problem with dynamic array and pointers in selection sort - arrays

Question

i make a program of selection sort using dynamic array and pointers but after running this code i found that array is being sorted if we give the size input like 4 and 6 but does't sort properly if the size input is like 5 and 7 etc ...i also did bubble sort program before this using same technique of pointers and dynamic array but it gave perfect sorted array in all the condition , i also try to debuge the code but still don't understand why this is happening if anyone having idea about this then please help me out .
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
int main()
{
int * ptr,temp,min;
int size,i,j,s;
printf("Enter the size of array:");
scanf("%d",&size);
ptr = (int *)(calloc (size,sizeof(int)));
if(ptr == NULL)
printf("No memory");
else
{
printf("\n=== RANDOM ELEMENTS OF ARRAY ===\n");
for(s=0;s<size;s++)
*(ptr+s) = rand()%100;
for(s=0;s<size;s++)
printf("\nElement [%d] = %d ",s,*(ptr+s));
// selection sort algorithm
for(i=0;i< size-1;i++)
{
min = i;
for(j=i+1;j<size;j++)
{
if(*(ptr+j) < *(ptr+min))
{
min = j;
}
temp = *(ptr+i);
*(ptr+i) = *(ptr+min);
*(ptr+min) = temp;
}
}
// End of algorithm
printf("\n\n======= SORTED ELEMENTS =======\n\n");
for(s=0;s<size;s++)
printf("Element [%d] = %d \n",s,*(ptr+s));
}
}

Answer 1

Your Selection Sort algorithm seems to be wrong. You have to replace the element in the current index with the minimum after the inner for loop finishes iteration:
for(i=0;i< size-1;i++)
{
min = i;
for(j=i+1;j<size;j++)
{
if(*(ptr+j) < *(ptr+min))
{
min = j;
}
}
temp = *(ptr+i);
*(ptr+i) = *(ptr+min);
*(ptr+min) = temp;
}
Now, it should work for all input sizes.

Answer 2

Clearing away a couple of unnecessary confusion factors...
*(ptr + index)
Is identical to
ptr[index]
But the second is much easier to read.
Next, in the following section the variable min is introduced,
if(*(ptr+j) < *(ptr+min))
{
min = j;
}
...but is not necessary for a simple sort. Just stick with i and j and the sort will proceed correctly for either even or odd sets of values. Finally, to eliminate the memory leak, call free(ptr); when ptr is no longer needed. Following is a cleaned up version, with corrections.
int main(void)//added void
{
int * ptr,temp/*,min*/;
int size,i,j,s;
printf("Enter the size of array:");
scanf("%d",&size);
ptr = calloc (size,sizeof(int));//casting is required in C++
//but unnecessary in C.
//(and can be problematic)
if(ptr == NULL)
{
printf("No memory");
}
else
{
printf("\n=== RANDOM ELEMENTS OF ARRAY ===\n");
for(s=0;s<size;s++)
ptr[s] = rand()%100;
for(s=0;s<size;s++)
printf("\nElement [%d] = %d ",s,ptr[s]);
// selection sort algorithm
for(i=0;i< size-1;i++)
{
for(j=i+1;j<size;j++)//removed if section introducing 'min'
{
if(ptr[j] < ptr[i])
{
temp = ptr[i];
ptr[i] = ptr[j];
ptr[j] = temp;
}
}
}
// End of algorithm
printf("\n\n======= SORTED ELEMENTS =======\n\n");
for(s=0;s<size;s++)
printf("Element [%d] = %d \n",s,*(ptr+s));
free(ptr);
}
return 0;//added
}
BTW, this was tested with size == 3 and
ptr[0] = 3;
ptr[1] = 2;
ptr[2] = 1;
along with several other odd and even counts of random values