如何在Java中制作二维数组的副本？

Question

Assuming I have a 2-D array as follow:

double[][] a={{1,0,0},{0,0,1},{0,1,0}};

I need to use this 'a' in a loop, each time as an input of a method. According to the output of the method, one element of this 2-D array may change. for example:

double [][] new_a=a;
new_a[0][0]=0;

I want to store the new-a in a Hash-map:

HashMap<Integer,double[][]> Store=new HashMap<Integer,double[][]>();
Store.put(size.Store(),new_a);

next time in the loop I need the original 'a' though. I don't know how I can make a copy from 2-D array 'a' in order to use the original one each time in the loop and store the new one in the Hash-map.

When I coded like above it changes the original 'a' as well and when I want to store in 'Store' it replaces new_a for all previous stored arrays.

I wonder if you can help me with this issue? Thanks.

Answer 1

Make sure you are defining double [][] new_a = a and HashMap<Integer,double[][]> Store=new HashMap<Integer,double[][]>() outside of the loop. I think that may be your problem that you are redefining everything in the loop. Can you show an extended version of your code to see the loop itself?

Answer 2

Two methods. 2nd is faster, should be O(n), I beleave.

for(int i=0; i<old.length; i++)
  for(int j=0; j<old[i].length; j++)
    old[i][j]=current[i][j];

or

 int[][] src //Old one you want a copy of
    int length = src.length;
        int[][] target = new int[length][src[0].length];
        for (int i = 0; i < length; i++) {
            System.arraycopy(src[i], 0, target[i], 0, src[i].length);
        }