WriteLine(list.AsString());
Console.WriteLine();

list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
Console.WriteLine(list.AsString());
list = overhandShuffle(list, 10);
Console.WriteLine(list.AsString());
Console.WriteLine();

list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
Console.WriteLine(list.AsString());
list = overhandShuffle(list, 1);
Console.WriteLine(list.AsString());
Console.WriteLine();
}
}
}
```

{{out}}

```[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[9, 2, 8, 3, 20, 15, 1, 13, 7, 18, 5, 16, 4, 19, 10, 6, 12, 14, 11, 17]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[12, 1, 2, 3, 4, 13, 14, 5, 15, 16, 6, 7, 17, 8, 18, 19, 9, 10, 20, 11]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[13, 18, 12, 17, 10, 9, 19, 11, 16, 15, 6, 8, 14, 1, 3, 2, 5, 4, 7, 20]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[20, 17, 18, 19, 15, 16, 11, 12, 13, 14, 10, 7, 8, 9, 5, 6, 2, 3, 4, 1]
```

## D

{{trans|Java}}

```import std.container.array;
import std.random;
import std.range;
import std.stdio;

auto riffleShuffle(T)(T[] list, int flips) {
auto newList = Array!T(list);

for (int n=0; n<flips; n++) {
//cut the deck at the middle +/- 10%, remove the second line of the formula for perfect cutting
int cutPoint = newList.length / 2
+ choice([-1, 1]) * uniform!"[]"(0, newList.length / 10);

//split the deck
auto left = newList[0..cutPoint];
auto right = newList[cutPoint..\$];

newList.clear();

while (left.length > 0 && right.length > 0) {
//allow for imperfect riffling so that more than one card can come form the same side in a row
//biased towards the side with more cards
//remove the if and else and brackets for perfect riffling
if (uniform01() >= (cast(real) left.length / right.length) / 2) {
newList.insertAfter(newList[], right.front);
right.popFront();
} else {
newList.insertAfter(newList[], left.front);
left.popFront();
}
}

//if either hand is out of cards then flip all of the other hand to the shuffled deck
if (!left.empty) newList ~= left;
if (!right.empty) newList ~= right;
}
return newList.array;
}

auto overhandShuffle(T)(T[] list, int passes) {
auto mainHand = Array!T(list);

for (int n=0; n<passes; n++) {
Array!T otherHand;

while (mainHand.length > 0) {
//cut at up to 20% of the way through the deck
int cutSize = uniform!"[]"(0, list.length / 5) + 1;

Array!T temp;

//grab the next cut up to the end of the cards left in the main hand
for (int i=0; i<cutSize && mainHand.length>0; i++) {
temp ~= mainHand;
mainHand.linearRemove(mainHand[0..1]);
}

//add them to the cards in the other hand, sometimes to the front sometimes to the back
if (uniform01() >= 0.1) {
//front most of the time
otherHand = temp ~ otherHand;
} else {
//end sometimes
otherHand ~= temp;
}
}

//move the cards back to the main hand
mainHand = otherHand;
}
return mainHand.array;
}

void main() {
auto list = iota(1,21).array;
writeln(list);
list = riffleShuffle(list, 10);
writeln(list);
writeln();

list = iota(1,21).array;
writeln(list);
list = riffleShuffle(list, 1);
writeln(list);
writeln();

list = iota(1,21).array;
writeln(list);
list = overhandShuffle(list, 10);
writeln(list);
writeln();

list = iota(1,21).array;
writeln(list);
list = overhandShuffle(list, 1);
writeln(list);
writeln();

list = iota(1,21).array;
writeln(list);
list.randomShuffle();
writeln(list);
}
```

{{out}}

```[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[7, 2, 4, 13, 19, 12, 9, 20, 6, 5, 17, 18, 1, 16, 3, 10, 14, 11, 8, 15]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[11, 12, 1, 2, 3, 4, 13, 5, 14, 15, 16, 6, 7, 17, 18, 8, 9, 19, 10, 20]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[20, 4, 1, 19, 13, 8, 17, 10, 15, 12, 6, 7, 2, 11, 9, 16, 18, 3, 5, 14]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[18, 19, 20, 16, 17, 11, 12, 13, 14, 15, 7, 8, 9, 10, 4, 5, 6, 1, 2, 3]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[10, 17, 19, 13, 5, 12, 1, 2, 14, 4, 9, 16, 7, 3, 15, 20, 8, 11, 6, 18]
```

## Go

{{trans|Kotlin}}

```package main

import (
"fmt"
"math/rand"
"time"
)

func reverse(s []int) {
for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
s[i], s[j] = s[j], s[i]
}
}

func riffle(deck []int, iterations int) []int {
le := len(deck)
pile := make([]int, le)
copy(pile, deck)
for i := 0; i < iterations; i++ {
mid := le / 2
tenpc := mid / 10
// choose a random number within 10% of midpoint
cut := mid - tenpc + rand.Intn(2*tenpc+1)
// split deck into two at cut point
deck1 := make([]int, cut)
deck2 := make([]int, le-cut)
copy(deck1, pile[:cut])
copy(deck2, pile[cut:])
pile = pile[:0]
fromTop := rand.Intn(2) // choose to draw from top (1) or bottom (0)
for len(deck1) > 0 && len(deck2) > 0 {
if fromTop == 1 {
el1 := deck1
deck1 = deck1[1:]
el2 := deck2
deck2 = deck2[1:]
pile = append(pile, el1, el2)
} else {
el1 := deck1[len(deck1)-1]
deck1 = deck1[:len(deck1)-1]
el2 := deck2[len(deck2)-1]
deck2 = deck2[:len(deck2)-1]
pile = append(pile, el1, el2)
}
}
// add any remaining cards to the pile and reverse it
if len(deck1) > 0 {
pile = append(pile, deck1...)
} else if len(deck2) > 0 {
pile = append(pile, deck2...)
}
reverse(pile) // as pile is upside down
}
return pile
}

func overhand(deck []int, iterations int) []int {
le := len(deck)
pile := make([]int, le)
copy(pile, deck)
pile2 := make([]int, 0)
twentypc := le / 5
for i := 0; i < iterations; i++ {
for len(pile) > 0 {
cards := 1 + rand.Intn(twentypc)
if cards > len(pile) {
cards = len(pile)
}
temp := make([]int, cards)
copy(temp, pile[:cards])
pile2 = append(temp, pile2...)
pile = pile[cards:]
}
pile = append(pile, pile2...)
pile2 = pile2[:0]
}
return pile
}

func main() {
rand.Seed(time.Now().UnixNano())
fmt.Println("Starting deck:")
deck := make([]int, 20)
for i := 0; i < 20; i++ {
deck[i] = i + 1
}
fmt.Println(deck)
const iterations = 10
fmt.Println("\nRiffle shuffle with", iterations, "iterations:")
fmt.Println(riffle(deck, iterations))
fmt.Println("\nOverhand shuffle with", iterations, "iterations:")
fmt.Println(overhand(deck, iterations))
fmt.Println("\nStandard library shuffle with 1 iteration:")
rand.Shuffle(len(deck), func(i, j int) {
deck[i], deck[j] = deck[j], deck[i] // shuffles deck in place
})
fmt.Println(deck)
}
```

{{out}} Sample output:

```
Starting deck:
[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20]

