Sorting in C

Question

I have array with 20 digits
I need arrange items that are in pair positions descending
The first "for" I set up correctly, but I do not know how to proceed
Help me please
Code:
#include <stdio.h>
#include <stdlib.h>
int a[20]={12,0,3,34,2,99,81,21,75,7,48,65,31,95,77,43,50,88,4,46};
int comp (const int *, const int *);
int main(void)
{
int i;
printf("Original array: ");
for (i=2; i<20; i+=2)
printf("%d ",a[i]);
printf ("\n");
qsort(a, 20, sizeof (int), (int(*) (const void *, const void *)) comp);
printf("Sorted array: ");
for(i = 2; i <20; i+=2)
printf("%d ", a[i]);
return 0;
}
int comp (const int *i, const int *j)
{
return *j - *i;
}

Answer 1

Are you wanting to sort the whole array or just the pairs?
(ie
{ 1, 5, 3, 2, 6, 9} >> {5, 1, 3, 2, 9, 6}
or
{ 1, 5, 3, 2, 6, 9} >> {9, 6, 5, 3, 2, 1}
)