Lesson 8.1. Basic syntax of C functions

Introduction

Sometimes called a subroutine, module or procedure, a function is a group of instructions that performs a given task. Every program in C consists of at least one function: main(). The idea of functions is to avoid repeating the same piece of code at different places in the program. We write this code in a function and we call this function each time it is needed. In the previous lessons, we have already used some functions:

printf()
scanf()

When a function is used, it is a call to a function. The following code is a call to the print function:

printf ("Hello");

Syntax

Consider, for the example, the following function max() which returns the larger of the two integers between a and b:

int max(int a, int b);

The syntax of this function is illustrated below:

Let's detail this syntax:

Return type: is the type of the value returned by the function (int, float, char ...).
Function name: is the name that will be used to call the function.
Parameters or arguments : these are the arguments that will be passed to the function when it is called. The type of each parameter must be specified.

Prototype and implementation

Writing a function is done in two parts:

The prototype informs the compiler about the name of the function and its use (arguments and returned type).
The implementation contains the code of the function.

Here is an example of the structure of a complete program with the implementation of a function :

#include <stdio.h>

// Prototype of the function
int max (int a, int b);

// Main function
int main(void)
{
  // Call the max function
  printf ("The greater is %d\n", max(12, 7));
  return 0;
}

// Implementation of the function
int max(int a, int b)
{
  if (a>b) return a; else return b;
}

Tester l'exemple en ligne

Exercises

Exercise 1

We provide two functions addition() and subtraction() which return the sum and subtraction of parameters a and b respectively:

// Return the sum of a and b
int addition (int a,int b) {
  return a+b;
}

// Subtract b from a and return the result
int subtraction (int a,int b) {
  return a-b;
}

Using these functions as models, complete the program with

the prototype,
the implementation
the call of a new function multiplication() which returns the product of its two parameters:

Function addition       : x + y = 12
Function subtraction   : x - y = 4
Function multiplication : x * y = 32

Do the exercise

Solution

Exercise 2

Write a program that contains a function int square (int n) that calculates the square of the integer n. Test your function with the following main program:

int main(void) {
  int x=8;
  printf("%d*%d = %d\n", x, x, carre(x));
  return 0;
}

Do the exercise

Solution

Exercise 3

Write a program that contains a function char parity(unsigned int n) that returns:

0 if n is an odd number ;
1 if n is an even number;

Test the function with a main program that asks the user to enter an integer before displaying whether it is even or odd.

Enter an integer: 15
15 is even.

The function even() should not display anything, but return 0 or 1 to the main program.

Do the exercise

Solution

Quiz

In C, a function allows

to lighten the main program

to define new types of variables

to factor code to use it several times

to better structure the code

Check

How many parameters does the following function have:

int My_function (float x, unsigned short Val);

No parameters.

1 argument

2 arguments

3 arguments

Check

The prototype of a function ...

informs the compiler about the name of the function

contains the code of the function

ends with a semicolon

is an unnecessary comment to the execution

Check

The prototype of a function is located ...

before the main function

inside the main function

after the main function

after the keyword function

Check

The implementation of a function ...

contains the code of the function

ends with a semicolon

must comply with the associated prototype

is executed when the function is called

Check

The implementation of a function is placed ...

before the main function

inside the main function

after the main function

after the keyword function

Check

Lesson 7.6 Summary Lesson 8.2

Lesson 8.1. Basic syntax of C functions

Introduction

Syntax

Prototype and implementation

Exercises

Exercise 1

Exercise 2

Exercise 3

Quiz

See also