Working with Functions in C: Arguments and Local Variables

When the printf function is called, you always supply one or more values to the func- tion, the first value being the format string and the remaining values being the specific program results to be displayed. These values, called  arguments, greatly increase the useful- ness and flexibility of a function. Unlike your printMessage routine, which displays the same message each time it is called, the printf function displays whatever you tell it to display.

You can define a function that accepts arguments. In Chapter 5, “Program Looping,” you developed an assortment of programs for calculating triangular numbers. Here, you define a function to generate a triangular number, called appropriately enough, calculateTriangularNumber. As an argument to the function, you specify which trian- gular number to calculate. The function then calculates the desired number and displays the results at the terminal. Program 8.4 shows the function to accomplish this task and a main routine to try it out.

Program 8.4   Calculating the nth Triangular Number

// Function to calculate the nth triangular number

#include <stdio.h>

void calculateTriangularNumber (int n)


int i, triangularNumber = 0;

for ( i = 1; i <= n; ++i )

triangularNumber += i;

printf (“Triangular number %i is %i\n”, n, triangularNumber);


int main (void)


calculateTriangularNumber (10);

calculateTriangularNumber (20);

calculateTriangularNumber (50);

return 0;


Program 8.4   Output

Triangular number 10 is 55

Triangular number 20 is 210

Triangular number 50 is 1275 

1. Function  Prototype  Declaration

The function calculateTriangularNumber requires a bit of explanation. The first line of the function:

void calculateTriangularNumber (int n)

is called the function prototype declaration. It tells the compiler that calculateTriangularNumber is a function that returns no value (the keyword void) and that takes a single argument, called n, which is an int. The name that is chosen for an argument, called its formal parameter name, as well as the name of the function itself, can be any valid name formed by observing the rules outlined in Chapter 4, “Variables, Data Types, and Arithmetic Expressions,” for forming variable names. For obvious rea- sons, you should choose meaningful names.

After the formal parameter name has been defined, it can be used to refer to the argument anywhere inside the body of the function.

The beginning of the function’s definition is indicated by the opening curly brace. Because you want to calculate the nth triangular number, you have to set up a variable

to store the value of the triangular number as it is being calculated.You also need a vari-able to act as your loop index. The variables triangularNumber and i are defined for these purposes and are declared to be of type int. These variables are defined and initial- ized in the same manner that you defined and initialized your variables inside the main routine in previous programs.

2. Automatic  Local Variables

Variables defined inside a function are known as automatic  local variables because they are automatically “created” each time the function is called, and because their values are local to the function. The value of a local variable can only be accessed by the function in which the variable is defined. Its value cannot be accessed by any other function. If an initial value is given to a variable inside a function, that initial value is assigned to the variable  each time the function is called.

When defining a local variable inside a function, it is more precise in C to use the keyword auto before the definition of the variable. An example of this is as follows:

auto int i, triangularNumber = 0;

Because the C compiler assumes by default that any variable defined inside a function is an automatic local variable, the keyword auto is seldom used, and for this reason it is not used in this book.

Returning to the program example, after the local variables have been defined, the function calculates the triangular number and displays the results at the terminal. The closing brace then defines the end of the function.

Inside the main routine, the value 10 is passed as the argument in the first call to calculateTriangularNumber. Execution is then transferred directly to the function where the value 10 becomes the value of the formal parameter n inside the function. The function then proceeds to calculate the value of the 10th triangular number and display the result.

The next time that calculateTriangularNumber is called, the argument 20 is passed. In a similar process, as described  earlier, this value becomes the value of n inside the function. The function then proceeds to calculate the value of the 20th triangular num- ber and display the answer at the terminal.

For an example of a function that takes more than one argument, rewrite the greatest common divisor program (Program 5.7) in function form. The two arguments to the function are the two numbers whose greatest common divisor (gcd) you want to calcu- late. See Program 8.5.

Program 8.5   Revising  the Program to Find the Greatest Common Divisor

/* Function to find the greatest common divisor

of two nonnegative integer values         */

#include <stdio.h>

void gcd (int u, int v)


int temp;

printf (“The gcd of %i and %i is “, u, v);

while ( v != 0 ) {

temp = u % v;

u = v;

v = temp;


printf (“%i\n”, u);


int main (void)


gcd (150, 35);

gcd (1026, 405);

gcd (83, 240);

return 0;



Program 8.5   Output

The gcd of 150 and 35 is 5

The gcd of 1026 and 405 is 27

The gcd of 83 and 240 is 1

The function gcd is defined to take two integer arguments. The function refers to these arguments through their formal parameter names u and v. After declaring the variable temp to be of type int, the program displays the values of the arguments u and v, together with an appropriate message at the terminal. The function then calculates and displays the greatest common divisor of the two integers.

You might be wondering why there are two printf statements inside the function gcd.You must display the values of u and v before you enter the while loop because their values are changed inside the loop. If you wait until after the loop has finished, the values displayed for u and v do not at all resemble the original values that were passed to the routine. Another solution to this problem is to assign the values of u and v to two vari- ables before entering the while loop. The values of these two variables can then be dis- played together with the value of u (the greatest common divisor) using a single printf statement after the while loop has completed.

Source: Kochan Stephen G. (2004), Programming in C: A Complete Introduction to the C Programming Language, Sams; Subsequent edition.

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