Step 1 of 6

Defining and calling functions

You've been writing one function all along: main. A program can have as many functions as you like. A function is a named block of code that you can run, or call, whenever you need it, by writing its name followed by ().

fn main() {
    println!("Match starting");
    print_divider();
    println!("Round 1");
    print_divider();
}

fn print_divider() {
    println!("----------");
}

Match starting
----------
Round 1
----------

• fn print_divider() { ... } defines the function: its name and what it does. Defining a function doesn't run it.
• print_divider(); calls it: the program jumps into the function, runs its body, then comes back and continues with the line after the call.
• print_divider is defined below main, and that's fine: in Rust, the order of functions in a file doesn't matter.
• Function names use snake_case, like variables. A good name says what the function does, usually with a verb: print_divider, save_game, reset_score.

Why write functions?

• No repetition: write the code once and call it many times. A fix then happens in one place.
• Names explain code: print_receipt(); tells the reader what happens without reading the body.
• Small pieces: each function can be understood, and tested, on its own. "This function is too long" is one of the most common code-review comments.

Functions can call functions

Any function can call any other. When a call finishes, the program returns to exactly where it was:

fn main() {
    start_round();
    start_round();
}

fn start_round() {
    println!("Fight!");
    cheer();
}

fn cheer() {
    println!("The crowd cheers");
}

Fight!
The crowd cheers
Fight!
The crowd cheers

Common mistakes

• Forgetting the (): print_divider; names the function but doesn't call it. Nothing happens, and the compiler warns path statement with no effect.
• A typo in the name: print_dividr(); gives error[E0425]: cannot find function `print_dividr` in this scope.
• Defining a function inside main's braces by accident: it works, but it can only be used inside main. Put functions side by side at the top level of the file.

Your task

Fill in the blanks: define a function called cheer and call it twice from main, so the program prints Go, Ferris! two times.

Challenge 2

Fill in the blanks so main calls intro, and intro calls rules. The program should print:

Welcome to the arena
Rule 1: no sandals

Challenge 3

Write a function print_banner that prints these three lines, and call it twice from main (six lines in total):

+--------+
| ARENA  |
+--------+

Next: Parameters