Riffle shuffle with 10 iterations:
[19 12 4 7 15 3 13 11 2 8 18 9 10 16 17 14 20 6 1 5]

Overhand shuffle with 10 iterations:
[5 7 9 4 13 3 15 11 2 8 18 12 1 17 16 6 14 19 10 20]

Standard library shuffle with 1 iteration:
[15 16 10 4 7 2 6 18 14 13 17 1 12 3 11 8 19 20 5 9]

```

## J

{{eff note|J|({~ ?~@#)}}

```NB. overhand cut
overhand=: (\: [: +/\ %@%:@# > # ?@# 0:)@]^:[

NB. Gilbert–Shannon–Reeds model
riffle=:  (({.~+/)`(I.@])`(-.@]#inv (}.~+/))} ?@(#&2)@#)@]^:[
```

The probability of a cut occurring between each pair of cards in this overhand shuffle is proportional to the reciprocal of the square root of the number of cards in the deck.

In other words, overhand cut breaks the deck into some number of pieces and reverses the order of those pieces.

Here are some examples of the underlying selection mechanism in action for a deck of 10 cards:

```   ([: +/\ %@%:@# > # ?@# 0:) i.10
0 0 0 0 0 0 0 0 1 1
([: +/\ %@%:@# > # ?@# 0:) i.10
1 1 2 2 2 3 3 3 3 3
([: +/\ %@%:@# > # ?@# 0:) i.10
0 1 1 2 3 3 3 3 4 5
([: +/\ %@%:@# > # ?@# 0:) i.10
0 1 1 1 1 2 2 3 3 3
```

The final step of a cut is to sort the deck in descending order based on the numbers we compute this way.

The left argument says how many of these cuts to perform.

```   1 riffle i.20
0 1 2 3 4 5 6 7 8 13 14 9 15 16 17 10 18 11 12 19
10 riffle i.20
6 10 13 8 2 14 15 9 19 3 18 16 11 1 12 17 5 4 0 7
1 overhand i.20
17 18 19 13 14 15 16 4 5 6 7 8 9 10 11 12 0 1 2 3
10 overhand i.20
15 11 2 4 5 12 16 10 17 19 9 8 6 13 3 18 7 1 0 14
```

## Java

{{works with|Java|1.5+}}

```import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Random;

public class CardShuffles{

private static final Random rand = new Random();

public static <T> LinkedList<T> riffleShuffle(List<T> list, int flips){

for(int n = 0; n < flips; n++){
//cut the deck at the middle +/- 10%, remove the second line of the formula for perfect cutting
int cutPoint = newList.size() / 2
+ (rand.nextBoolean() ? -1 : 1 ) * rand.nextInt((int)(newList.size() * 0.1));

//split the deck

newList.clear();

while(left.size() > 0 && right.size() > 0){
//allow for imperfect riffling so that more than one card can come form the same side in a row
//biased towards the side with more cards
//remove the if and else and brackets for perfect riffling
if(rand.nextDouble() >= ((double)left.size() / right.size()) / 2){
}else{
}
}

//if either hand is out of cards then flip all of the other hand to the shuffled deck
}
return newList;
}

public static <T> LinkedList<T> overhandShuffle(List<T> list, int passes){

for(int n = 0; n < passes; n++){

while(mainHand.size() > 0){
//cut at up to 20% of the way through the deck
int cutSize = rand.nextInt((int)(list.size() * 0.2)) + 1;

//grab the next cut up to the end of the cards left in the main hand
for(int i = 0; i < cutSize && mainHand.size() > 0; i++){
}

//add them to the cards in the other hand, sometimes to the front sometimes to the back
if(rand.nextDouble()  >= 0.1){
//front most of the time
}else{
//end sometimes
}
}

//move the cards back to the main hand
mainHand = otherHand;
}
return mainHand;
}

public static void main(String[] args){
List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
System.out.println(list);
list = riffleShuffle(list, 10);
System.out.println(list + "\n");

list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
System.out.println(list);
list = riffleShuffle(list, 1);
System.out.println(list + "\n");

list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
System.out.println(list);
list = overhandShuffle(list, 10);
System.out.println(list + "\n");

list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
System.out.println(list);
list = overhandShuffle(list, 1);
System.out.println(list + "\n");

list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
System.out.println(list);
Collections.shuffle(list);
System.out.println(list + "\n");
}
}
```

{{out}}

```[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[20, 11, 1, 9, 15, 4, 19, 16, 8, 13, 7, 2, 14, 12, 10, 3, 17, 18, 6, 5]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[1, 12, 2, 3, 4, 5, 13, 14, 15, 6, 16, 7, 8, 9, 17, 18, 10, 19, 20, 11]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[20, 3, 10, 4, 2, 8, 1, 18, 13, 19, 14, 6, 9, 12, 16, 15, 5, 7, 11, 17]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[18, 19, 20, 17, 13, 14, 15, 16, 9, 10, 11, 12, 8, 6, 7, 3, 4, 5, 1, 2]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[18, 12, 13, 14, 2, 3, 15, 5, 9, 19, 7, 11, 1, 6, 4, 20, 16, 17, 10, 8]
```

## Julia

{{works with|Julia|0.6}}

```function riffleshuffle!(list::Vector, flips::Integer)
len = length(list)
# pre-allocate the left and right part for efficiency
llist = similar(list, len÷2 + fld(len, 10))
rlist = similar(list, len÷2 + fld(len, 10))
for _ in Base.OneTo(flips)
# cut the deck at the middle +/- 10%,
# remove the second line of the formula for perfect cutting
cut = len ÷ 2 + rand(-1:2:1) * rand(0:fld(len, 10))

# split the deck and copy it to left and right
copy!(llist, 1, list, 1, cut)
copy!(rlist, 1, list, cut + 1, len - cut)

ind, indl, indr = len, cut, len - cut
while indl ≥ 1 && indr ≥ 1
if rand() < indl / 2indr
list[ind] = llist[indl]
indl -= 1
else
list[ind] = rlist[indr]
indr -= 1
end
ind -= 1
end

copy!(list, 1, rlist, 1, indr)
copy!(list, 1, llist, 1, indl)
end
return list
end

function overhandshuffle!(list::Vector, passes::Integer)
len = length(list)
otherhand = similar(list)
for _ in Base.OneTo(passes)
ind = 1
while ind ≤ endof(list)
chklen = min(rand(1:cld(len, 5)), len - ind + 1)
copy!(otherhand, ind, list, len - ind - chklen + 2, chklen)
ind += chklen
end
list .= otherhand
end
return list
end

v = collect(1:20)
println("# Riffle shuffle (1):\n", v)
println(" -> ", riffleshuffle!(v, 1), "\n")

v = collect(1:20)
println("# Riffle shuffle (10):\n", v)
println(" -> ", riffleshuffle!(v, 10), "\n")

v = collect(1:20)
println("# Overhand shuffle (1):\n", v)
println(" -> ", overhandshuffle!(v, 1), "\n")

v = collect(1:20)
println("# Overhand shuffle (10):\n", v)
println(" -> ", overhandshuffle!(v, 10), "\n")

v = collect(1:20)
println("# Default shuffle:\n", v)
println(" -> ", shuffle!(v), "\n")
```

{{out}}

```# Riffle shuffle (1):
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
-> [11, 1, 12, 2, 3, 13, 14, 4, 5, 15, 6, 16, 17, 7, 8, 18, 9, 10, 19, 20]

