Reversing arrays in C

Question

I am attempting to make a program that takes an array and reverts it backwards however the program must do this to the array in groups of three. So if the user enters the numbers 1, 2, 3, 4, 5, 6 into the array the program will then output: 3, 2, 1, 6, 5, 4.

When I run the current program I get: 3 2 1 4 5 6. If anyone could help me figure out why that would be great as I am a little confused.

Here is my code:

int * numbersProcessFour(int *x, int size) 
{
    int i = 0, three = 3, six = 6, nine = 9;
    if (size < 4) 
    {
        for (i; i < three; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
    }else if (size > 3 && size < 7) 
    {
        for (i; i < three; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
        for (i; i < 6; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
    }
    else if (size > 6 && size < 10) 
    {
        for (i; i < three; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
        for (i; i < 6; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
        for (i; i < 9; i++)
        {
            reverse_array(x, three);
            printf("%d ", x[i]);
        }
    }
}
void reverse_array(int *x, int length)
{
    int i, temp;
    for (i = 0; i<length / 2; i++)
    {
        temp = x[i];
        x[i] = x[length - i - 1];
        x[length - i - 1] = temp;
    }
}

Answer 1

Continuing from your comment to fluter's answer, you may be over thinking it a bit. In order to swap the 1st and 3rd element in each 3-element partition of an array, you simply need to step though the array 3-elements at a time. You need to decide how you will handle any final partial partition, but since your goal is to swap the 1st and 3rd, there is no 3rd in anything less than a full partition, so the logical choice is to ignore any final partial partition.

A variant of what you and fluter have done incorporating a swap would be:

/* reverse 1st and 3rd element in each group of 3 */
void rev3 (int *a, size_t sz)
{
    if (sz < 3) return;
    size_t i;

    for (i = 0; i < sz; i += 3) {
        if (sz - i < 3) break;
        swap (&a[i], &a[i+2]);
    }
}

You can put it together with:

#include <stdio.h>

void rev3 (int *a, size_t sz);
void swap (int *a, int *b);

int main (void) {

    int a[] = {1,2,3,4,5,6,7,8,9};
    size_t i;

    rev3 (a, sizeof a/sizeof *a);

    for (i = 0; i < sizeof a/sizeof *a; i++) printf (" %2d", a[i]);
    putchar ('\n');

    return 0;
}

void swap (int *a, int *b)
{
    int t = *a;
    *a = *b;
    *b = t;
}

Example Use

When compiled and run it will give you the swap (reversal) of the 1st and 3rd elements throughout the array that you specify in your problem.

$ ./bin/revarr3
  3  2  1  6  5  4  9  8  7

There is no difference whether you use a separate swap or whether you include that operation in your reversal function. There is also no need to incur the additional overheard of calling a recursive function when a procedural approach will work. Look over all the answers and compare/contrast the differing ways to accomplish your goal.

Answer 2

A generalized version of numberProcessFour might look like this.

int reverse_array_mod(int *input, size_t size, int mod)
{
        int i, smod;

        /* Error: return modulus if size cannot be divided by mod */
        if(size%mod)
                return size%mod;

        smod = size/mod;

        for(i=0; i<smod; i++)
                reverse_array(input+i*mod, mod);

        /* return 0 on success */
        return 0;
}

Test

int main(int argc, char **argv)
{
        int a[] = {0, 1, 2, 3, 4, 5};
        int i, err, mod;

        for(mod=1; mod<5; mod++) {
                err = reverse_array_mod(a, 6, mod);
                if(err) {
                        fprintf(stderr, "Error %d, modulus %d invalid\n", err, mod);
                        return err;
                }

                for(i=0; i<6; i++)
                        printf("%d\n", a[i]);
                printf("\n");
        }

        return 0;
}

Result:

0
1
2
3
4
5

1
0
3
2
5
4

3
0
1
4
5
2

Error 2, modulus 4 invalid

Answer 3

try this

int *numbersProcessFour(int *x, int size) {
    int i;
    for(i = 0; i + 3 < size; i += 3){
        reverse_array(x + i, 3);
    }
    if(size - i > 1)
        reverse_array(x + i, size - i);
    return x;
}

Answer 4

You have branches for each multiple of 3, that is inefficient. One way to solve it is you can take the array as a split by 3 smaller arrays, and reverse on them. Also, reversing an array of 3 elements is the same as swap the 1st and the 3rd element.

int i;
int temp;
for (i = 0; i < count; i += 3) {
    if (i+2 >= count)
        break;
    temp = arr[i];
    arr[i] = arr[i+2];
    arr[i+2] = temp;
}