Creating Your Own Functions

What is a function?

A function is a named block of code that does one job. You have already used some: printf, scanf and sqrt are functions. main is a function too!

Think of it like this: A function is like a recipe card with a name, such as "Make Tea". Once it is written, you just say "Make Tea" whenever you want tea – you don't write out all the steps again.

Writing a function

#include <stdio.h>

void sayHello() {
    printf("Hello there!\n");
    printf("Welcome to C.\n");
}

int main() {
    sayHello();
    sayHello();
    return 0;
}

Output

Hello there!
Welcome to C.
Hello there!
Welcome to C.

• void means the function gives nothing back (more on that soon).
• sayHello is the name we chose.
• The code inside { } runs every time the function is called.
• sayHello(); calls it. We call it twice, so its lines print twice.

Why use functions?

• No copy-paste: write the code once, use it many times.
• Easier to read: printReceipt(); says what it does.
• Easier to fix: a bug only needs fixing in one place.

Write functions before main

C reads your file from top to bottom. A function must be known before it is called, so put your functions above main. (The next lessons show another way.)

Remember: Define a function once, above main. Call it by writing its name followed by ();.