How do I copy a 2 Dimensional array in Java

Question

I need to make a copy of a fairly large 2 dimensional array for a project I am working on  I have two 2D arrays   int    current  int    old    I also have two methods to do the copying  I need to copy the array because current is regularly being updated   public void old      old current     and  public void keepold      current old     However  this does not work  If I were to call old  make an update on current  and then call keepold  current is not equal to what it was originally  Why would this be   Thanks

User · Answer

public  static byte     arrayCopy byte     arr       if arr  null           int     arrCopy   new int arr length              System arraycopy arr  0  arrCopy  0  arr length           return arrCopy       else   return new int

User · Answer

current old or old current makes the two array refer to the same thing  so if you subsequently modify current  old will be modified too  To copy the content of an array to another array  use the for loop  for int i 0  i lt old length  i      for int j 0  j lt old i  length  j        old i  j  current i  j     PS  For a one-dimensional array  you can avoid creating your own for loop by using Arrays copyOf

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I am using this function   public static int     copy final int     array        if  array    null            final int     copy   new int array length              for  int i   0  i  lt  array length  i                  final int   row   array i                copy i    new int row length               System arraycopy row  0  copy i   0  row length                      return copy             return null      The big advantage of this approach is that it can also copy arrays that don t have the same row count such as   final int     array   new int         5  3  6      1

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You can also do as follows   public static int     copy int     src        int     dst   new int src length         for  int i   0  i  lt  src length  i              dst i    Arrays copyOf src i   src i  length             return dst

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I solved it writing a simple function to copy multidimensional int arrays using System arraycopy  public static void arrayCopy int     aSource  int     aDestination        for  int i   0  i  lt  aSource length  i              System arraycopy aSource i   0  aDestination i   0  aSource i  length             or actually I improved it for for my use case          Clones the provided array         param src     return a new clone of the provided array     public static int     cloneArray int     src        int length   src length      int     target   new int length  src 0  length       for  int i   0  i  lt  length  i              System arraycopy src i   0  target i   0  src i  length             return target

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current   old     Assignment operations doesnot copy elements of one array to another  You are just making the current matrix refer to the old matrix  You need to do a member wise copy

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you could also use a for each loop  int r 0  for int   array old       int c 0      for int element array          current r  c    array      r        Or  int c 0  for int array   old       System arraycopy array 0 current c    0 array length       However something like   int c 0  for int   array old       current c    array      would be wrong as it would just copy references of the subarrays of old and changes made to old would be reflected in current

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Here s how you can do it by using loops   public static int     makeCopy int     array       b new int array length          for int row 0  row lt array length    row           b row  new int array row  length           for int col 0  col lt b row  length    col               b row  col  array row  col                       return b

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Since Java 8  using the streams API   int     copy   Arrays stream matrix  map int    clone  toArray int      new

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Arrays in java are objects  and all objects are passed by reference  In order to really  copy  an array  instead of creating another name for an array  you have to go and create a new array and copy over all the values  Note that System arrayCopy will copy 1-dimensional arrays fully  but NOT 2-dimensional arrays  The reason is that a 2D array is in fact a 1D array of 1D arrays  and arrayCopy copies over pointers to the same internal 1D arrays

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Using java 8 this can be done with   int     destination Arrays stream source                       map a - gt   Arrays copyOf a  a length                        toArray int      new

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You can give below code a try   public void multiArrayCopy int     source int     destination   destination source clone       Hope it works

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Clones the provided array         param src     return a new clone of the provided array     public static int     cloneArray int     src        int length   src length      int     target   new int length  src 0  length       for  int i   0  i  lt  length  i              System arraycopy src i   0  target i   0  src i  length             return target       Is it possible to modify this code to support n-dimensional arrays of Objects   You would need to support arbitrary lengths of arrays and check if the src and destination have the same dimensions  and you would also need to copy each element of each array recursively  in case the Object was also an array   It s been a while since I posted this  but I found a nice example of one way to create an n-dimensional array class  The class takes zero or more integers in the constructor  specifying the respective size of each dimension  The class uses an underlying flat array Object   and calculates the index of each element using the dimensions and an array of multipliers   This is how arrays are done in the C programming language     Copying an instance of NDimensionalArray would be as easy as copying any other 2D array  though you need to assert that each NDimensionalArray object has equal dimensions  This is probably the easiest way to do it  since there is no recursion  and this makes representation and access much simpler

[java] How do I copy a 2 Dimensional array in Java?

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