Rust: from idea to program · Lesson 01 / 08

Values that can change

In the Rullst quiz, points change when someone answers correctly. Now let’s look at the language used to write that rule: Rust.

Let's discover together

1
A name can hold a value: let energy = 2;. Read it as “the name energy starts with value 2.” The semicolon ends this statement.
2
By default, we cannot assign a new value to the same binding. let mut energy = 2; allows changes. mut means mutable; use it when change belongs in the rule.
3
energy += 3; adds 3 to the current value. It is short for energy = energy + 3;. The right-hand side is calculated before storing the result.
4
fn main() { ... } is this program’s entry point. println! displays a message in the terminal. Here we assemble and predict the snippet; the final project can run on your computer.

✦

YOUR MISSION

Build a small energy update and predict the final value.

Now it's your turn

Discovery workshop

Power up the robot

Starting state: 2 units. The station adds 3. Choose the pieces and predict the printed number.

01Word after let to allow the update02Operator that adds 3 to the current value03Value that will appear in the terminal

Your snippet in progress

let {1} energy = 2;
energy {2} 3;
println!("{energy}");
// {3} ?

Build your answers and check when you are ready.

A moment to understand

Why did we add mut?

To allow assigning a new value to energy.To make every program faster.

I need a hint

mut is about allowing value changes, not speed.

Loading your journey…

Take it with you

Predict the result if energy starts at 7 and gains 2. Then ask: does a name that never changes need mut?

Show me the Rust behind it

This part is optional. You don't need to memorize the code to continue.

fn main() {
    let mut energy = 2;
    energy += 3;
    println!("{energy}");
}

At your own pace

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