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Rust: from idea to program · Lesson 05 / 08
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One function, one responsibility

The same scoring rule appears on several screens. We can write it once, give it a name and test its behavior.

Learning paths9 min

Let's discover together

  1. 1

    fn total(items: u32, each: u32) -> u32 defines a function named total. It receives two nonnegative integers and returns another. The names describe each input’s role.

  2. 2

    The body { items * each } multiplies the inputs. The final expression, without a semicolon, becomes the returned value. return also exists, but is unnecessary here.

  3. 3

    Calling total(3, 4) gives items the value 3 and each the value 4. The function returns 12. Returning is not the same as displaying: println! handles display.

  4. 4

    A small function makes normal and boundary cases easier to test. Our example uses small quantities; a real application would also need limits and handling for possible numeric overflow.

✦
YOUR MISSION

Read a function’s inputs and output and calculate three calls, including a zero input.

Now it's your turn

Discovery workshop

One rule, three calls

Function: fn total(items: u32, each: u32) -> u32 { items * each }. All values in this activity fit the type.
Build your answers and check when you are ready.
A moment to understand

Which body returns the multiplication as its final expression?

I need a hint

In this return form, the final expression does not end with ;.

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Take it with you

Name a function that receives a quantity and returns double. Write two input/output examples before writing its body.

Show me the Rust behind it

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

fn total(items: u32, each: u32) -> u32 {
    items * each
}

fn main() {
    println!("{}", total(3, 4));
}
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