4.4 Loops

Explanation

Rust for loops can iterate over ranges or over borrowed data. Ranges such as 0..xs.len() are useful when the algorithm needs positions. Rust indexing is 0-based, so the first element has index 0.

fn sum_by_index(xs: &[f64]) -> f64 {
    let mut total = 0.0;
    for i in 0..xs.len() {
        total += xs[i];
    }
    total
}

When the algorithm only needs values, iterate over the slice directly.

fn sum_by_value(xs: &[f64]) -> f64 {
    let mut total = 0.0;
    for x in xs {
        total += *x;
    }
    total
}

Both versions read the same slice. The choice is about whether the position i is part of the algorithm.

Things to look up

for loop
Range 0..n
Slice iteration
0-based indexing

Exercise

For the slice [2.0, 4.0, 6.0], compute the sum by hand and write down the value of total after each loop step. Then implement a small sum(xs: &[f64]) -> f64 function and compare the result.

Notes for the exercise

Prefer iterator forms when indices are not needed.
Use indices when the algorithm depends on positions.
Include the empty slice as a boundary case when testing a sum function.