Nth element

Suppose you have a Vector in Rust, and wish to filter out every other element, or every N elements. This is similar to the Nth-child selector from CSS.

The retain function in Rust acts upon an element value, not its index. But we can pass a local variable (and increment that variable) to retain().

Output requirement

Consider a Vector of 4 i32 integers in Rust. When we get every other integer, we want a Vector that has the first, and third, elements remaining.

Input:  1, 2, 3, 4
Output: 1, 3
        (Every Nth(2))

To start, we introduce the every_nth_element—this receives a Vector of integers. We call every_nth_element twice in the program.

Part 1 We filter the Vector and get every other element—we must pass the argument 2 to indicate "every other."

Part 2 We get every third element from the Vector. Notice how the first and fourth elements, 10 and 40, are the ones remaining here.

Info In every_nth_element we start the index counter "c" at -1 as it is incremented before being tested.

fn every_nth_element(values: &mut Vec<i32>, n: i32) {
    // Retain elements with evenly divisible indexes.
    let mut c = -1;
    values.retain(|_| { c += 1; return c % n == 0 });
}

fn main() {
    // Part 1: retain every other element.
    // Be sure to borrow vectors mutably.
    let mut values1 = vec![1, 2, 3, 4];
    every_nth_element(&mut values1, 2);
    println!("EVERY OTHER: {:?}", values1);

    // Part 2: retain every third element.
    let mut values2 = vec![10, 20, 30, 40, 50, 60];
    every_nth_element(&mut values2, 3);
    println!("EVERY NTH(3): {:?}", values2);
}EVERY OTHER: [1, 3]
EVERY NTH(3): [10, 40]

We can pass a function argument to retain() on a vector to keep certain elements in the vector. And we can filter on indexes with a local variable.