# Riffle shuffle (10):
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
-> [13, 11, 7, 2, 12, 1, 3, 16, 19, 5, 4, 14, 9, 10, 18, 15, 6, 17, 8, 20]

# Overhand shuffle (1):
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
-> [19, 20, 17, 18, 15, 16, 14, 13, 10, 11, 12, 6, 7, 8, 9, 2, 3, 4, 5, 1]

# Overhand shuffle (10):
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
-> [2, 8, 12, 4, 1, 5, 7, 11, 17, 6, 14, 19, 3, 9, 10, 15, 18, 13, 16, 20]

# Default shuffle:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
-> [3, 11, 18, 14, 2, 12, 13, 4, 10, 19, 8, 16, 20, 5, 1, 6, 9, 15, 17, 7]
```

## Kotlin

```// version 1.1.51

import java.util.Random
import java.util.Collections.shuffle

val r = Random()

fun riffle(deck: List<Int>, iterations: Int): List<Int> {
val pile = deck.toMutableList()

repeat(iterations) {
val mid = deck.size / 2
val tenpc = mid / 10
// choose a random number within 10% of midpoint
val cut = mid - tenpc + r.nextInt(2 * tenpc + 1)
// split deck into two at cut point
val deck1 = pile.take(cut).toMutableList()
val deck2 = pile.drop(cut).toMutableList()
pile.clear()
val fromTop = r.nextBoolean() // choose to draw from top or bottom
while (deck1.size > 0 && deck2.size > 0) {
if (fromTop) {
}
else {
}
}
// add any remaining cards to the pile and reverse it
else if (deck2.size > 0) pile.addAll(deck2)
pile.reverse() // as pile is upside down
}
return pile
}

fun overhand(deck: List<Int>, iterations: Int): List<Int> {
val pile = deck.toMutableList()
val pile2 = mutableListOf<Int>()
val twentypc = deck.size / 5
repeat(iterations) {
while (pile.size > 0) {
val cards = minOf(pile.size, 1 + r.nextInt(twentypc))
repeat(cards) { pile.removeAt(0) }
}
pile2.clear()
}
return pile
}

fun main(args: Array<String>) {
println("Starting deck:")
var deck = List<Int>(20) { it + 1 }
println(deck)
val iterations = 10
println("\nRiffle shuffle with \$iterations iterations:")
println(riffle(deck, iterations))
println("\nOverhand shuffle with \$iterations iterations:")
println(overhand(deck, iterations))
println("\nStandard library shuffle with 1 iteration:")
shuffle(deck) // shuffles deck in place
println(deck)
}
```

Sample output:

```
Starting deck:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

Riffle shuffle with 10 iterations:
[5, 18, 3, 12, 15, 6, 8, 16, 2, 20, 17, 11, 13, 1, 9, 14, 19, 7, 10, 4]

Overhand shuffle with 10 iterations:
[2, 8, 1, 4, 11, 7, 3, 5, 17, 12, 10, 9, 6, 18, 20, 14, 19, 13, 15, 16]

Standard library shuffle with 1 iteration:
[17, 9, 12, 15, 7, 13, 18, 8, 2, 20, 5, 10, 16, 6, 14, 4, 19, 3, 11, 1]

```

## Lua

```-- Return a table respresenting a standard deck of cards in order
function newDeck ()
local cards, suits = {}, {"C", "D", "H", "S"}
for _, suit in pairs(suits) do
for value = 2, 14 do
if value == 10 then value = "T" end
if value == 11 then value = "J" end
if value == 12 then value = "Q" end
if value == 13 then value = "K" end
if value == 14 then value = "A" end
table.insert(cards, value .. suit)
end
end
return cards
end

-- Display all cards (strings) in a given deck (table)
function show (deck)
for _, card in pairs(deck) do io.write(card .. " ") end
print("\n")
end

-- Perform a riffle shuffle on deck and return it as a new table
function riffle (deck)
local pile1, pile2, pos = {}, {}, 1
for i, card in ipairs(deck) do
if i < math.ceil(#deck / 2) + 1 then
table.insert(pile1, card)
else
table.insert(pile2, card)
end
end
deck = {}
while pile2[pos] do
table.insert(deck, pile1[pos])
table.insert(deck, pile2[pos])
pos = pos + 1
end
return deck
end

-- Perform an overhand shuffle on a deck and return it as a new table
function overhand (deck)
local newDeck, twentyPercent, groupSize, pos = {}, math.floor(#deck / 5)
repeat
repeat
groupSize = math.random(twentyPercent)
until groupSize <= #deck
for pos = #deck - groupSize, #deck do
table.insert(newDeck, deck[pos])
deck[pos] = nil
end
until #deck == 0
return newDeck
end

-- Main procedure
math.randomseed(os.time())
local deck1, deck2 = newDeck(), newDeck()
deck1 = riffle(deck1)
print("Sorted deck after one riffle shuffle:")
show(deck1)
deck2 = overhand(deck2)
print("Sorted deck after one overhand shuffle:")
show(deck2)
```

{{out}}

```Sorted deck after one riffle shuffle:
2C 2H 3C 3H 4C 4H 5C 5H 6C 6H 7C 7H 8C 8H 9C 9H TC TH JC JH QC QH KC KH AC AH 2D
2S 3D 3S 4D 4S 5D 5S 6D 6S 7D 7S 8D 8S 9D 9S TD TS JD JS QD QS KD KS AD AS

Sorted deck after one overhand shuffle:
QS KS AS 3S 4S 5S 6S 7S 8S 9S TS JS JH QH KH AH 2S 4H 5H 6H 7H 8H 9H TH 2H 3H 4D
5D 6D 7D 8D 9D TD JD QD KD AD QC KC AC 2D 3D 4C 5C 6C 7C 8C 9C TC JC 2C 3C

```

```MODULE CardShuffles;
FROM FormatString IMPORT FormatString;
FROM RandomNumbers IMPORT Random;

PROCEDURE WriteCard(c : CARDINAL);
VAR buf : ARRAY[0..15] OF CHAR;
BEGIN
FormatString("%c", buf, c);
WriteString(buf)
END WriteCard;

PROCEDURE WriteInteger(i : INTEGER);
VAR buf : ARRAY[0..15] OF CHAR;
BEGIN
FormatString("%02i", buf, i);
WriteString(buf)
END WriteInteger;

PROCEDURE WriteIntArray(array : ARRAY OF INTEGER);
VAR i : CARDINAL;
BEGIN
WriteString("[");
FOR i:=0 TO HIGH(array) DO
IF i>0 THEN
WriteString(", ");
END;
WriteInteger(array[i]);
END;
WriteString("]")
END WriteIntArray;

(*---------------------------------------*)

TYPE Deck_t = ARRAY[0..20] OF INTEGER;

PROCEDURE InitDeck(VAR deck : ARRAY OF INTEGER);
VAR i : CARDINAL;
BEGIN
FOR i:=0 TO HIGH(deck) DO
deck[i] := i + 1
END
END InitDeck;

PROCEDURE RiffleShuffle(VAR deck : Deck_t; flips : CARDINAL);
VAR
n,cutPoint,nlp,lp,rp,bound : CARDINAL;
nl : Deck_t;
BEGIN
FOR n:=1 TO flips DO
cutPoint := HIGH(deck) / 2;
IF Random(0, 2) > 0 THEN
cutPoint := cutPoint + Random(0, HIGH(deck) / 10);
ELSE
cutPoint := cutPoint - Random(0, HIGH(deck) / 10);
END;

nlp := 0;
lp := 0;
rp := cutPoint;

