Question

I wrote a short code that receives an array and sorts it.
This was my first time and I have so many error – please if you can
explain to me how to fix and where/what are my mistakes…
All I want to do is to sort the array and print it after sorting.

#include <stdio.h>

int merge_sort(int *a,int first, int last)
{
    int middle;
    if(first < last)
    {
        middle=(first+last)/2;
        merge_sort(a,first,middle);
        merge_sort(a,middle+1,last);
        merge(a,first,middle,last);
    {
}

void main()
{
    int x[] = {1, 2, 3, 1, 3, 0, 6};
    int xsize= (sizeof x / sizeof x[0])
    merge_sort(x, 0, sizeof x / sizeof x[0]);

    for (int i = 0; i < xsize; i++) { printf("%d ", x[i]); } putchar('\n');
}

Answer 1

I could see multiple errors in your program

1. int xsize= (sizeof x / sizeof x[0]) must have a semicolon ; at the end.
2. for(int i=0;i<xsize;printf("%d ",x[i]), i++); is not the way to print the elements of the array.
3. your merge() is missing – merge(a,first,middle,last);

So I went ahead and wrote this sample mergesort for your reference. Hope this helps!!

#include <stdio.h>

/*

NOTE: 
The mergesort boils downs to this.. 
Given two sorted array's how do we merge this?

We need a new array to hold the result of merging
otherwise it is not possible to do it using array, 
so we may need a linked list

*/

void merge(int a[], int i, int j)
{
    int mid = (i+j)/2;
    int ai = i;
    int bi = mid+1;

    int newa[j-i+1], newai = 0;

    while(ai <= mid && bi <= j) {
        if (a[ai] > a[bi])
            newa[newai++] = a[bi++];
        else                    
            newa[newai++] = a[ai++];
    }

    while(ai <= mid) {
        newa[newai++] = a[ai++];
    }

    while(bi <= j) {
        newa[newai++] = a[bi++];
    }

    for (ai = 0; ai < (j-i+1) ; ai++)
        a[i+ai] = newa[ai];

}

void mergesort(int a[], int i, int j)
{
    int mid = (i+j) / 2;

    if (i >= j) return;

    mergesort(a, i, mid);
    mergesort(a, mid+1, j);
    merge(a, i, j);
    return;
}

int main()
{
    //int a[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    int a[] = {9, 7, 2, 3, 5, 4, 1, 8, 6, 10};
    int i;

    mergesort(a, 0, 9);

    for (i = 0; i < 10; i++)
        printf ("%d ", a[i]);

    printf ("\n");

    return 0;
}