CODEWITHSUNDEEP
cpointers
Jin Kwon
Jin Kwon

Reputation: 21978

How can I reverse an array using pointers?

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?

Upvotes: 1

Views: 717

Answers (1)

almightyGOSU
almightyGOSU

Reputation: 3739

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

Upvotes: 2

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