Step 1 of 7

Define and call a function

As programs grow, putting everything in main gets messy fast. A function packages a piece of work under a name, so you can write it once and use it (call it) wherever it's needed. You've been calling functions all along: printf and scanf are functions someone else wrote.

A function definition has four parts:

int square(int x) {
    return x * x;
}

• Return type (int): the type of value the function hands back.
• Name (square): how you call it.
• Parameters (int x): inputs, each with a type and a name. Inside the function, x is a variable holding whatever value the caller passed.
• Body: the code between the braces. return ends the function and sends a value back.

#include <stdio.h>

int square(int x) {
    return x * x;
}

int main(void) {
    int a = square(7);
    printf("%d\n", a);
    printf("%d\n", square(3) + square(4));
    return 0;
}

49
25

What happens during a call

When main reaches square(7), it pauses. The value 7 is copied into the parameter x, the body of square runs, and return x * x sends 49 back. The call square(7) is then replaced by that value, so int a = square(7); stores 49, and main carries on. A call is an expression, so it can be used anywhere a value can, like inside printf or another calculation.

Function steps on this site

From now on, some steps are function steps: you write only the function, and hidden tests call it with many different inputs and check what it returns. Don't write a main on those steps (the tests bring their own), and don't print anything the task didn't ask for; the tests look at the return value.

Your turn: complete add so it returns the sum of its two parameters.

Next: Write a function