WHILE (lp <= cutPoint) AND (rp < HIGH(deck)) DO
(* Allow for an imperfect riffling so that more than one card can come from the same side in a row
biased towards the side with more cards. Remove the IF statement for perfect riffling. *)
bound := (cutPoint - lp) * 50 / (HIGH(deck) - rp);
IF Random(0, 50)>= bound THEN
nl[nlp] := deck[rp];
INC(nlp);
INC(rp);
ELSE
nl[nlp] := deck[lp];
INC(nlp);
INC(lp);
END
END;
WHILE lp <= cutPoint DO
nl[nlp] := deck[lp];
INC(nlp);
INC(lp);
END;
WHILE rp < HIGH(deck) DO
nl[nlp] := deck[rp];
INC(nlp);
INC(rp);
END;

deck := nl
END
END RiffleShuffle;

PROCEDURE OverhandShuffle(VAR mainHand : Deck_t; passes : CARDINAL);
VAR
n,cutSize,mp,op,tp,i : CARDINAL;
otherHand,temp : Deck_t;
BEGIN
FOR n:=1 TO passes DO
mp := 0;
op := 0;
FOR i:=0 TO HIGH(otherHand) DO
otherHand[i] := 9999
END;

WHILE mp < HIGH(mainHand) DO
(* Cut at up to 20% of the way through the deck *)
cutSize := Random(0, HIGH(mainHand) / 5) + 1;
tp := 0;

(* Grab the next cut up to the end of the cards left in the main hand *)
i:=0;
WHILE (i < cutSize) AND (mp < HIGH(mainHand)) DO
temp[tp] := mainHand[mp];
INC(tp);
INC(mp);
INC(i);

IF mp = HIGH(mainHand) THEN
temp[tp] := mainHand[mp];
INC(tp);
INC(mp);
END
END;

(* Add them to the cards in the other hand, sometimes to the front and sometimes to the back *)
IF Random(0, 10) >= 1 THEN
(* otherHand = temp + otherHand *)

(* copy other hand elements up by temp spaces *)
i := op;
WHILE (i > 0) AND (op > 0) DO
otherHand[tp + i] := otherHand[i];
DEC(i)
END;
IF op > 0 THEN
otherHand[tp] := otherHand
END;

(* copy the elements of temp into the front of other hand *)
FOR i:=0 TO tp-1 DO
otherHand[i] := temp[i]
END
ELSE
(* otherHand = otherHand + temp *)
FOR i:=0 TO tp DO
otherHand[op+i] := temp[i]
END
END;
op := op + tp
END;

(* Move the cards back to the main hand *)
mainHand := otherHand
END
END OverhandShuffle;

(* Main *)
VAR deck : Deck_t;
BEGIN
WriteString("Riffle shuffle");
WriteLn;
InitDeck(deck);
WriteIntArray(deck);
WriteLn;
RiffleShuffle(deck, 10);
WriteIntArray(deck);
WriteLn;
WriteLn;

WriteString("Riffle shuffle");
WriteLn;
InitDeck(deck);
WriteIntArray(deck);
WriteLn;
RiffleShuffle(deck, 1);
WriteIntArray(deck);
WriteLn;
WriteLn;

WriteString("Overhand shuffle");
WriteLn;
InitDeck(deck);
WriteIntArray(deck);
WriteLn;
OverhandShuffle(deck, 10);
WriteIntArray(deck);
WriteLn;
WriteLn;

WriteString("Overhand shuffle");
WriteLn;
InitDeck(deck);
WriteIntArray(deck);
WriteLn;
OverhandShuffle(deck, 1);
WriteIntArray(deck);
WriteLn;

END CardShuffles.
```

## PARI/GP

Riffle shuffle:

```riffle(v)=
{
my(n=#v,k,t,deck=vector(n),left,right);
t=random(2^n);
for(i=0,n,
t -= binomial(n,i);
if(t<0, k=i; break)
);
if(k==0||k==n, return(v));
left=k;
right=n-k;
deck=vector(n,i,
t=random(n+1-i);
v[if(t<left, k-left--, n-right--)]
);
vecextract(v, deck);
}
addhelp(riffle, "riffle(v): Riffle shuffles the vector v, following the Gilbert-Shannon-Reeds model.");
```

Overhand shuffle:

```overhand(v)=
{
my(u=[],t,n=2*#v\5);
while(#v,
t=min(random(n)+1,#v);
u=concat(v[1..t],u);
v=if(t<#v,v[t+1..#v],[]);
);
u;
}
addhelp(overhand, "overhand(v): Overhand shuffles the vector v.");
```

Usage:

```riffle([1..52])
overhand([1..52])
```

{{out}}

```%1 = [1, 2, 3, 21, 4, 22, 23, 5, 24, 25, 26, 6, 27, 28, 29, 30, 7, 31, 32, 33, 34, 35, 36, 8, 37, 38, 39, 40, 9, 10, 11, 12, 41, 42, 43, 13, 44, 45, 14, 46, 47, 48, 15, 16, 17, 49, 50, 18, 51, 19, 20, 52]
%2 = [44, 45, 46, 47, 48, 49, 50, 51, 52, 43, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 23, 24, 25, 26, 27, 28, 29, 30, 31, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 1, 2, 3, 4]
```

## Perl

Follows the Perl 6 implementation for the overhand shuffle, but uses classic one-liner for riffle.

```sub overhand {
our @cards; local *cards = shift;
my(@splits,@shuffle);
my \$x = int +@cards / 5;
push @splits, (1..\$x)[int rand \$x] for 1..+@cards;
while (@cards) {
push @shuffle, [splice @cards, 0, shift @splits];
}
@cards = flatten(reverse @shuffle);
}

sub flatten { map { ref \$_ eq 'ARRAY' ? @\$_ : \$_ } @_ }

sub riffle {
our @cards; local *cards = shift;
splice @cards, @cards/2 - \$_, 0, pop @cards for 0 .. (@cards/2)-1;
}

@cards = 1..20;
overhand(\@cards) for 1..10;
print join ' ', @cards, "\n";

@cards = 1..20;
riffle(\@cards) for 1..10;
print join ' ', @cards, "\n";
```

{{out}}

```9 11 5 2 4 14 1 3 8 6 15 13 16 12 19 20 7 18 10 17
1 10 19 9 18 8 17 7 16 6 15 5 14 4 13 3 12 2 11 20
```

## Perl 6

```use v6;

sub overhand ( @cards ) {
my @splits = roll 10, ^( @cards.elems div 5 )+1;
@cards.rotor( @splits  ,:partial ).reverse.flat
}

sub riffle ( @pile is copy ) {
my @pile2 = @pile.splice: @pile.elems div 2 ;

roundrobin(
@pile.rotor(  (1 .. 3).roll(7), :partial ),
@pile2.rotor( (1 .. 3).roll(9), :partial ),
).flat
}

my @cards = ^20;
@cards.=&overhand for ^10;
say @cards;

my @cards2 = ^20;
@cards2.=&riffle for ^10;
say @cards2;

say (^20).pick(*);

```

## Phix

```function riffle(sequence s)
sequence res = {}
integer l = length(s)
integer r = rand(l)
for i=1 to l do
if r+i<=l then
res &= s[r+i]
end if
if i<=r then
res &= s[i]
end if
end for
return res
end function

function overhand(sequence s)
sequence res = {}
integer l = length(s)
while length(s) do
integer r = rand(l*0.2)
if r>length(s) then
r = length(s)
end if
res = s[1..r]&res
s = s[r+1..\$]
end while
return res
end function

