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Actual parameters and formal parameters

There are two other categories that you should know about that are also referred to as "parameters". They are called "parameters" because they define information that is passed to a function.

• Actual parameters are parameters as they appear in function calls.
• Formal parameters are parameters as they appear in function declarations.

A parameter cannot be both a formal and an actual parameter, but both formal parameters and actual parameters can be either value parameters or variable parameters.

Let's look at calculate_bill again:

#include <stdio.h>

int main (void);
int calculate_bill (int, int, int);

int main()
{
  int bill;
  int fred = 25;
  int frank = 32;
  int franny = 27;

  bill = calculate_bill (fred, frank, franny);
  printf("The total bill comes to $%d.00.\n", bill);

  exit (0);
}

int calculate_bill (int diner1, int diner2, int diner3)
{
  int total;

  total = diner1 + diner2 + diner3;
  return total;
}

In the function main in the example above, fred, frank, and franny are all actual parameters when used to call calculate_bill. On the other hand, the corresponding variables in calculate_bill (namely diner1, diner2 and diner3, respectively) are all formal parameters because they appear in a function definition.

Although formal parameters are always variables (which does not mean that they are always variable parameters), actual parameters do not have to be variables. You can use numbers, expressions, or even function calls as actual parameters. Here are some examples of valid actual parameters in the function call to calculate_bill:

bill = calculate_bill (25, 32, 27);

bill = calculate_bill (50+60, 25*2, 100-75);

bill = calculate_bill (fred, franny, (int) sqrt(25));

(The last example requires the inclusion of the math routines in math.h, and compilation with the -lm option. sqrt is the square-root function and returns a double, so it must be cast into an int to be passed to calculate_bill.)