Defining Functions

In this lesson, you will learn to write your own functions. Let's start with a problem. A game prints the same message every time you finish a level. The program in the editor does it for two levels:

#include <stdio.h>

int main(void) {
    printf("Level 1 finished\n");
    printf("==========\n");
    printf("Well done!\n");
    printf("==========\n");

    printf("Level 2 finished\n");
    printf("==========\n");
    printf("Well done!\n");
    printf("==========\n");
    return 0;
}

It works, but the same three lines are there twice. A real game has a hundred levels. Imagine writing those lines a hundred times! And if you wanted to change the message, you would have to change it in a hundred places, and you would surely miss one.

Programmers try not to repeat code. They write the repeated part once, give it a name and then use it wherever they need it. Such a named piece of code is called a function. A good name also says what the code does, so the program is easier to read.

In fact, you have been writing one function since the first lesson: main. The line int main(void) { starts the definition of a function named main, and the statements between the curly braces are its body. When you run a program, it starts by running main. You define your own function in a similar way:

void greet(void) {
    printf("Hello!\n");
    printf("Have a nice day!\n");
}

A function definition has these parts:

• void (English for empty) is the return type. It says that the function gives no value back. It only does something: here, it prints two lines. Later in this lesson, you will write functions that do give a value back.
• The name of the function, here greet. The same rules apply as for variable names: letters, digits and _, no spaces, and upper and lower case matter. Names made of several words are written in snake_case, such as print_banner. Choose a name that says what the function does.
• Parentheses with void inside. Here, void means that the function takes no values. You will soon find out what else can go in the parentheses.
• The body of the function in curly braces { }, indented by 4 spaces like the body of main. It holds the statements that the function runs.

Write the function above main, not inside it. In C, a function can't be defined inside another function. If you write it in the body of main, the program doesn't compile, and the compiler reports error: function definition is not allowed here.

When you run the program with the new function, it still prints only what main prints. That's not a mistake! A definition only says what the function should do, like a recipe written down in a cookbook: nobody has cooked anything yet. In the next exercise, you will make the function run.