CODEWITHSUNDEEP
c
manubmxnsb
manubmxnsb

Reputation: 151

Pointer array sorting in C has unexpected output

#include <stdio.h>

void sort(int *ptr, int n) {
    int i,j,tmp;
    for (i=0;i<n;i++)
        for (j=0;j<n;j++)
                if (ptr[i] < ptr[j])
                {
                    tmp=ptr[i];
                    ptr[i]=ptr[j];
                    ptr[j]=tmp;
                }
}
int main() {
    int i,n;
    int *ptr;
    printf("Nr. of elements : 5 \n");
    n=5;

    ptr=(int*)malloc( n * sizeof(int));
    for (i=0;i<n;i++) {
        scanf("%d",&ptr[i]);
    }

    printf("Initial array is : ");
    for (i=0;i<n;i++) {
        printf("%d ",ptr[i]);
    }

    sort(ptr,n);

    printf("Sorted array is : ");
    for (i=0;i<n;i++) {
        printf("%d ",ptr[i]);
    }
    return 0;
}

This is my code. I'm trying to sort a pointer array using a function. Whatever the (int) input, it sorts out fine. My confusion is that i'm using ptr[i] < ptr[j] instead of ptr[i] > ptr[j] as it should normally be to sort it ascending. Why is that?

Upvotes: 2

Views: 168

Answers (3)

Doug Currie
Doug Currie

Reputation: 41180

No, your confusion is misplaced. Look at the for loops, and the relation between i and j. There are times when i < j and times when i > j, so what constitutes being "out of order" and requiring a swap?

The inner loop should start at i+1 not at '0'; that will make the relation between i and j invariant.

Upvotes: 4

Bill Lynch
Bill Lynch

Reputation: 81926

Given your loops go from i = 0 .. n and j = 0 .. n, there is no guarantee in your code that i < j.

There's two ways to fix this:

void sort(int *ptr, int n) {
  int i,j,tmp;
  for (i=0; i<n; i++) {
    for (j=0; j<n; j++) {
      if (i < j && ptr[i] < ptr[j]) { // Note the changed conditional
        tmp=ptr[i];
        ptr[i]=ptr[j];
        ptr[j]=tmp;
      }
    }
  }
}

or

void sort(int *ptr, int n) {
  int i,j,tmp;
  for (i=0; i<n; i++) {
    for (j=i+1; j<n; j++) { // Note the changed start value
      if (ptr[i] < ptr[j]) {
        tmp=ptr[i];
        ptr[i]=ptr[j];
        ptr[j]=tmp;
      }
    }
  }
}

Upvotes: 4

Piyush Kumar
Piyush Kumar

Reputation: 191

As we can see, you are using bubble sort. In bubble sort, our main intention is either transfer the heavier element to the end or lighter element to the top.

(ptr[i] < ptr[j])

what you are doing is moving the heavy elements to the end of the array, this is why whenever you are finding ptr[j](j is an inner loop variable) which is bigger than the ptr[i] (outer loop variable), you are doing a swap.

Upvotes: -1

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