Reverse an array

This is a discussion on Reverse an array within the C Programming forums, part of the General Programming Boards category; Hi i have 2 function for reversing arrays. One work and the other doesn't. can help me analyse why the ...

  1. #1
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    Red face 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!

  2. #2
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    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.

  3. #3
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    Quote Originally Posted by anduril462 View Post
    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.

  4. #4
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    Quote Originally Posted by wonderwall View Post
    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.

  5. #5
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    What the first function does is overwrite every value with the value from every other position in the array. It looks to me like a coding-first, think-later attempt.
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