-- to shorten the output, all 2..7 have been removed from the deck
constant DECKSIZE=52-24

procedure show_deck(sequence s)
for i=1 to DECKSIZE do
integer c = s[i]-1
--      puts(1,"23456789TJQKA"[remainder(c,13)+1]&"HCDS"[floor(c/13)+1]&" ")
puts(1,"89TJQKA"[remainder(c,7)+1]&"HCDS"[floor(c/7)+1]&" ")
end for
puts(1,"\n")
end procedure

show_deck(riffle(tagset(DECKSIZE)))
show_deck(overhand(tagset(DECKSIZE)))
show_deck(shuffle(tagset(DECKSIZE)))
```

{{out}}

```
TC 8H JC 9H QC TH KC JH AC QH 8D KH 9D AH TD 8C JD 9C QD KD AD 8S 9S TS JS QS KS AS
KS AS JS QS TS AD 8S 9S 9D TD JD QD KD QC KC AC 8D AH 8C 9C TC JC JH QH KH TH 8H 9H
KH TH AH QH 8D JC QC 8C JH 8H 9D KS TD AS KD 8S TC AD TS AC 9C KC 9H QD JD JS 9S QS

```

## PicoLisp

```(load "@lib/simul.l")

(de riffle (Lst)
(let N (/ (setq @@ (length Lst)) 2)
(conc
(mapcan list (head N Lst) (tail (- N) Lst))
(and (bit? 1 @@) (tail 1 Lst)) ) ) )
(de overhand (Lst)
(let N (/ (* (length Lst) 20) 100)
(make
(while (flip (cut N 'Lst))
(for I @
(yoke I) ) ) ) ) )

(println 'riffle (riffle (range 1 19)) )
(println 'overhand (overhand (range 1 19)) )
(println 'shuffle (shuffle (range 1 19)) )
```

{{out}}

```
riffle (1 10 2 11 3 12 4 13 5 14 6 15 7 16 8 17 9 18 19)
overhand (19 16 17 18 13 14 15 10 11 12 7 8 9 4 5 6 1 2 3)
shuffle (5 3 13 15 17 12 14 11 2 1 19 7 6 9 18 8 10 4 16)

```

## Python

{{trans|D}}

```import random

def riffleShuffle(va, flips):
nl = va
for n in range(flips):
#cut the deck at the middle +/- 10%, remove the second line of the formula for perfect cutting
cutPoint = len(nl)/2 + random.choice([-1, 1]) * random.randint(0, len(va)/10)

# split the deck
left = nl[0:cutPoint]
right = nl[cutPoint:]

del nl[:]
while (len(left) > 0 and len(right) > 0):
#allow for imperfect riffling so that more than one card can come form the same side in a row
#biased towards the side with more cards
#remove the if and else and brackets for perfect riffling
if (random.uniform(0, 1) >= len(left) / len(right) / 2):
nl.append(right.pop(0))
else:
nl.append(left.pop(0))
if (len(left) > 0):
nl = nl + left
if (len(right) > 0):
nl = nl + right
return nl

def overhandShuffle(va, passes):
mainHand = va
for n in range(passes):
otherHand = []
while (len(mainHand) > 0):
#cut at up to 20% of the way through the deck
cutSize = random.randint(0, len(va) / 5) + 1
temp = []

#grab the next cut up to the end of the cards left in the main hand
i=0
while (i<cutSize and len(mainHand) > 0):
temp.append(mainHand.pop(0))
i = i + 1

#add them to the cards in the other hand, sometimes to the front sometimes to the back
if (random.uniform(0, 1) >= 0.1):
#front most of the time
otherHand = temp + otherHand
else:
otherHand = otherHand + temp
#move the cards back to the main hand
mainHand = otherHand
return mainHand

print "Riffle shuffle"
nums = [x+1 for x in range(21)]
print nums
print riffleShuffle(nums, 10)
print

print "Riffle shuffle"
nums = [x+1 for x in range(21)]
print nums
print riffleShuffle(nums, 1)
print

print "Overhand shuffle"
nums = [x+1 for x in range(21)]
print nums
print overhandShuffle(nums, 10)
print

print "Overhand shuffle"
nums = [x+1 for x in range(21)]
print nums
print overhandShuffle(nums, 1)
print

print "Library shuffle"
nums = [x+1 for x in range(21)]
print nums
random.shuffle(nums)
print nums
print
```

{{out}}

```Riffle shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[4, 16, 5, 19, 3, 14, 2, 9, 20, 13, 17, 10, 6, 7, 1, 18, 12, 11, 8, 21, 15]

Riffle shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[13, 14, 15, 1, 16, 2, 3, 17, 4, 5, 18, 6, 7, 19, 8, 9, 20, 10, 11, 21, 12]

Overhand shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[21, 12, 5, 16, 7, 2, 15, 14, 20, 6, 8, 11, 13, 1, 4, 17, 19, 9, 3, 18, 10]

Overhand shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[21, 20, 19, 18, 16, 17, 14, 15, 11, 12, 13, 4, 5, 6, 7, 1, 2, 3, 8, 9, 10]

Library shuffle
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[14, 12, 2, 17, 18, 21, 8, 4, 15, 9, 11, 10, 3, 1, 7, 19, 20, 6, 5, 16, 13]
```

## Racket

These implementations are in `typed/racket`, which means that additional annotations are needed which looks like hard work.

On the bright side, if you want to add a new `Cutter` or `Riffler`, DrRacket will let you know immediately if you're consuming lists of lists of lists at the right depth and in the right quantities.

Racket has a built in `shuffle` function. Frankly, I'd go with that in your own code!

```#lang typed/racket
;; ---------------------------------------------------------------------------------------------------
;; Types and shuffle builder

;; A cutter separates the deck into more than one sub-decks -- the last one of these is "left in the
;; hand", as per the overhand shuffle (since it is the last strip to be stripped). The riffler
;; indicates this in its second (non-null) return value
(define-type (Cutter A) (-> (Listof A) (Pair (Listof A) (Listof (Listof A)))))
;; A riffler takes taking hand and the cut deck parts. returns a newly merged deck in the "taking"
;; hand and the deck left in the "giving" hand. The shuffler will keep taking,
;; until there is nothing to give
(define-type (Riffler A) ((Listof A) (Listof A) (Listof A) * -> (Values (Listof A) (Listof A))))
;; "The shuffler will keep taking until there is nothing to give"... and will do this
;; the number of times specified by its second argument
(define-type (Shuffler A) ((Listof A) Natural -> (Listof A)))

;; makes a shuffler from the cutter and the riffler
(: shuffler-composer (All (A) (Cutter A) (Riffler A) -> (Shuffler A)))
(define ((shuffler-composer cut riffle) deck n)
(: one-shuffle : (Listof A) -> (Listof A))
(define (one-shuffle g)
(let: shuff ((t : (Listof A) null) (g : (Listof A) g))
(let-values (((t+ g-) (apply riffle t (cut g))))
(if (null? g-) t+ (shuff t+ g-)))))
(for/fold : (Listof A) ((d deck)) ((i (in-range n)))
(one-shuffle d)))

