1 · Values and control flow · 25 MIN

Bindings, expressions and functions

Rust distinguishes mutable bindings from immutable ones.

let creates an immutable binding unless mut is requested. A function’s final expression can be its returned value without a return keyword. Adding a semicolon changes that expression into a statement, which can make the function return the unit type instead of the declared integer. The example keeps calculation separate from println. Fixed-width integers have explicit ranges, and overflow behavior varies by build settings; use checked arithmetic when overflow is part of your input contract.

Read the compiler’s expected and found types before changing code.

Read the example

fn total(price: i32, quantity: i32) -> i32 { price * quantity }
fn main() {
    let price = 12;
    let mut quantity = 2;
    quantity += 1;
    println!("{}", total(price, quantity));
    println!("{}", total(price, 0));
}

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Your challenge

Add a function returning a discounted total while rejecting negative inputs with Option.

Solution cost: O(1) fixed-width arithmetic. time · O(1) auxiliary state. space

Common trap

A trailing semicolon on the last expression can remove the intended return value.

fn total(price: i32, quantity: i32) -> i32 { price * quantity }
fn main() {
    let price = 12;
    let mut quantity = 2;
    quantity += 1;
    println!("{}", total(price, quantity));
    println!("{}", total(price, 0));
}

Further reading: Official documentation