Thread: Reverse an array

Hi i have 2 function for reversing arrays. One work and the other doesn't. can help me analyse why the first is wrong?

Code:

void reverse_array(int arr[], int length)
{
    int i,j;
    for(i=length; i>0; i--)
        for(j=0; j<length; j++)
        {    
            arr[j]=arr[i];
            printf("%d ", arr[j]);
        }
}

The above can't work.

Code:

void reverse_array(int arr[], int length)
{
    int i, temp;
    for (i=0; i<length/2; i++)
    {
        temp = arr[i];
        arr[i] = arr[length-i-1];
        arr[length-i-1] = temp;
    }
}

this works (why they use length/2 ??)

thanks!

Work through it by hand on a small array, say [1, 2, 3, 4, 5]. See what happens if you go stop at length/2 and what happens if you go all the way to length.

Originally Posted by anduril462

Work through it by hand on a small array, say [1, 2, 3, 4, 5]. See what happens if you go stop at length/2 and what happens if you go all the way to length.

Oh it is because if i swap till length, not length/2, array will still be 1,2,3,4,5?

In what kind of circumstances, should i use length instead of length/2?
what about the first code? i didnt use temp.

Originally Posted by wonderwall

Oh it is because if i swap till length, not length/2, array will still be 1,2,3,4,5?

Yes. You're working from both ends towards the middle (think of the relationship between i and length-i-1). If you swap length times, you swap, e.g., i = 0 with length-0-1, then when i is length-1, you swap that with length-(length-1)-1 = 0, effectively swapping everything back.

In what kind of circumstances, should i use length instead of length/2?

Not this circumstance. You will use length much more often than length/2, almost any time you need to access each element of the array (except swap, and maybe some search and sort methods).

what about the first code? i didnt use temp.

Your first code is horribly broken. The first time through your i loop, i = length (which is one element past the end of your array, so it is garbage data), your j loop assigns that garbage data to all the elements of the array.