;; convenient wrapper around the above (otherwise we'd need the inst every time we
;; wanted to compose a cut and a riffle
(define-syntax-rule (define-composed-shuffler s (c r))
(define: (A) (s [x : (Listof A)] [n : Natural]) : (Listof A)
((#{shuffler-composer @ A} c r) x n)))

;; ---------------------------------------------------------------------------------------------------
;; Overhand (and, as far as I can tell, Indian)
(: overhand-cutter (All (A) (Cutter A)))
(: overhand-riffler (All (A) (Riffler A)))

(define (overhand-cutter l)
(define spl (match (length l) [0 0] [1 1] [len (add1 (random (sub1 len)))]))
(list (take l spl) (drop l spl)))

(define (overhand-riffler t p1 . rest)
(values (append p1 t) (append* rest)))

(define-composed-shuffler overhand-shuffle (overhand-cutter overhand-riffler))

;; ---------------------------------------------------------------------------------------------------
;; Riffle (with optional "drop" where two cards are riffled
(: half-deck-cutter (All (A) (Cutter A)))
(: mk-riffle-riffler (All (A) ((#:p-drop Nonnegative-Real) -> (Riffler A))))

(define (half-deck-cutter l)
(define spl (quotient (length l) 2))
(list (take l spl) (drop l spl)))

;; All the "reverse"ing is to emulate a physical shuffle... it's not
;; necessary for the "randomising" effect (which there isn't really on
;; a pure riffle anyway)
;;
;; Additional complexity added by ability to drop cards on both taking
;; and giving hand
(define ((mk-riffle-riffler #:p-drop (p-drop 0)) t p1 . rest)
(define g-/rev
(let R : (Listof A)
((r1 : (Listof A) p1)
(r2 : (Listof A) (append* rest))
(rv : (Listof A) t)) ; although t should normaly be null
(define drop-t? (< (random) p-drop))
(define drop-g? (< (random) p-drop))
(match* (r1 r2 drop-t? drop-g?)
[((list) (app reverse 2r) _ _) (append 2r rv)]
[((app reverse 1r) (list) _ _) (append 1r rv)]
[((list a1.1 a1.2 d1 ...) (list a2.1 a2.2 d2 ...) #t #t)
(R d1 d2 (list* a2.2 a2.1 a1.2 a1.1 rv))]
[((list a1.1 a1.2 d1 ...) (list a2.1 d2 ...) #t _)
(R d1 d2 (list* a2.1 a1.2 a1.1 rv))]
[((list a1.1 d1 ...) (list a2.1 a2.2 d2 ...) _ #t)
(R d1 d2 (list* a2.2 a2.1 a1.1 rv))]
[((list a1.1 d1 ...) (list a2.1 d2 ...) _ _)
(R d1 d2 (list* a2.1 a1.1 rv))])))
(values (reverse g-/rev) null))

(define-composed-shuffler pure-riffle-shuffle (half-deck-cutter (mk-riffle-riffler)))
(define-composed-shuffler klutz-riffle-shuffle (half-deck-cutter (mk-riffle-riffler #:p-drop 0.5)))

;; ---------------------------------------------------------------------------------------------------
;; Pile Shuffle
;; Also Wash Shuffle, if pile-height=1 and random-gather=#t
(: mk-pile-cutter (All (A) (#:pile-height Positive-Integer -> (Cutter A))))
(: mk-pile-riffler (All (A) ((#:random-gather? Boolean) -> (Riffler A))))

(define ((mk-pile-cutter #:pile-height pile-height) l)
(define len-l (length l))
(define n-piles (add1 (quotient (sub1 len-l) pile-height)))
(: make-pile (Integer -> (Listof A)))
(define (make-pile n)
(for/list : (Listof A) ((i (in-range n len-l n-piles)))
(list-ref l i)))
(define pile-0 (make-pile 0))
(define piles-ns (for/list : (Listof (Listof A)) ((n (in-range 1 n-piles))) (make-pile n)))
(list* pile-0 piles-ns))

(define ((mk-pile-riffler #:random-gather? (random-gather? #f)) t p1 . rest)
(: piles (Listof (Listof A)))
(define piles (cons p1 rest))
(define gather (if random-gather? (shuffle piles) piles))
(values (append* (cons t (if random-gather? (shuffle piles) piles))) null))

(define-composed-shuffler 4-high-pile-shuffle ((mk-pile-cutter #:pile-height 4) (mk-pile-riffler)))
(define-composed-shuffler wash-pile-shuffle
((mk-pile-cutter #:pile-height 1) (mk-pile-riffler #:random-gather? #t)))

;; ---------------------------------------------------------------------------------------------------
(define unshuffled-pack
(for*/list : (Listof String)
((s '(♥ ♦ ♣ ♠))
(f '(2 3 4 5 6 7 8 9 T J Q K A)))
(format "~a~a" f s)))

;; ---------------------------------------------------------------------------------------------------
;; TEST/OUTPUT
(module+ test
(require typed/rackunit)
(check-equal? (overhand-shuffle null 1) null)
(check-equal? (overhand-shuffle '(a) 1) '(a))
(check-equal? (overhand-shuffle '(a b) 1) '(b a))
(check-equal? (pure-riffle-shuffle '(1 2 3 4) 1) '(1 3 2 4))
(error-print-width 80))

(module+ main
(printf "deck (original order):          ~.a~%" unshuffled-pack)
(printf "overhand-shuffle (2 passes):    ~.a~%" (overhand-shuffle unshuffled-pack 2))
(printf "overhand-shuffle (1300 passes): ~.a~%" (overhand-shuffle unshuffled-pack 1300))
(printf "riffle: pure                    ~.a~%" (pure-riffle-shuffle unshuffled-pack 1))
(printf "riffle: klutz                   ~.a~%" (klutz-riffle-shuffle unshuffled-pack 1))
(printf "4-high piles:                   ~.a~%" (4-high-pile-shuffle unshuffled-pack 1))
(printf "4-high piles (7 passes):        ~.a~%" (4-high-pile-shuffle unshuffled-pack 7))
(printf "4-high piles (7 passes again):  ~.a~%" (4-high-pile-shuffle unshuffled-pack 7))
(printf "wash piles:                     ~.a~%" (wash-pile-shuffle unshuffled-pack 1))
;; Or there is always the built-in shuffle:
(printf "shuffle:                        ~.a~%" (shuffle unshuffled-pack)))
```

{{out}}

You see no output from the tests... that's a good thing, they're all passing.

Output is truncated by the `~.a` format in `printf`. However, this should give you some idea of what's going on.

