Question

I am trying to write the following code for mergesort ,Can anybody please tell me what is wrong with the code:-

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

int arr[100];

void merge(int low,int mid,int high)
{
    int temp[100],lowind = low,highind = mid+1,i=0;
    while(lowind <= mid && highind<=high)
    {
        if(arr[lowind]<arr[highind])
        {
            temp[i] = arr[lowind];
            i++;
            lowind++;
        }
        else
        {
            temp[i] = arr[highind];
            i++;
            highind++;
        }
    }

    while(lowind<=mid)
    {
        temp[i] = arr[lowind];
        i++;
        lowind++;
    }

    while(highind<=high)
    {
        temp[i] = arr[highind];
        i++;
        highind++;
    }

    for(i=low;i<=high;i++)
        arr[i] = temp[i-low];
}

void divider(int low,int high)
{
    int mid = (low+high)/2;
    if(low<mid)
    {
        divider(low,mid);
        divider(mid+1,high);
        merge(low,mid,high);
    }
}

int main()
{
    int i,j,temp;

    for(i=0;i<20;i++)
        arr[i] = rand()%21;

    for(i=0;i<20;i++)
        printf("%d ",arr[i]);
    printf("\n");

    divider(0,19);

    for(i=0;i<20;i++)
            printf("%d ",arr[i]);
    printf("\n");

    return 0;
}

Answer 1

In void divider(int low,int high), it should be if(low<high) instead of if(low<mid) as you have written.

To know why, try to understand cases like low=2, high=3, here mid=(low+high)/2=5/2=2. Hence your condition fails.