Question

I tried executing this code in gcc as well as turboc. In GCC it gives me a segmentation fault error at run time and in turbo it gives a null pointer assignment error again at run time.

I tried tracing it but i cant see where the problem is.

#include<stdio.h>

int heapsize;

void heapify(int a[],int i)
{
    int l=2*i,r=2*i+1,largest,temp;
    if(l<=heapsize && a[l]>a[i])
        largest=l;
    else
        largest=i;
    if(r<=heapsize && a[r]>a[largest])
        largest=r;
    if(largest!=i)
    {
        temp=a[i];
        a[i]=a[largest];
        a[largest]=temp;
        heapify(a,largest);
    }
}

void buildheap(int a[],int length)
{
    int i;  
    heapsize=length;    
    for(i=length/2;i>=0;i--)
        heapify(a,i);
}

void heapsort(int a[],int length)
{
    int i,temp; 
    buildheap(a,length);
    for(i=length-1;i>=1;i++)
    {
        temp=a[0];
        a[0]=a[i];
        a[i]=temp;
        heapsize--;         
        heapify(a,0);   
    }
}

void main()
{
    int a[20],i,length;
    printf("ENTER THE SIZE OF YOUR ARRAY:");
    scanf("%d",&length);
    printf("ENTER THE ARRAY ELEMENTS: \n");
    for(i=0;i<length;i++)
        scanf("%d",&a[i]);
    heapsort(a,length);
    printf("THE SORTED ARRAY IS:");
    for(i=0;i<length;i++)
        printf("%d /t",a[i]);
}

NOTE: I coded this using the heapsort algorithm given in CLRS.

Edit: here’s the input i gave and the error I got.

chaitanya@chaitanya-laptop:~/Desktop/My prog$ ./a.out
ENTER THE SIZE OF YOUR ARRAY:5
ENTER THE ARRAY ELEMENTS:
9
5
8
7
6
Segmentation fault

Edit: apparently a silly mistake of i++ instead of i– was causing the problem. but now there seems to be a logical error since the program is not giving the sorted array as the output.

ENTER THE SIZE OF YOUR ARRAY:5
ENTER THE ARRAY ELEMENTS:
2
4
3
1
9
THE SORTED ARRAY IS:3013077 4 1 9 2

Answer 1

The following for loop will never terminate. The index keeps on increasing causing array to go out of bounds and that’s the reason you get the above error.

for(i=length-1;i>=1;i++)
{
    temp=a[0];
    a[0]=a[i];
    a[i]=temp;
    heapsize--;         
    heapify(a,0);   
}

Update : Your heapify() is incorrect.. try this instead

if(l<heapsize && a[l]>a[i])
    largest=l;
else
    largest=i;
if(r<heapsize && a[r]>a[largest])
    largest=r;