Step 1 of 6

Storing values with let

A variable is a name for a value, so you can use the value later without typing it again. You create one with let:

fn main() {
    let player = "Ferris";
    let score = 120;
    println!("{} scored {} points", player, score);
}

Ferris scored 120 points

Piece by piece:

• let starts a variable declaration. Rust calls this binding a name to a value: after this line, the name score is bound to 120.
• The = means "is bound to", not "equals" in the math sense. The right side is worked out first, then the result gets the name on the left.
• The ; ends the statement, just like after println!.
• From then on, writing score (without quotes) means "the value bound to score". Here it fills a {} placeholder.

Calculating with variables

The right side can be any calculation, including one that uses other variables:

fn main() {
    let price = 250;
    let quantity = 3;
    let total = price * quantity;
    println!("{} x {} = {}", quantity, price, total);
}

3 x 250 = 750

Names make code explain itself: price * quantity says what is being computed, 250 * 3 doesn't.

A variable exists from its let line onward, and only inside the pair of braces { } it was declared in. That region of code is the variable's scope. Code runs from top to bottom, so a name can't be used before its let:

fn main() {
    println!("{}", score);
    let score = 120;
}

error[E0425]: cannot find value `score` in this scope
 --> src/main.rs:2:20
  |
2 |     println!("{}", score);
  |                    ^^^^^ not found in this scope

Common mistakes

• Forgetting let: score = 120; on its own means "change the existing variable score". If there isn't one, you get the same E0425 error as above.
• Quotes around the name: println!("{}", "score") prints the word score. A bare name is a variable; text in double quotes is a string literal.
• Spaces or dashes in names: let total price and let total-price don't work. Names are one word; join words with _: total_price.

Your task

Fill in the blanks: declare a variable coins holding 25, then print it, so the program prints Coins: 25.

Challenge 2

Fill in the blanks so the program adds up the length of two songs and prints Total: 7 minutes. Let Rust do the adding.

Challenge 3

Write it yourself: declare width as 7 and height as 6, then a third variable area that multiplies them, and print Area: 42. Let Rust work out the 42: don't type it yourself.

Next: Immutable by default, and mut