Although vectors are best suited for procedural programming, I would like to use a map function on them. The following snippet works:
fn map<A, B>(u: &Vec<A>, f: &Fn(&A) -> B) -> Vec<B> {
let mut res: Vec<B> = Vec::with_capacity(u.len());
for x in u.iter() {
res.push(f(x));
}
res
}
fn f(x: &i32) -> i32 {
*x + 1
}
fn main() {
let u = vec![1, 2, 3];
let v = map(&u, &f);
println!("{} {} {}", v[0], v[1], v[2]);
}
Why isn't there any such function in the standard library? (and also in std::collections::LinkedList). Is there another way to deal with it?
Answer
Rust likes to be more general than that; mapping is done over iterators, rather than over solely vectors or slices.
A couple of demonstrations:
let u = vec![1, 2, 3];
let v: Vec<_> = u.iter().map(f).collect();
let u = vec![1, 2, 3];
let v = u.iter().map(|&x| x + 1).collect::<Vec<_>>();
.collect() is probably the most magic part of it, and allows you to collect all the elements of the iterator into a large variety of different types, as shown by the implementors of FromIterator. For example, an iterator of Ts can be collected to Vec<T>, of chars can be collected to a String, of (K, V) pairs to a HashMap<K, V>, and so forth.
This way of working with iterators also means that you often won’t even need to create intermediate vectors where in other languages or with other techniques you would; this is more efficient and typically just as natural.