Merge Sort Program

Question

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

void merge(int a[],int l,int m,int r)
{
    int i,j,k;
    int n1= m - l + 1;
    int n2= r - m;
    int L[n1],R[n2];
    for(i=0;i<n1;i++)
        L[i]=a[l+i];
    for(j=0;j<n2;j++)
        R[j]=a[m + 1+ j];
    i=0;j=0;k=l;
    while(i<n1 && j<n2)
    {
        if(L[i]<R[j])
        {
            a[k]=L[i];
            i++;
        }
        else
        {
            a[k]=R[j];
            j++;
        }
        k++;
    }
    while(i<n1)
    {
        a[k]=L[i];
        i++;
        k++;
    }
    while(j<n2)
    {
        a[k]=R[j];
        j++;
        k++;
    }
}

void mergeSort(int a[],int l,int r)
{
    if(l<r)
    {
        int m = l+(r-1)/2;
        mergeSort(a,l,m);
        mergeSort(a,m+1,r);
        merge(a,l,m,r);
    }   
}

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

int main()
{
    int a[]= {
                12,76,34,45,63,98
             };
    int n = sizeof(a)/sizeof(a[0]);
    printf("The element entered in array   ");
    printArray(a,n);
    mergeSort(a,0,n-1);
    printf("The element after sorting    ");
    printArray(a,n);
    return 0;
}

This is a merge sort program. Why it's giving runtime error if anyone can explain? There is no error in this program

I have tried this 10 times. And unable to find the solution.

Answer 1

When you call mergesort(a, 3, 5) the following happens:

int m = l+(r-1)/2;  // 3 + (5-1)/2 -> 3 + 4/2 -> 3 + 2 -> 5
mergeSort(a,l,m);   // So this will call: mergesort(a, 3, 5) again

In other words: An endless loop.

Maybe you want this instead:

int m = (l+(r-1))/2;

        ^       ^
         notice

So how did I find the bug?

Very easy - just use some printf-debug.

First I added a print in the start of mergesort - like:

void mergeSort(int a[],int l,int r)
{
    printf("mergesort %d %d\n", l, r);  // Debug print
    if(l<r)
    {
        int m = (l+(r-1))/2;
        mergeSort(a,l,m);
        mergeSort(a,m+1,r);
        merge(a,l,m,r);
    }   
}

and got the output:

mergesort 3 5
mergesort 3 5
mergesort 3 5
...
...

which told me that there was an endless loop for input values 3 and 5.

Then I added a print of m

void mergeSort(int a[],int l,int r)
{
  printf("mergesort %d %d\n", l, r);
    if(l<r)
    {
        int m = (l+(r-1))/2;
        printf("m %d\n", m);
        mergeSort(a,l,m);
        mergeSort(a,m+1,r);
        merge(a,l,m,r);
    }   
}

and got the output:

mergesort 3 5
m 5
mergesort 3 5
m 5
mergesort 3 5
m 5

so obviously m was calculated wrongly.

Looking close at

int m = l+(r-1)/2;

is was clear that the addition should be before the division. A set of (....) was missing.

Hope you can use this debug example for your own debug.