Solution

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

/// Calculate the magnitude of the given vector.
fn magnitude(vector: &[f64; 3]) -> f64 {
    let mut mag_squared = 0.0;
    for coord in vector {
        mag_squared += coord * coord;
    }
    mag_squared.sqrt()
}

/// Change the magnitude of the vector to 1.0 without changing its direction.
fn normalize(vector: &mut [f64; 3]) {
    let mag = magnitude(vector);
    for item in vector {
        *item /= mag;
    }
}

fn main() {
    println!("Magnitude of a unit vector: {}", magnitude(&[0.0, 1.0, 0.0]));

    let mut v = [1.0, 2.0, 9.0];
    println!("Magnitude of {v:?}: {}", magnitude(&v));
    normalize(&mut v);
    println!("Magnitude of {v:?} after normalization: {}", magnitude(&v));
}

Note that in normalize we were able to do *item /= mag to modify each element. This is because we’re iterating using a mutable reference to an array, which causes the for loop to give mutable references to each element.
It is also possible to take slice references here, e.g., fn magnitude(vector: &[f64]) -> f64. This makes the function more general, at the cost of a runtime length check.

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Solution