# Card shuffles

Card shuffles is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
This task has been clarified. Its programming examples are in need of review to ensure that they still fit the requirements of the task.

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

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:

1. Split the deck into two piles
2. Merge the two piles by taking one card from the top of either pile in proportion to the number of cards remaining in the pile. To start with the probability for both piles will be 26/52 (50-50), then 25/51-26/51 etc etc as the riffle progresses.
3. The merged deck is now the new "shuffled" deck

One iteration of the overhand shuffle is defined as:

1. Take a group of consecutive cards from the top of the deck. For our purposes up to 20% of the deck seems like a good amount.
2. Place that group on top of a second pile
3. Repeat these steps until there are no cards remaining in the original deck
4. The second pile is now the new "shuffled" deck

Bonus

Implement other methods described in the Wikipedia article:   card shuffling.

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

## C

Translation of: 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 20int 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;}`
Output:
```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;} `
Output:
```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#

Translation of: 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) {                        newList.Add(right.First());                        right.RemoveAt(0);                    }                    else {                        newList.Add(left.First());                        left.RemoveAt(0);                    }                }                 //if either hand is out of cards then flip all of the other hand to the shuffled deck                if (left.Count > 0) newList.AddRange(left);                if (right.Count > 0) newList.AddRange(right);            }             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++) {                        temp.Add(mainHand.First());                        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                        temp.AddRange(otherHand);                        otherHand = temp;                    }                    else {                        //end sometimes                        otherHand.AddRange(temp);                    }                }                 //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();        }    }}`
Output:
```[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

Translation of: 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[0];                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);}`
Output:
```[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

Translation of: 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[0]                deck1 = deck1[1:]                el2 := deck2[0]                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)}`
Output:

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

Generally, this task should be accomplished in J using `({~ [email protected]#)`. Here we take an approach that's more comparable with the other examples on this page.
`NB. overhand cutoverhand=: (\: [: +/\ %@%:@# > # [email protected]# 0:)@]^:[ NB. Gilbert–Shannon–Reeds modelriffle=:  (({.~+/)`([email protected]])`([email protected]]#inv (}.~+/))} [email protected](#&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:

`   ([: +/\ %@%:@# > # [email protected]# 0:) i.100 0 0 0 0 0 0 0 1 1   ([: +/\ %@%:@# > # [email protected]# 0:) i.101 1 2 2 2 3 3 3 3 3   ([: +/\ %@%:@# > # [email protected]# 0:) i.100 1 1 2 3 3 3 3 4 5   ([: +/\ %@%:@# > # [email protected]# 0:) i.100 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.200 1 2 3 4 5 6 7 8 13 14 9 15 16 17 10 18 11 12 19   10 riffle i.206 10 13 8 2 14 15 9 19 3 18 16 11 1 12 17 5 4 0 7   1 overhand i.2017 18 19 13 14 15 16 4 5 6 7 8 9 10 11 12 0 1 2 3   10 overhand i.2015 11 2 4 5 12 16 10 17 19 9 8 6 13 3 18 7 1 0 14`

## Java

Works with: Java version 1.5+
`import java.util.Arrays;import java.util.Collections;import java.util.LinkedList;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){		LinkedList<T> newList = new LinkedList<T>(); 		newList.addAll(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.size() / 2 				+ (rand.nextBoolean() ? -1 : 1 ) * rand.nextInt((int)(newList.size() * 0.1)); 			//split the deck			List<T> left = new LinkedList<T>();			left.addAll(newList.subList(0, cutPoint));			List<T> right = new LinkedList<T>();			right.addAll(newList.subList(cutPoint, newList.size())); 			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){					newList.add(right.remove(0));				}else{					newList.add(left.remove(0));				}			} 			//if either hand is out of cards then flip all of the other hand to the shuffled deck			if(left.size() > 0) newList.addAll(left);			if(right.size() > 0) newList.addAll(right);		}		return newList;	} 	public static <T> LinkedList<T> overhandShuffle(List<T> list, int passes){		LinkedList<T> mainHand = new LinkedList<T>(); 		mainHand.addAll(list);		for(int n = 0; n < passes; n++){			LinkedList<T> otherHand = new LinkedList<T>(); 			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; 				LinkedList<T> temp = new LinkedList<T>(); 				//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++){					temp.add(mainHand.remove());				} 				//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.addAll(0, temp);				}else{					//end sometimes					otherHand.addAll(temp);				}			} 			//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");	}}`
Output:
```[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 version 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 listend 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 listend 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")`
Output:
```# 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.Randomimport 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) {                pile.add(deck1.removeAt(0))                 pile.add(deck2.removeAt(0))            }            else {                pile.add(deck1.removeAt(deck1.lastIndex))                           pile.add(deck2.removeAt(deck2.lastIndex))            }        }        // add any remaining cards to the pile and reverse it        if (deck1.size > 0) pile.addAll(deck1)        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))            pile2.addAll(0, pile.take(cards))            repeat(cards) { pile.removeAt(0) }        }        pile.addAll(pile2)        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 orderfunction 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 cardsend -- 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 tablefunction 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 deckend -- Perform an overhand shuffle on a deck and return it as a new tablefunction 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 newDeckend -- Main proceduremath.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)`
Output:
```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
```

## Modula-2

`MODULE CardShuffles;FROM FormatString IMPORT FormatString;FROM RandomNumbers IMPORT Random;FROM Terminal IMPORT WriteString,WriteLn,ReadChar; 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    ENDEND 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    ENDEND 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[0]                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    ENDEND 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;     ReadChar;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])`
Output:
```%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";`
Output:
```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 resend 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 resend function -- to shorten the output, all 2..7 have been removed from the deckconstant 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)))`
Output:
```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)) )`
Output:
```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

Translation of: 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 numsprint riffleShuffle(nums, 10)print print "Riffle shuffle"nums = [x+1 for x in range(21)]print numsprint riffleShuffle(nums, 1)print print "Overhand shuffle"nums = [x+1 for x in range(21)]print numsprint overhandShuffle(nums, 10)print print "Overhand shuffle"nums = [x+1 for x in range(21)]print numsprint overhandShuffle(nums, 1)print print "Library shuffle"nums = [x+1 for x in range(21)]print numsrandom.shuffle(nums)print numsprint`
Output:
```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)))`
Output:

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:     [email protected].1;        do j=2  for #-1;   _=_ @.j;   end /*j*/;           L = length(_)          say center( arg(1), L, '═');     say _;     say;           return  /*show deck*/`
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.

` 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_deckend 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_deckend def bonus deck  deck.sort { |a, b| Time.now.to_i % a <=> Time.now.to_i % b }end deck = [*1..20] puts riffle(deck).inspectputs overhand(deck).inspectputs bonus(deck).inspect `

## 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

Translation of: C#
`Imports System.Runtime.CompilerServicesImports System.Text Module Module1     <Extension()>    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                    newList.Add(right.First())                    right.RemoveAt(0)                Else                    newList.Add(left.First())                    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                newList.AddRange(left)            End If            If right.Count > 0 Then                newList.AddRange(right)            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                    temp.Add(mainHand.First())                    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                    temp.AddRange(otherhand)                    otherhand = temp                Else                    'end sometimes                    otherhand.AddRange(temp)                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`
Output:
```[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.

`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()}`
`riffle(  [1..19].walk()).println();overHand([1..19].walk()).println();[1..19].walk().shuffle().println();`
Output:
```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)
```