在C程序中使用指针排序

Question

I written a sorting to an ascending order program using pointers in C.Logically this program works an ascending order,but output is descending order.What happened to this program.

This is my code :

#include<stdio.h>
#include<stdlib.h>
main()
{
int n,i,j,t,*ptr,sum=0;
printf("Enter number of elements to sort : ");
scanf("%d",&n);
ptr=(int*)malloc(n*sizeof(int));
if(ptr==NULL)
{ printf("Error occurred memory not allocated \n"); exit(0);  }
printf("Enter the elements of Array \n");
for(i=0;i<n;i++)
{
scanf("%d",ptr+i);
}

for(i=0;i<n;i++){

       for(j=0;j<n;j++)
       {

           if( *(ptr+i) > *(ptr+j))
           {
               t = *(ptr+i);

               *(ptr+i) = *(ptr+j);
               *(ptr+j) = t;
           }
       }
}
for(i=0;i<n;i++)
{
printf("\n%d\n",*(ptr+i));
}

free(ptr);
}

Answer 1

Two things

1) For ascending order, change the if statement to

if( *(ptr+i) < *(ptr+j) )

2) You don't need to check the entire array every time. You can start from the position of the previous lowest value.

for(i=0; i<(n-1); i++){
   for(j=i+1; j<n; j++) {
      ...
   }
}

Answer 2

OK let's do it step by step by a testcase that reproduces the "error".

Suppose that the size of the array is 5, and the user input is 5, 3, 4, 2, 0 , so we have after iniliazization:

a[0]=5;
a[1]=3;
a[2]=4;
a[3]=2;
a[4]=0;

1) i = 0 ,j = 0 : (5 is not > 5) so no change
2) i = 0, j = 1 : (5 is > than 3) so a[0] -> 3 , a[1] -> 5
3) i = 0, j = 2 : (3 is not > 4) so no change
4) i = 0, j = 3 : (3 is > than 2) so a[0] -> 2 , a[3] -> 3
5) i = 0, j = 4 : (2 is > than 0) so a[0] -> 0 , a[4] -> 2

Now i=1;

1) i = 1, j = 0 : (5 is > than 0) so a[1] -> 0 , a[0] -> 5

You see, that you write a[0] = 5 again. This is the main error of your algorithm. If bubbleshort is what you want to implement have a better look at the algorithm (and after that check some more effective sorting algorithms like quicksort .

Answer 3

your program is correct but the line:

if( *(ptr+i) > *(ptr+j))

makes the arrangement in descending order, just make that line:

if( *(ptr+i) < *(ptr+j)) 

and the output will be in ascending order.

Answer 4

I think you are trying to implement Bubble Sort. There is a logical problem in the code in second for loop

for(j=0;j<n;j++)

Change it to

for(j=1;j<n-1;j++). 

I think this will fix the issue