Rust: from idea to program · Lesson 03 / 08

The decision at the boundary

A door should open with ten points or more. Nine, ten and eleven are neighbors, but reveal whether we wrote the right rule.

Let's discover together

1
if takes one path when a condition is true. else describes the other path. The condition must be bool, not just “any nonzero number.”
2
points >= 10 means “points is greater than or equal to 10.” points > 10 excludes 10. One symbol changes the boundary.
3
In Rust, if can also produce a value: let open = if points >= 10 { true } else { false };. Here the comparison already produces bool and can be used directly.
4
Test below, exactly at and above the boundary. These small cases reveal mistakes a single distant value could hide.

✦

YOUR MISSION

Test both sides of the boundary and the boundary itself before approving the condition.

Take it with you

If the rule becomes “fewer than five attempts,” which three numbers would you test? Remember that < excludes the boundary.

Show me the Rust behind it

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

fn main() {
    for points in [9, 10, 11] {
        let open = points >= 10;
        println!("{points}: {open}");
    }
}

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The decision at the boundary

Let's discover together

Now it's your turn

The ten-point door

Take it with you

My files and GitHub

Your file for this lesson

Save to my GitHub

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