Random shuffling of an array

Question

I need to randomly shuffle the following Array   int   solutionArray    1  2  3  4  5  6  6  5  4  3  2  1     Is there any function to do that

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Using Guava s Ints asList   it is as simple as   Collections shuffle Ints asList array

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This is knuth shuffle algorithm   public class Knuth            this class should not be instantiated     private Knuth                       Rearranges an array of objects in uniformly random order         under the assumption that  lt tt gt Math random   lt  tt gt  generates independent        and uniformly distributed numbers between 0 and 1           param a the array to be shuffled             public static void shuffle Object   a            int n   a length          for  int i   0  i  lt  n  i                     choose index uniformly in  i  n-1              int r   i    int   Math random      n - i                Object swap   a r               a r    a i               a i    swap                                  Reads in a sequence of strings from standard input  shuffles        them  and prints out the results              public static void main String   args                read in the data         String   a   StdIn readAllStrings                shuffle the array         Knuth shuffle a               print results          for  int i   0  i  lt  a length  i                StdOut println a i

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Collections class has an efficient method for shuffling  that can be copied  so as not to depend on it          Usage        int   array    1  2  3         Util shuffle array       public class Util        private static Random random                  Code from method java util Collections shuffle                public static void shuffle int   array            if  random    null  random   new Random            int count   array length          for  int i   count  i  gt  1  i--                swap array  i - 1  random nextInt i                         private static void swap int   array  int i  int j            int temp   array i           array i    array j           array j    temp

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Look at the Collections class  specifically shuffle

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Box from int   to Integer   Wrap an array into a list with the Arrays asList method Shuffle with Collections shuffle method  int   solutionArray     1  2  3  4  5  6  6  5  4  3  2  1     Integer   boxed   Arrays stream solutionArray  boxed   toArray Integer    new   Collections shuffle Arrays asList boxed     System out println Arrays toString boxed        1  5  5  4  2  6  1  3  3  4  2  6

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import java util ArrayList  import java util Random  public class shuffle       public static void main String   args            int a       1 2 3 4 5 6 7 8 9            ArrayList b   new ArrayList           int i 0 q 0         Random rand   new Random            while a length  b size                        int l   rand nextInt a length     this is one option to that but has a flaw on 0              if a l    0                                   b add a l                     a l  0                                                    this works for every no                   if   b contains a l                                           b add a l                                             for  int j   0  j  lt b size    j                    System out println b get j                               System out println b

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A simple solution for Groovy   solutionArray sort  new Random   nextInt       This will sort all elements of the array list randomly which archives the desired result of shuffling all elements

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Here is a working and efficient Fisher   Yates shuffle array function   private static void shuffleArray int   array        int index      Random random   new Random        for  int i   array length - 1  i  gt  0  i--                index   random nextInt i   1           if  index    i                        array index     array i               array i     array index               array index     array i                       or  private static void shuffleArray int   array        int index  temp      Random random   new Random        for  int i   array length - 1  i  gt  0  i--                index   random nextInt i   1           temp   array index           array index    array i           array i    temp

