Rust by Example

8.3 Higher Order Functions

Rust provides Higher Order Functions (HOF). These are functions that take one or more functions and/or produce a more useful function. HOFs and lazy iterators give Rust its functional flavor.

fn is_odd(n: u32) -> bool {
    n % 2 == 1
}

fn main() {
    println!("Find the sum of all the squared odd numbers under 1000");
    let upper = 1000;

    // Imperative approach
    // Declare accumulator variable
    let mut acc = 0;
    // Iterate: 0, 1, 2, ... to infinity
    for n in 0.. {
        // Square the number
        let n_squared = n * n;

        if n_squared >= upper {
            // Break loop if exceeded the upper limit
            break;
        } else if is_odd(n_squared) {
            // Accumulate value, if it's odd
            acc += n_squared;
        }
    }
    println!("imperative style: {}", acc);

    // Functional approach
    let sum_of_squared_odd_numbers: u32 =
        (0..).map(|n| n * n)             // All natural numbers squared
             .take_while(|&n| n < upper) // Below upper limit
             .filter(|&n| is_odd(n))     // That are odd
             .fold(0, |sum, i| sum + i); // Sum them
    println!("functional style: {}", sum_of_squared_odd_numbers);
}

Option and Iterator implement their fair share of HOFs.