Functions, parameters, and return values

Why functions

• A function is a named piece of work. You write it once and call it whenever you need it.
• Functions keep programs short and clear, and let you build big jobs from small ones.
• You have already used printf — a function from <stdio.h>. Now you write your own.

Parameters and return values

• The values you pass in are the parameters. The value you hand back is the return value.
• int max2(int a, int b) takes two int parameters and returns one int.
• return x; ends the function right away and gives x back to the caller.

void functions, and building big from small

• A function that returns nothing has the return type void. It does work but hands back no value.
• One function can call another. max3 can call max2 twice to do its job.
• Small functions that each do one thing are easier to read and to fix.

C copies your arguments

• When you call a function, C gives it a copy of each argument.
• So changing a parameter inside the function does not change the caller's variable.
• Keep this in mind — in a later lesson, pointers let a function reach the original.

#include <stdio.h>

void try_double(int x) {
    x = x * 2;            // changes only the COPY
}

int main(void) {
    int n = 5;
    try_double(n);
    printf("%d\n", n);    // still 5 — the original did not change
    return 0;
}

Common mistakes

• A function used before it is defined needs a prototype above main.
• Arguments pass by value — pass a pointer to change the caller's variable.

Now you try

• Write each function with the exact name and parameters shown, and return the right value.
• You may call one of your functions from another.
• Do not write a main — the checker provides one.