Functions and control flow

In plain English

A function is declared with fn and can return a value; the last expression in a function is its return value when there is no semicolon. A statement does something and returns nothing, while an expression produces a value, which is why an if can be used as an expression that returns a value. if and else choose between branches, loop repeats forever until it breaks, while repeats while a condition holds and for walks over a range or collection. A range is written a..b for a half-open range that includes a but not b, and a..=b for an inclusive range that includes both ends.

Step by step

1. 1Write an fn that returns the value of its last expression.
2. 2Use if/else if/else as an expression and store the result.
3. 3Break out of a loop when a condition is met.
4. 4Walk a range with for, choosing 1..4 or 1..=3 depending on whether the end is included.

fn level(minutes: u32) -> u32 {
    if minutes < 10 {
        1
    } else if minutes < 20 {
        2
    } else {
        3
    }
}

fn main() {
    let level = level(12);
    println!("level {}", level);

    let mut count = 0;
    loop {
        count += 1;
        if count >= 3 {
            break;
        }
    }
    println!("looped {} times", count);

    while count < 5 {
        count += 1;
    }
    println!("count now {}", count);

    for i in 1..4 {
        println!("lesson {}", i);
    }

    for i in 1..=3 {
        println!("inclusive {}", i);
    }
}

Common mistakes

• Putting a semicolon after the final expression of a function turns it into a statement, so the function returns the unit type instead of your value.
• Writing if as a statement when you need its value forces repeated assignments, when the whole if could be one expression.
• Using 1..5 expecting it to include 5 is wrong, because a..b excludes b; use a..=b to include it.
• Forgetting to break inside a loop that should end makes the loop run forever.