Function Pointers, Parameters, and Arrays

Working with Functions

As with several programming languages, C allows variables to refer to functions, and functions can be passed as parameters. However, since C requires the type of each variable or parameter be declared, variables that may refer to functions must indicate the function's signature (number and type of parameters, return type).

Initial Example

Suppose you are asked to write a program to compute a circle's circumference and area for radii between 0 and 9. The desired output might be:

      radius   circumference   area
      0.0000      0.0000      0.0000
      1.0000      6.2832      3.1416
      2.0000     12.5664     12.5664
      3.0000     18.8496     28.2743
      4.0000     25.1327     50.2655
      5.0000     31.4159     78.5398
      6.0000     37.6991    113.0973
      7.0000     43.9823    153.9380
      8.0000     50.2655    201.0619
      9.0000     56.5487    254.4690

Program circle-cirum-area-1.c illustrates one simple approach for writing this type of program.

/* program to compute a circle's circumference and area
 */

#include <stdio.h>

const double pi = 3.1415926535;

/* circumference function */
double circum (double radius)
{
  return 2 * pi * radius;
}

/* area function */
double area (double radius)
{
  return pi * radius * radius;
}

int main ()
{
  printf ("      radius   circumference   area\n");
  double radius;
  for (radius = 0; radius < 10; radius++)
    {
      printf ("%12.4lf", radius);
      printf ("%12.4lf", circum(radius));
      printf ("%12.4lf", area(radius));
      printf ("\n");
    }
  return 0;
}

Function Parameters

The above C program works fine and likely is completely satisfactory for the simple problem given. However, several elements in the program contain some common elements. For example,

• The first three printf statements all use the same format string, and they each print just one value.

• The second and third printf statements call a function, the function requires one parameter, and radius is used in the function call.

• Arguably, the first printf statement also has this format, where the function called is the identify function x = iden(x)

Although these common elements are hardly earth shaking, it can be helpful to take advantage of such common elements in more complicated programs.

Printing Function

In exploiting the similarities in the printf statements, we might write a function that takes a radius and a function as parameters and then performs the required printing. The following code is an example:

/* printing function */
double myPrint (double x, double f (double))
{
  printf ("%12.4lf", f(x));
}

This function has two formal parameters, the number x and a function f. Further, f is identified as a function that will utilize one double value as an input parameter, and f will return a double when it completes execution.

The function myPrint utilizes the function f when it performs its printing.

Full Program

The following program circle-cirum-area-2.c replaces the printf statements in the main loop by calls to myPrint.

/* program to compute a circle's circumference and area

   example using function with a function parameter
 */

#include <stdio.h>

const double pi = 3.1415926535;

/* identity function */
double iden (double radius)
{
  return radius;
}

/* circumference function */
double circum (double radius)
{
  return 2 * pi * radius;
}

/* area function */
double area (double radius)
{
  return pi * radius * radius;
}

/* printing function */
void myPrint (double x, double f (double))
{
  printf ("%12.4lf", f(x));
}

int main ()
{
  printf ("      radius   circumference   area\n");
  double radius;
  for (radius = 0; radius < 10; radius++)
    {
      myPrint (radius, iden); 
      /* could also be
         printf ("%12.4lf", radius);
      */
      myPrint (radius, circum);
      myPrint (radius, area);
      printf ("\n");
    }
  return 0;
}

Arrays of Functions

As noted earlier in this lab, functions as parameters provide one mechanism to take advantage of common elements within an algorithm. A second approach involves utilizing an array of functions.

Declaration

Already, we have observed that the main loop in our example utilizes the same printf statement — with different functions being called. This suggests that we might declare an array of the relevant functions:

  double (*funcarr[3]) (double)= {iden, circum, area};

Full Program

The following program circle-cirum-area-3.c uses this declaration of funcarr in the main loop by iterating through the three functions of interest for printing.

/* program to compute a circle's circumference and area

   example using an array of functions
 */

# include <stdio.h>

const double pi = 3.1415926535;

/* identity function */
double iden (double radius)
{
  return radius;
}

/* circumference function */
double circum (double radius)
{
  return 2 * pi * radius;
}

/* area function */
double area (double radius)
{
  return pi * radius * radius;
}

int main ()
{
  double (*funcarr[3]) (double) = {iden, circum, area};
  printf ("      radius   circumference   area\n");
  double radius;
  for (radius = 0; radius < 10; radius++)
    {
      int i;
      for (i = 0; i < 3; i++)
        {
          printf ("%12.4lf", funcarr[i](radius));
        }
      printf ("\n");
    }
  return 0;
}