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You have a couple options here  A list is a bit different than an array when it comes to shuffling   As you can see below  an array is faster than a list  and a primitive array is faster than an object array   Sample Durations  List lt Integer gt  Shuffle  43133ns     Integer   Shuffle  31884ns         int   Shuffle  25377ns     Below  are three different implementations of a shuffle  You should only use Collections shuffle if you are dealing with a collection  There is no need to wrap your array into a collection just to sort it  The methods below are very simple to implement   ShuffleUtil Class  import java lang reflect Array  import java util     public class ShuffleUtil lt T gt        private static final int   EMPTY INT ARRAY   new int 0       private static final int SHUFFLE THRESHOLD   5       private static Random rand    Main Method      public static void main String   args            List lt Integer gt  list   null          Integer   arr   null          int   iarr   null           long start   0          int cycles   1000          int n   1000              Shuffle List lt Integer gt          start   System nanoTime            list   range n           for  int i   0  i  lt  cycles  i                  ShuffleUtil shuffle list                     System out printf   22s   dns n    List lt Integer gt  Shuffle    System nanoTime   - start    cycles               Shuffle Integer           start   System nanoTime            arr   toArray list           for  int i   0  i  lt  cycles  i                  ShuffleUtil shuffle arr                     System out printf   22s   dns n    Integer   Shuffle    System nanoTime   - start    cycles               Shuffle int           start   System nanoTime            iarr   toPrimitive arr           for  int i   0  i  lt  cycles  i                  ShuffleUtil shuffle iarr                     System out printf   22s   dns n    int   Shuffle    System nanoTime   - start    cycles           Shuffling a Generic List                                                                                 Shuffle List lt T gt   java lang Collections                                                                               SuppressWarnings  unchecked       public static  lt T gt  void shuffle List lt T gt  list            if  rand    null                rand   new Random                      int size   list size            if  size  lt  SHUFFLE THRESHOLD    list instanceof RandomAccess                for  int i   size  i  gt  1  i--                    swap list  i - 1  rand nextInt i                            else               Object arr     list toArray                 for  int i   size  i  gt  1  i--                    swap arr  i - 1  rand nextInt i                               ListIterator lt T gt  it   list listIterator                int i   0               while  it hasNext                      it next                    it set  T  arr i                                         public static  lt T gt  void swap List lt T gt  list  int i  int j            final List lt T gt  l   list          l set i  l set j  l get i                public static  lt T gt  List lt T gt  shuffled List lt T gt  list            List lt T gt  copy   copyList list           shuffle copy           return copy          Shuffling a Generic Array                                                                                 Shuffle T                                                                               public static  lt T gt  void shuffle T   arr            if  rand    null                rand   new Random                       for  int i   arr length - 1  i  gt  0  i--                swap arr  i  rand nextInt i   1                         public static  lt T gt  void swap T   arr  int i  int j            T tmp   arr i           arr i    arr j           arr j    tmp             public static  lt T gt  T   shuffled T   arr            T   copy   Arrays copyOf arr  arr length           shuffle copy           return copy          Shuffling a Primitive Array                                                                                 Shuffle int                                                                               public static  lt T gt  void shuffle int   arr            if  rand    null                rand   new Random                       for  int i   arr length - 1  i  gt  0  i--                swap arr  i  rand nextInt i   1                         public static  lt T gt  void swap int   arr  int i  int j            int tmp   arr i           arr i    arr j           arr j    tmp             public static int   shuffled int   arr            int   copy   Arrays copyOf arr  arr length           shuffle copy           return copy          Utility Methods  Simple utility methods to copy and convert arrays to lists and vice-versa                                                                                  Utility methods                                                                             protected static  lt T gt  List lt T gt  copyList List lt T gt  list            List lt T gt  copy   new ArrayList lt T gt  list size             for  T item   list                copy add item                     return copy             protected static int   toPrimitive Integer   array            if  array    null                return null            else if  array length    0                return EMPTY INT ARRAY                    final int   result   new int array length           for  int i   0  i  lt  array length  i                  result i    array i  intValue                      return result             protected static Integer   toArray List lt Integer gt  list            return toArray list  Integer class              protected static  lt T gt  T   toArray List lt T gt  list  Class lt T gt  clazz             SuppressWarnings  unchecked           final T   arr   list toArray  T    Array newInstance clazz  list size              return arr          Range Class  Generates a range of values  similar to Python s range function                                                                                  Range class for generating a range of values                                                                              protected static List lt Integer gt  range int n            return toList new Range n   new ArrayList lt Integer gt                 protected static  lt T gt  List lt T gt  toList Iterable lt T gt  iterable            return toList iterable  new ArrayList lt T gt                 protected static  lt T gt  List lt T gt  toList Iterable lt T gt  iterable  List lt T gt  destination            addAll destination  iterable iterator              return destination             protected static  lt T gt  void addAll Collection lt T gt  collection  Iterator lt T gt  iterator            while  iterator hasNext                  collection add iterator next                          private static class Range implements Iterable lt Integer gt            private int start          private int stop          private int step           private Range int n                this 0  n  1                      private Range int start  int stop                this start  stop  1                      private Range int start  int stop  int step                this start   start              this stop   stop              this step   step                      Override         public Iterator lt Integer gt  iterator                 final int min   start              final int max   stop   step               return new Iterator lt Integer gt                      private int current   min                    Override                 public boolean hasNext                         return current  lt  max                                      Override                 public Integer next                         if  hasNext                              return current     step                        else                           throw new NoSuchElementException  Range reached the end                                                              Override                 public void remove                         throw new UnsupportedOperationException  Can t remove values from a Range

