Lesson 8.4. Return keyword

Introduction

return is a C keyword that has a double purpose:

it ends the execution of the function;
it returns a value or a variable to the entity that called the function (the main program or another function).

In a function of type void fct(), the return is optional. If it is present, it is necessarily written as return; without value.

A function can only return one value. We will see later on other mechanisms (pointers or structures) that allow a function to return several values.

Syntax

Note that return is a C keyword that must be common to all ANSI compliant compilers. Since return is not a function, parentheses are not necessary:

return (5);

should rather be written :

return 5;

The return keyword can be followed by

a value ;
a calculation;
- a symbolic constant
a variable;
a call to a function.

Here are some examples:

// Value
return 35.65;

// Calculation
return 2*3.1415;

// Constante
return M_PI;

// Variable 
return maVariable;

// Function
return cos(3.1415);

Exercise

Write a function int isPrime(unsigned int n); which receives a positive integer n as parameter.

If n = 0 : the function returns 0.
If n = 1 : the function returns 0.
If n is divisible by an integer in the interval [2;n[, the function returns this integer.
If n is a prime number, the function returns 1.

A prime number is a natural number that has exactly two distinct positive integer divisors (1 and itself).

Test the function with the following code in the main program:

unsigned int x, p;

// Ask for a positive integer
printf("Enter a positive integer: ");
scanf ("%u", &x);

// Check if the integer entered is a prime number
p = isPrime (x);
switch (p) {
  case 0: printf ("By convention, 0 and 1 are not prime numbers.\n"); break;
  case 1: printf ("%u is prime.\n",x); break;
  default: printf ("%u is a divisor of %d.\n", p, x); break;
}

Here are some examples of expected outputs:

Enter a positive integer: 1
By convention, 0 and 1 are not prime numbers.

Enter a positive integer: 125
5 is a divisor of 125.

Enter a positive integer: 11
11 is prime.

Do the exercise

Solution

Quiz

In C, the return keyword is used to :

systematically end the execution of the program

systematically end the execution of a function

return a value

to return to the previous line of code

Check Bravo! return ends the function and eventually returns a value.

Let's consider the following function:

void fct(int i);

there can be no return in this function.

return will return a value in the argument i.

a return in the code of this function is optional.

return is mandatory in all functions.

Check Bravo! The return is optional in functions of type void.

return is...

in the stdio.h library

a C keyword

a C function C

standard on all compilers

Check Bravo! return is a keyword in standard C

What the following function displays:

void test() {
  printf ("Hello");
  return;
  print f("Bye");
}

nothing

Hello

Bye

HelloBye

Check Bravo! return exits the funcion

Which syntaxes allow to return several values?

return 1,2;

return 1.2

return {1,2}

None of the other answers

Check

Lesson 8.3 Summary Lesson 8.5

Lesson 8.4. Return keyword

Introduction

Syntax

Exercise

Quiz

See also