4.5 Functions

Explanation

A function signature defines an input and output contract. The name, argument types, and return type say what data the function expects and what it returns.

Small functions are easier to test and review. They also make it easier to avoid hidden global mutable state, because data must be passed in through the function arguments or returned as a value.

fn square(x: f64) -> f64 {
    x * x
}

fn sum(xs: &[f64]) -> f64 {
    xs.iter().sum()
}

The sum function reads a slice and does not take ownership of the underlying data.

Things to look up

• Function signature
• Return type
• Slice argument
• Option
• Unit test

Exercise

Implement mean(xs: &[f64]) -> Option<f64>. Return None for an empty slice and Some(value) for a non-empty slice.

Notes for the exercise

• The empty slice is a boundary case.
• Use a small hand-checkable input such as [1.0, 2.0, 3.0].
• Keep the function independent of hidden global state.