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Using Collections to shuffle an array of primitive types is a bit of an overkill     It is simple enough to implement the function yourself  using for example the Fisher   Yates shuffle   import java util    import java util concurrent ThreadLocalRandom   class Test     public static void main String args            int   solutionArray     1  2  3  4  5  6  16  15  14  13  12  11         shuffleArray solutionArray       for  int i   0  i  lt  solutionArray length  i                System out print solutionArray i                    System out println              Implementing Fisher   Yates shuffle   static void shuffleArray int   ar             If running on Java 6 or older  use  new Random    on RHS here     Random rnd   ThreadLocalRandom current        for  int i   ar length - 1  i  gt  0  i--              int index   rnd nextInt i   1            Simple swap       int a   ar index         ar index    ar i         ar i    a

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similar without using swap b          Random r   new Random        int n   solutionArray length      List lt Integer gt  arr    Arrays stream solutionArray  boxed   collect Collectors toList         for  int i   0  i  lt  n-1  i              solutionArray i    arr remove  r nextInt arr size         randomize base on size           solutionArray n-1    arr get 0

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Here is a Generics version for arrays   import java util Random   public class Shuffle lt T gt         private final Random rnd       public Shuffle             rnd   new Random                          Fisher   Yates shuffle              public void shuffle T   ar            for  int i   ar length - 1  i  gt  0  i--                int index   rnd nextInt i   1               T a   ar index               ar index    ar i               ar i    a                      Considering that ArrayList is basically just an array  it may be advisable to work with an ArrayList instead of the explicit array and use Collections shuffle    Performance tests however  do not show any significant difference between the above and Collections sort     Shuffe lt Integer gt  shuffle      performance  576084 shuffles per second Collections shuffle ArrayList lt Integer gt   performance  629400 shuffles per second MathArrays shuffle int    performance  53062 shuffles per second   The Apache Commons implementation MathArrays shuffle is limited to int   and the performance penalty is likely due to the random number generator being used

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You can use java 8 now   Collections addAll list  arr   Collections shuffle list   cardsList toArray arr

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public class ShuffleArray   public static void shuffleArray int   a        int n   a length      Random random   new Random        random nextInt        for  int i   0  i  lt  n  i              int change   i   random nextInt n - i           swap a  i  change            private static void swap int   a  int i  int change        int helper   a i       a i    a change       a change    helper     public static void main String   args        int   a   new int     1  2  3  4  5  6  6  5  4  3  2  1        shuffleArray a       for  int i   a            System out println i

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The most simple solution for this Random Shuffling in an Array   String location       delhi   banglore   mathura   lucknow   chandigarh   mumbai    int index  String temp  Random random   new Random    for int i 1 i lt location length i          index   random nextInt i 1       temp   location index       location index    location i       location i    temp      System out println  Location Based On Random Values    location i

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Here is a solution using Apache Commons Math 3 x  for int   arrays only    MathArrays shuffle array     http   commons apache org proper commons-math javadocs api-3 6 1 org apache commons math3 util MathArrays html shuffle int     Alternatively  Apache Commons Lang 3 6 introduced new shuffle methods to the ArrayUtils class  for objects and any primitive type    ArrayUtils shuffle array     http   commons apache org proper commons-lang javadocs api-release org apache commons lang3 ArrayUtils html shuffle-int A-

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Here is a simple way using an ArrayList   List lt Integer gt  solution   new ArrayList lt  gt     for  int i   1  i  lt   6  i          solution add i     Collections shuffle solution

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The following code will achieve a random ordering on the array       Shuffle the elements in the array Collections shuffle Arrays asList array      from  http   www programcreek com 2012 02 java-method-to-shuffle-an-int-array-with-random-order

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One of the solution is using the permutation to pre-compute all the permutations and stored in the ArrayList  Java 8 introduced a new method  ints    in the java util Random class  The ints   method returns an unlimited stream of pseudorandom int values  You can limit the random numbers between a specified range by providing the minimum and the maximum values   Random genRandom   new Random    int num   genRandom nextInt arr length     With the help of generating the random number  You can iterate through the loop and swap with the current index with the random number   That s how you can generate a random number with O 1  space complexity

