Pointers to Functions

        One of those things beginners in C find difficult is the concept of pointers. The purpose of this tutorial is to provide an introduction to pointers and their use to these beginners.

Up to this point we have been discussing pointers to data objects. C also permits the declaration of pointers to functions. Pointers to functions have a variety of uses and some of them will be discussed here.
Consider the following real problem. You want to write a function that is capable of sorting virtually any collection of data that can be stored in an array. This might be an array of strings, or integers, or floats, or even structures. The sorting algorithm can be the same for all. For example, it could be a simple bubble sort algorithm, or the more complex shell or quick sort algorithm. We' use a simple bubble sort for demonstration  purposes.
Sedgewick [1] has described the bubble sort using C code by setting up a function which when passed a pointer to the array would sort it. If we call that function bubble(), a sort program is described by bubble_1.c, which follows:

/*-------------------- bubble_1.c --------------------*/
/* Program bubble_1.c from PTRTUT10.HTM                  6/13/97 */
#include <stdio.h>
int arr[10] = { 3,6,1,2,3,8,4,1,7,2};
void bubble(int a[], int N);
int main(void)
{
     int i;
     putchar('\n');
     for (i = 0; i < 10; i++)
     {
           printf("%d ", arr[i]);
     }
     bubble(arr,10);
     putchar('\n');
     for (i = 0; i < 10; i++)
     {
           printf("%d ", arr[i]);
     }
     return 0;
}
void bubble(int a[], int N)
{
     int i, j, t;
     for (i = N-1; i >= 0; i--)
     {
           for (j = 1; j <= i; j++)
           {
                 if (a[j-1] > a[j])
                 {
                      t = a[j-1];
                      a[j-1] = a[j];
                      a[j] = t;
                 }
           }
     }
}
/*---------------------- end bubble_1.c -----------------------*/

The bubble sort is one of the simpler sorts. The algorithm scans the array from the second to the last element comparing each element with the one which precedes it. If the one that precedes it is larger than the current element, the two are swapped so the larger one is closer to the end of the array. On the first pass, this results in the largest element ending up at the end of the array. The array is now limited to all elements except the last and the process repeated. This puts the next largest element at a point preceding the largest element. The process is repeated for a number of times equal to the number of elements minus 1. The end result is a sorted array.
Here our function is designed to sort an array of integers. Thus in line 1 we are comparing integers and in lines 2 through 4 we are using temporary integer storage to store integers. What we want to do now is see if we can convert this code so we can use any data type, i.e. not be restricted to integers.
At the same time we don'want to have to analyze our algorithm and the code associated  with it each time we use it. We start by removing the comparison from within the function bubble() so as to make it relatively easy to modify the comparison function without having to re-write portions related to the actual algorithm. This results in

bubble_2.c:
/*---------------------- bubble_2.c -------------------------*/
/* Program bubble_2.c from PTRTUT10.HTM                  6/13/97 */
    /* Separating the comparison function */
#include <stdio.h>
int arr[10] = { 3,6,1,2,3,8,4,1,7,2};
void bubble(int a[], int N);
int compare(int m, int n);
int main(void)
{
     int i;
     putchar('\n');
      for (i = 0; i < 10; i++)
      {
            printf("%d ", arr[i]);
      }
      bubble(arr,10);
      putchar('\n');
      for (i = 0; i < 10; i++)
      {
            printf("%d ", arr[i]);
      }
      return 0;
}
void bubble(int a[], int N)
{
      int i, j, t;
      for (i = N-1; i >= 0; i--)
      {
            for (j = 1; j <= i; j++)
            {
                 if (compare(a[j-1], a[j]))
                 {
                       t = a[j-1];
                       a[j-1] = a[j];
                       a[j] = t;
                 }
            }
      }
}
int compare(int m, int n)
{
      return (m > n);
}
/*--------------------- end of bubble_2.c -----------------------*/