How can I reverse an array using pointers?

Question

I'm trying to write a function which reverses an array.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void * reverse(void * array, int length, size_t size) {
    void * temp = malloc(size);
    int s = 0;          // start
    int e = length - 1; // end
    const int halp = length / 2;
    for (; s < halp; s++) {
        memcpy(temp, (array + s), size);
        memcpy((array + (s * size)), (array + (e * size)), size);
        memcpy((array + (e * size)), temp, size);
        e--;
    }
    free(temp);
    return array;
}

int main(int argc, char * argv[]) {
    int array[] = {0, 1, 2, 3, 4};
    for (int i = 0; i < 5; i++) {
        printf("array[%d]: %d\n", i, array[i]);
    }
    printf("------------------------------------------\n");
    reverse(array, 5, sizeof(int));
    printf("------------------------------------------\n");
    for (int i = 0; i < 5; i++) {
        printf("array[%d]: %d\n", i, array[i]);
    }
}

Result:

array[0]: 0
array[1]: 1
array[2]: 2
array[3]: 3
array[4]: 4
------------------------------------------
------------------------------------------
array[0]: 4
array[1]: 3
array[2]: 2
array[3]: 16777216
array[4]: 0

Why the result has an unexpected value?

Shouldn't it be memcpy(temp, (array + s * size), size); in your first memcpy? I mean, that seems to be the case for all the other lines. — almightyGOSU
– almightyGOSU, Commented Jun 7, 2015 at 9:31
Just added it as an answer, I had to test it first just to be sure :) — almightyGOSU
– almightyGOSU, Commented Jun 7, 2015 at 9:37

almightyGOSU · Accepted Answer · 2015-06-07 09:41:32Z

2

Change from

memcpy(temp, (array + s), size);

to

memcpy(temp, (array + s * size), size); // Note: s * size

Tested, and it works.

Output:

array[0]: 0
array[1]: 1
array[2]: 2
array[3]: 3
array[4]: 4
------------------------------------------
------------------------------------------
array[0]: 4
array[1]: 3
array[2]: 2
array[3]: 1
array[4]: 0

edited Jun 7, 2015 at 9:41

answered Jun 7, 2015 at 9:36

almightyGOSU

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