Function Pointers

As with Scheme, 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).

In this unit, we begin with a reading from your textbook, supplemented by a demonstration of passing functions as parameters to procedures, just as we did in Scheme.

Textbook reading

• King: Section 17.7, pages 439-445

Function Pointers and Arrays

The remainder of this reading presents a simple example that illustrates the use of functions as parameters.

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

One simple approach for writing this type of program might utilize these features:

• Separate functions are defined for the circumference and the area of a circle.
• The first line of the main program prints a header.
• A main loop iterates through the desired radius values.
• The loop itself contains three printf states, one for each value to be printed on a given line.

The resulting C program follows:

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

#include <stdio.h>
#include <math.h> /* Provides constant M_PI */

/* calculate the circumference of a circle given its radius */
double
circumference (double radius)
{
  return 2 * M_PI * radius;
} // circum


/* calculate the area of a circle given its radius */
double
area (double radius)
{
  return M_PI * radius * radius;
} // area


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

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

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 replaces the printf statements in the main loop by calls to myPrint.

Note that myPrint is also used for printing the value of radius by using an identify function. The code observes, however, that a simple printf for radius could be used as a reasonable alternative.

/* program to compute a circle's circumference and area example using function
   with a function parameter */

#include <stdio.h>
#include <math.h>

/* identity function (return the value given) */
double
identity (double radius)
{
  return radius;
} // identity


/* calculate the circumference of a circle given its radius */
double
circumference (double radius)
{
  return 2 * M_PI * radius;
} // circum


/* calculate the area of a circle given its radius */
double
area (double radius)
{
  return M_PI * radius * radius;
} // area


/* print the value of a given function at the given value */
void
myPrint (double x, double func (double))
{
  printf ("%12.4lf", func (x));
} // myPrint


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