Rust: apply the function to each element of vector

November 2, 2022

Given the following function:

fn some_function<K, F: Fn(K) -> K>(f: F, vs: Vec<K>) -> Vec<K> {
    let mut index = 0;
    let new_vec = vs.iter().map(|x| {
        index += 1;
        for _ in 1 .. index {
            x = f(x); // <- error here: mismatched types expected reference `&K` found type parameter `K`
        }
        *x
    }).collect();
    new_vec
}

How can i make it work ?

>Solution :

This example doesn’t work because .iter() will not produce an iterator of owned variables, but references to elements of vs. You could patch your code by turning it into into_iter():

fn some_function<K>(f: impl Fn(K) -> K, vs: Vec<K>) -> Vec<K> {
    let mut index = 0;
    vs.into_iter()
    .map(|mut x| {
        index += 1;
        for _ in 1..index { x = f(x); }
        x
    })
    .collect()
}

But you don’t need to keep track of index:

fn some_function<K>(f: impl Fn(K) -> K, vs: Vec<K>) -> Vec<K> {
    vs
    .into_iter()
    .enumerate()
    .map(|(i, mut x)| {
        for _ in 0..i { x = f(x); }
        x
    })
   .collect()
}