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Here s another way to shuffle a list  public List lt Integer gt  shuffleArray List lt Integer gt  a    List lt Integer gt  b   new ArrayList lt Integer gt         while  a size      0            int arrayIndex    int   Math random      a size              b add a get arrayIndex            a remove a get arrayIndex              return b      Pick a random number from the original list and save it in another list Then remove the number from the original list The size of the original list will keep decreasing by one until all elements are moved to the new list

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Using ArrayList lt Integer gt  can help you solving the problem of shuffling without applying much of logic and consuming less time  Here is what I suggest   ArrayList lt Integer gt  x   new ArrayList lt Integer gt     for int i 1  i lt  add length    i          x add i     Collections shuffle x

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Using the Random Class    public static void randomizeArray int   arr           Random rGenerator   new Random       Create an instance of the random class        for  int i  0  i lt  arr length i                   Swap the positions               int rPosition   rGenerator nextInt arr length      Generates an integer within the range  Any number from 0 - arr length            int temp   arr i      variable temp saves the value of the current array index            arr i    arr rPosition       array at the current position  i  get the value of the random generated            arr rPosition    temp     the array at the position of random generated gets the value of temp                 for int i   0  i lt arr length  i                System out print arr i      Prints out the array

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I m weighing in on this very popular question because nobody has written a shuffle-copy version  Style is borrowed heavily from Arrays java  because who isn t pillaging Java technology these days  Generic and int implementations included                Shuffles elements from   code original  into a newly created array               param original the original array        return the new  shuffled array        throws NullPointerException if   code original    null             SuppressWarnings  unchecked      public static  lt T gt  T   shuffledCopy T   original          int originalLength   original length     For exception priority compatibility        Random random   new Random          T   result    T    Array newInstance original getClass   getComponentType    originalLength          for  int i   0  i  lt  originalLength  i               int j   random nextInt i 1            result i    result j            result j    original i                  return result                      Shuffles elements from   code original  into a newly created array               param original the original array        return the new  shuffled array        throws NullPointerException if   code original    null            public static int   shuffledCopy int   original          int originalLength   original length        Random random   new Random          int   result   new int originalLength          for  int i   0  i  lt  originalLength  i               int j   random nextInt i 1            result i    result j            result j    original i                  return result

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Here is a complete solution using the Collections shuffle approach   public static void shuffleArray int   array      List lt Integer gt  list   new ArrayList lt  gt       for  int i   array        list add i          Collections shuffle list      for  int i   0  i  lt  list size    i          array i    list get i               Note that it suffers due to Java s inability to smoothly translate between int   and Integer    and thus int   and List lt Integer gt

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Without Random solution      static void randomArrTimest int   some           long startTime   System currentTimeMillis            for  int i   0  i  lt  some length  i                  long indexToSwap   startTime  i 1               long tmp   some  int  indexToSwap               some  int  indexToSwap    some i               some i     int  tmp                    System out println Arrays toString some

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I saw some miss information in some answers so i decided to add a new one   Java collections Arrays asList takes var-arg of type T  T       If you pass a primitive array  int array   asList method will infer and generate a List lt int   gt   which is a one element list  the one element is the primitive array   if you shuffle this one element list  it won t change any thing   So  first you have to convert you primitive array to Wrapper object array  for this you can use ArrayUtils toObject method from apache commons lang  then pass the generated array to a List and finaly shuffle that     int   intArr    1 2 3     List lt Integer gt  integerList   Arrays asList ArrayUtils toObject array      Collections shuffle integerList       now  elements in integerList are shuffled

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There is another way also  not post yet    that way  send many object types diferentes public anotherWayToReciveParameter Object    objects          ready with array     final int length  objects length      System out println length         for ready same list     Arrays asList objects       that way more easy  depended of the context

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Random rnd   new Random    for  int i   ar length - 1  i  gt  0  i--      int index   rnd nextInt i   1        Simple swap   int a   ar index     ar index    ar i     ar i    a      By the way  I ve noticed that this code returns a ar length - 1 number of elements  so if your array has 5 elements  the new shuffled array will have 4 elements  This happens because the for loop says i gt 0  If you change to i gt  0  you get all elements shuffled

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Simplest code to shuffle   import java util    public class ch       public static void main String args                  Scanner sc new Scanner System in           ArrayList lt Integer gt  l new ArrayList lt Integer gt  10           for int i 0 i lt 10 i                l add sc nextInt             Collections shuffle l           for int j 0 j lt 10 j                System out println l get j

[java] Random shuffling of an array

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