How to get & replace a value in Rust Vec?

Question

What I'm looking for is a replace method:

pub fn replace(&mut self, index: usize, element: T) -> T

Replaces an element at position index within the vector and returns the existing value.

Calling remove+insert seems wasteful to me.

Answer 1

You can emulate this easily on any container that gives you mutable references to its elements using std::mem::replace:

fn main() {
    let mut v = vec![1, 2, 3, 4, 5, 6];

    let got = std::mem::replace(&mut v[3], 42);

    println!("v = {:?}", v);
    println!("got = {:?}", got);
}

(Permalink to the playground)

Result:

v = [1, 2, 3, 42, 5, 6]
got = 4

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In the case of Vec, should you want to replace a range of elements, you would be interested in the method splice, which replaces values, and returns the old one. However, it is likely to be less efficient for a single value.