```deck (original order):          (2♥ 3♥ 4♥ 5♥ 6♥ 7♥ 8♥ 9♥ T♥ J♥ Q♥ K♥ A♥ 2♦ 3♦ 4...
overhand-shuffle (2 passes):    (2♥ 6♠ 5♠ J♦ Q♦ K♦ A♦ 2♣ 3♣ 4♣ 5♣ 6♣ 7♣ 8♣ 9♣ T...
overhand-shuffle (1300 passes): (J♦ J♥ J♠ A♥ K♦ 5♥ J♣ 8♣ 2♥ 4♠ 9♥ A♠ K♣ Q♥ 4♥ 7...
riffle: pure                    (2♥ 2♣ 3♥ 3♣ 4♥ 4♣ 5♥ 5♣ 6♥ 6♣ 7♥ 7♣ 8♥ 8♣ 9♥ 9...
riffle: klutz                   (2♥ 2♣ 3♥ 3♣ 4♥ 4♣ 5♣ 5♥ 6♥ 6♣ 7♥ 7♣ 8♥ 8♣ 9♥ 9...
4-high piles:                   (2♥ 2♦ 2♣ 2♠ 3♥ 3♦ 3♣ 3♠ 4♥ 4♦ 4♣ 4♠ 5♥ 5♦ 5♣ 5...
4-high piles (7 passes):        (2♥ 6♥ T♥ A♥ 5♦ 9♦ K♦ 4♣ 8♣ Q♣ 3♠ 7♠ J♠ 3♥ 7♥ J...
4-high piles (7 passes again):  (2♥ 6♥ T♥ A♥ 5♦ 9♦ K♦ 4♣ 8♣ Q♣ 3♠ 7♠ J♠ 3♥ 7♥ J...
wash piles:                     (4♣ K♠ 4♠ Q♥ J♣ A♣ 6♦ 6♥ 7♥ A♠ T♠ T♥ Q♣ 8♠ 3♣ J...
shuffle:                        (J♣ 2♠ 4♦ A♦ K♥ 6♦ 5♦ 8♣ 2♦ T♥ 4♠ 3♣ 7♦ 9♠ T♦ J...

```

## REXX

A little extra effort was put into the '''create''' subroutine to build any sort of deck, even a multiple deck as in canasta and samba (with/without jokers). Adding options for short decks, pinochle, schmear, six-handed 500, and the like would be prohibitive and muddy up the code and be distracting.

Six-handed 500 has additional cards of: ♣11 ♣12 ♠11 ♠12 ♦11 ♦12 ♦13 ♦11 ♦12 ♦13

```/*REXX program simulates various types of shuffling a deck of cards  (any kind of deck).*/
call create;  call show  'new deck'              /*build and display a new card deck.   */

call create;  call riffle     1                  /*invoke a riffle shuffle  (N times).  */
call show  'riffle shuffle'        /*display the results from last shuffle*/

call create;  call overhand  1/5                 /*invoke overhand shuffle with 20% cuts*/
call show  'overhand shuffle'      /*display the results from last shuffle*/

call create;  call barnYard  13                  /*also called a washing machine shuffle*/
call show  'barn yard shuffle'     /*display the results from last shuffle*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
create: if 9=='f9'x  then suit= "cdhs"           /*EBCDIC?   Then use letters for suits.*/
else suit= "♣♦♥♠"           /* ASCII?     "   "  symbols  "    "   */
jokers= 0                                /*number of jokers in the card deck.   */
wild= copies("jH jL", jokers)            /*a large # of high jokers, low jokers.*/
rank= 'A23456789tJQK'                    /*t  in the rank represents a ten (10).*/
decks= 1                                 /*the number of decks, building a shoe?*/
\$=                                       /*the initial (null) card deck (string)*/
do   s=1  for length(suit)        /*process each of the card deck suits. */
_= substr(suit, s, 1)             /*extract a single suit to build + pips*/
do r=1  for length(rank)        /*process each of the card deck pips.  */
\$= \$  _ || substr(rank, r, 1)   /*build a card, then append it to deck.*/
end   /*r*/                     /*Note: some decks have more pips, >13.*/
end     /*s*/                     /*  "     "    "     "    "  suits, >4.*/
\$= space(\$  subword(wild, 1, jokers) )   /*keep a new card deck for each shuffle*/
\$= copies(\$, decks)                      /*maybe build multiple decks for a shoe*/
#= words(\$)                              /*set the number of cards in the deck. */
/*another entry point for this function*/
build:  @.=;         do j=1  for words(\$)        /*build an array for the card deck.    */
@.j= word(\$, j)             /*construct an card from the deck list.*/
end   /*j*/
return \$                                 /*elide the leading blank in the deck. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
?:        return random(1, word( arg(1) #, 1) )  /*gen a random number from  1 ──► arg. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
barnYard:   do j=1  for arg(1);       x=?();
do until y\==x | #<2;   y=?()
end   /*until*/
parse value   @.x  @.y     with     @.y  @.x
end     /*j*/;                                           return
/*──────────────────────────────────────────────────────────────────────────────────────*/
riffle:   \$A= subword(\$, 1, #%2);     \$B= subword(\$, #%2 + 1);   \$= /*split deck in half*/
do j=1  for max( words(\$A), words(\$B) );       \$= \$  word(\$A, j)   word(\$B, j)
end   /*j*/
\$= space(\$);   call build;                                 return
/*──────────────────────────────────────────────────────────────────────────────────────*/
overhand: parse arg pc .;  if pc==''  then pc= 1/5;   chunk= # * pc % 1;       \$B=
do while words(\$)\==0;    \$B= \$B subword(\$, 1, chunk); \$= subword(\$, chunk +1)
end   /*while*/
\$= space(\$B);               call build;                    return
/*──────────────────────────────────────────────────────────────────────────────────────*/
show:     _=@.1;        do j=2  for #-1;   _=_ @.j;   end /*j*/;           L = length(_)
say center( arg(1), L, '═');     say _;     say;           return  /*show deck*/
```

{{out|output}}

```═════════════════════════════════════════════════════════════════════════new deck══════════════════════════════════════════════════════════════════════════
♣A ♣2 ♣3 ♣4 ♣5 ♣6 ♣7 ♣8 ♣9 ♣t ♣J ♣Q ♣K ♦A ♦2 ♦3 ♦4 ♦5 ♦6 ♦7 ♦8 ♦9 ♦t ♦J ♦Q ♦K ♥A ♥2 ♥3 ♥4 ♥5 ♥6 ♥7 ♥8 ♥9 ♥t ♥J ♥Q ♥K ♠A ♠2 ♠3 ♠4 ♠5 ♠6 ♠7 ♠8 ♠9 ♠t ♠J ♠Q ♠K

══════════════════════════════════════════════════════════════════════riffle shuffle═══════════════════════════════════════════════════════════════════════
♣A ♥A ♣2 ♥2 ♣3 ♥3 ♣4 ♥4 ♣5 ♥5 ♣6 ♥6 ♣7 ♥7 ♣8 ♥8 ♣9 ♥9 ♣t ♥t ♣J ♥J ♣Q ♥Q ♣K ♥K ♦A ♠A ♦2 ♠2 ♦3 ♠3 ♦4 ♠4 ♦5 ♠5 ♦6 ♠6 ♦7 ♠7 ♦8 ♠8 ♦9 ♠9 ♦t ♠t ♦J ♠J ♦Q ♠Q ♦K ♠K

═════════════════════════════════════════════════════════════════════overhand shuffle══════════════════════════════════════════════════════════════════════
♣A ♣2 ♣3 ♣4 ♣5 ♣6 ♣7 ♣8 ♣9 ♣t ♣J ♣Q ♣K ♦A ♦2 ♦3 ♦4 ♦5 ♦6 ♦7 ♦8 ♦9 ♦t ♦J ♦Q ♦K ♥A ♥2 ♥3 ♥4 ♥5 ♥6 ♥7 ♥8 ♥9 ♥t ♥J ♥Q ♥K ♠A ♠2 ♠3 ♠4 ♠5 ♠6 ♠7 ♠8 ♠9 ♠t ♠J ♠Q ♠K

═════════════════════════════════════════════════════════════════════barn yard shuffle═════════════════════════════════════════════════════════════════════
♣A ♣2 ♣3 ♠2 ♥6 ♣6 ♣7 ♣8 ♦8 ♥8 ♣J ♣Q ♣K ♦A ♦2 ♦3 ♦4 ♥t ♦6 ♦7 ♠A ♦9 ♦t ♦J ♦Q ♠t ♥A ♥2 ♣t ♥4 ♠5 ♣5 ♦K ♥Q ♥9 ♦5 ♥J ♥3 ♥K ♣9 ♣4 ♠3 ♠4 ♠K ♠6 ♥7 ♠8 ♠9 ♠7 ♠J ♠Q ♥5

```

## Ruby

Two methods to solve the requirements, and a third one as bonus.

