modifying the copied 2d array in java

Question

I am trying to copy a 2d array of ints to another,such that the original remains unaffected when copied array is modified

 int[][] t3 = new int[][]{ {1,2,3}, {0,4,6}, {7,8,9} }; show(t3); int[][] t4 = new int[t3.length][t3[0].length]; System.arraycopy(t3, 0,t4 ,0, t3.length-1); show(t4); t3[0][0]=99; show(t3); show(t4);

However,the original gets modified here

t3 1 2 3 0 4 6 7 8 9 t4 1 2 3 0 4 6 0 0 0 t3 modified 99 2 3 0 4 6 7 8 9 t4 99 2 3 0 4 6 0 0 0

I tried clone() ,still the behaviour is the same

Why is this? Any idea how to keep the original unmodified?

Already answered question -- stackoverflow.com/questions/6101684/system-arraycopy-question — Joe2013
– Joe2013, Commented Mar 11, 2013 at 8:40
Already answered. stackoverflow.com/questions/6101684/system-arraycopy-question — Joe2013
– Joe2013, Commented Mar 11, 2013 at 8:42

msi · Accepted Answer · 2013-03-11 08:41:47Z

You are just coping 1d array references to your new array t4. That is why your reference t4 still points to your old 1d arrays.

You should use a loop to replace 'deeper' arrays:

for (int i = 0; i < t3.length; i++) { System.arraycopy(t3[i], 0, t4[i], 0, t3[0].length); }

Achintya Jha · Accepted Answer · 2013-03-11 08:41:00Z

1

for(int i = 0; i < t3.length; i++) t4[i] = t3[i].clone();

Try this:

answered Mar 11, 2013 at 8:41

Achintya Jha

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Comments

Jean Logeart · Accepted Answer · 2013-03-11 08:41:51Z

You need to copy each array of your 2d array.

Using Arrays.copyOf(int[], int):

int[][] t4 = new int[t3.length][]; for(int i = 0 ; i < t3.length ; ++i) { t4[i] = Arrays.copyOf(t3[i], t3[i].length); } show(t4);

Ahmed Nabil · Accepted Answer · 2013-03-11 08:51:20Z

Just use this method

public static int[][] produceAcopy2DArray(int[][] original) { int [][] copied = new int[original.length][]; for(int i = 0; i < original.length; i++) copied[i] = original[i].clone(); return copied; }

Then call it where you like as

t4 = produceAcopy2DArray(t3);

Then you will keep the original unmodified as you need

Kevin Bowersox · Accepted Answer · 2013-03-11 08:53:32Z

The copy utilities in Java will not copy a multidimensional array. You must iterate over the array and assign the values into a new array.

 public static void main(String[] args) { int[][] t3 = new int[][] { { 1, 2, 3 }, { 0, 4, 6 }, { 7, 8, 9 } }; int[][] t4 = new int[t3.length][t3[0].length]; for (int x = 0; x < t3.length - 1; x++) { for (int y = 0; y < t3[x].length - 1; y++) { t4[x][y] = t3[x][y]; } } t4[0][0] = 99; System.out.println(t3[0][0]); //Prints 1 System.out.println(t4[0][0]); //Prints 99 }

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