Copy array into another array many times in java

Question

I wanted to copy array from one Integer array into another array many times.

int a[6]={1,2,3};
int b[]=new int[12];
for(int i=0;i<12;i++)
{
b[i]=a[i];
System.out.println(b[i]);
}

I wanted output like this:

1,2,3,1,2,3,1,2,3,1,2,3

How should i copy all element from a[] to b[] for as many times i want.

You are aware that this code does not work yet? You are accessing the index of a[6] for example and that exceeds the size of the array. Also your array initialization is not correct. Now to prepare a good answer: Do you really need the println output? This will prevent batch copies. — Nitram
– Nitram, Commented Jun 25, 2015 at 12:53
Please have a look at my answer. This may clarify why I was asking. — Nitram
– Nitram, Commented Jun 25, 2015 at 13:04
int a[6]={1,2,3}; int b[]=new int[12]; for(int i=0;i<12;i++) { b[i]=a[i]; System.out.println(b[i]); } I wanted output like this 1,2,3,1,2,3,1,2,3,1,2,3 and i dont know how to do this Sir. thats why I was asking. And The initialization was correct. — Pramod yadav
– Pramod yadav, Commented Jun 25, 2015 at 13:14
You got two answers. If you want the println output Erans answer is the way to go. That doesn't work with my proposal. If you want to transfer the data from array a to b both answer work. — Nitram
– Nitram, Commented Jun 25, 2015 at 13:17

Eran · Accepted Answer · 2015-06-25 12:50:57Z

6

You can use modulus operator :

for(int i=0;i<12;i++)
{
    b[i]=a[i%a.length];
    System.out.println(b[i]);
}

i%a.length will iterate repeatedly from 0 to a.length-1.

answered Jun 25, 2015 at 12:50

Eran

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2 Comments

Hindol Over a year ago

The simplest and closest to OP's approach. +1

mstelz Over a year ago

if you pull a.length out of the loop it will also decrease that latency which could matter if this is being done at high enough iterations.

Nitram · Accepted Answer · 2015-06-25 13:03:29Z

The most efficient way to do what you want (that I can think of) is to copy batches.

So we got our two input arrays:

int a[] = {1, 2, 3};
int b[] = new int[12];

What we want to do is to copy the first array into the second until again and again in sequence until the second array is filled.

The first batch is simple. Just copy the first array once into the second.

System.arraycopy(a, 0, b, 0, a.length);

And now we copy what we append the data in the second array to the free fields of the second array.

int usedSize = a.length; 
while (usedSize < b.length) {
    System.arraycopy(b, 0, b, usedSize, Math.min(b.length - usedSize, usedSize);
    usedSize *= 2; 
}

Your array b will change in the following way:

{1,2,3,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x}
{1,2,3,1,2,3,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x}
{1,2,3,1,2,3,1,2,3,1,2,3,x,x,x,x,x,x,x,x,x,x,x,x}
{1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,1,2,3}

So it reduced the copy step required and will work very fast even for big arrays. System.arraycopy is in the end the fastest way to copy array contents around.

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