```Ruby

def riffle deck
left, right = deck.partition{rand(10).odd?}
new_deck    = []

# the condition below is true when both left and right stacks are empty
until ((left_card=left.pop).to_i + (right_card=right.shift).to_i).zero? do
new_deck << left_card  if left_card
new_deck << right_card if right_card
end

new_deck
end

def overhand deck
new_deck = []

until deck.empty? do
stack = deck[-rand(deck.size * 0.2), deck.size]
new_deck += stack
deck     -= stack
end

new_deck
end

def bonus deck
deck.sort { |a, b| Time.now.to_i % a <=> Time.now.to_i % b }
end

deck = [*1..20]

puts riffle(deck).inspect
puts overhand(deck).inspect
puts bonus(deck).inspect

```

## Tcl

```Tcl

proc riffle deck {
set length [llength \$deck]
for {set i 0} {\$i < \$length/2} { incr i} {
lappend temp [lindex \$deck \$i] [lindex \$deck [expr {\$length/2+\$i}]]}
set temp}
proc overhand deck {
set cut [expr {[llength \$deck] /5}]
for {set i \$cut} {\$i >-1} {incr i -1} {
lappend temp [lrange \$deck [expr {\$i *\$cut}] [expr {(\$i+1) *\$cut -1}] ]}
concat {*}\$temp}
puts [riffle [list 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52]]
puts [overhand [list 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52]]

```

## Visual Basic .NET

{{trans|C#}}

```vbnet
Imports System.Runtime.CompilerServices
Imports System.Text

Module Module1

Function AsString(Of T)(c As ICollection(Of T)) As String
Dim sb = New StringBuilder("[")
sb.Append(String.Join(", ", c))
Return sb.Append("]").ToString()
End Function

Private rand As New Random()

Function RiffleShuffle(Of T)(list As ICollection(Of T), flips As Integer) As List(Of T)
Dim newList As New List(Of T)(list)

For n = 1 To flips
'cut the deck at the middle +/- 10%, remove the second line of the formula for perfect cutting
Dim cutPoint As Integer = newList.Count / 2 + If(rand.Next(0, 2) = 0, -1, 1) * rand.Next(newList.Count * 0.1)

'split the deck
Dim left As New List(Of T)(newList.Take(cutPoint))
Dim right As New List(Of T)(newList.Skip(cutPoint))

newList.Clear()

While left.Count > 0 AndAlso right.Count > 0
'allow for imperfect riffling so that more than one card can come form the same side in a row
'biased towards the side with more cards
'remove the if And else And brackets for perfect riffling
If rand.NextDouble() >= left.Count / right.Count / 2 Then
right.RemoveAt(0)
Else
left.RemoveAt(0)
End If
End While

'if either hand is out of cards then flip all of the other hand to the shuffled deck
If left.Count > 0 Then
End If
If right.Count > 0 Then
End If
Next

Return newList
End Function

Function OverhandShuffle(Of T)(list As ICollection(Of T), passes As Integer) As List(Of T)
Dim mainHand As New List(Of T)(list)

For n = 1 To passes
Dim otherhand = New List(Of T)

While mainHand.Count > 0
'cut at up to 20% of the way through the deck
Dim cutSize = rand.Next(list.Count * 0.2) + 1

Dim temp = New List(Of T)

'grab the next cut up to the end of the cards left in the main hand
Dim i = 0
While i < cutSize AndAlso mainHand.Count > 0
mainHand.RemoveAt(0)
i = i + 1
End While

'add them to the cards in the other hand, sometimes to the front sometimes to the back
If rand.NextDouble() >= 0.1 Then
'front most of the time
otherhand = temp
Else
'end sometimes
End If
End While

'move the cards back to the main hand
mainHand = otherhand
Next

Return mainHand
End Function

Sub Main()
Dim list = New List(Of Integer)(Enumerable.Range(1, 20))
Console.WriteLine(list.AsString())
list = RiffleShuffle(list, 10)
Console.WriteLine(list.AsString())
Console.WriteLine()

list = New List(Of Integer)(Enumerable.Range(1, 20))
Console.WriteLine(list.AsString())
list = RiffleShuffle(list, 1)
Console.WriteLine(list.AsString())
Console.WriteLine()

list = New List(Of Integer)(Enumerable.Range(1, 20))
Console.WriteLine(list.AsString())
list = OverhandShuffle(list, 10)
Console.WriteLine(list.AsString())
Console.WriteLine()

list = New List(Of Integer)(Enumerable.Range(1, 20))
Console.WriteLine(list.AsString())
list = OverhandShuffle(list, 1)
Console.WriteLine(list.AsString())
Console.WriteLine()
End Sub

End Module
```

{{out}}

```txt
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[1, 5, 15, 8, 3, 7, 17, 12, 14, 6, 19, 18, 13, 16, 2, 20, 11, 10, 4, 9]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[1, 2, 12, 13, 14, 3, 15, 4, 5, 16, 17, 6, 7, 8, 9, 18, 10, 19, 20, 11]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[15, 16, 20, 14, 17, 9, 10, 5, 6, 3, 12, 18, 11, 4, 1, 2, 8, 13, 19, 7]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[19, 20, 15, 16, 17, 18, 13, 14, 10, 11, 12, 7, 8, 9, 4, 5, 6, 1, 2, 3]
```

## zkl

A much better shuffle is List's shuffle method.

```zkl
fcn riffle(deck){
len,N:=deck.len(),len/2;
newDeck:=N.pump(List,'wrap(n){ return(Void.Write,deck[n],deck[N+n]) });
if(len.isOdd) return(newDeck.append(deck[-1]));
newDeck
}
fcn overHand(deck){
len,N,piles:=deck.len(),(0.2*len).toInt(),(len.toFloat()/N).ceil().toInt();
piles.pump(List,'wrap(n){ deck[n*N,N] }).reverse().flatten()
}
```

```zkl
riffle(  [1..19].walk()).println();
overHand([1..19].walk()).println();
[1..19].walk().shuffle().println();
```

{{out}}

```txt

L(1,10,2,11,3,12,4,13,5,14,6,15,7,16,8,17,9,18,19)
L(19,16,17,18,13,14,15,10,11,12,7,8,9,4,5,6,1,2,3)
L(9,11,12,6,17,18,5,10,8,19,2,15,4,3,13,1,7,14,16)

```

```