What is the idiomatic way to pop the last N elements in a mutable Vec?

Question

I am contributing Rust code to RosettaCode to both learn Rust and contribute to the Rust community at the same time. What is the best idiomatic way to pop the last n elements in a mutable Vec?

Here's roughly what I have written but I'm wanting to see if there's a better way:

fn main() {
    let mut nums: Vec<u32> = Vec::new();
    nums.push(1);
    nums.push(2);
    nums.push(3);
    nums.push(4);
    nums.push(5);

    let n = 2;
    for _ in 0..n {
        nums.pop();
    }

    for e in nums {
        println!("{}", e)
    }
}

(Playground link)

Answer 1

I'd recommend using Vec::truncate:

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

    let n = 2;
    let final_length = nums.len().saturating_sub(n);
    nums.truncate(final_length);

    println!("{:?}", nums);
}

Additionally, I

• used saturating_sub to handle the case where there aren't N elements in the vector
• used vec![] to construct the vector of numbers easily
• printed out the entire vector in one go

---

Normally when you "pop" something, you want to have those values. If you want the values in another vector, you can use Vec::split_off:

let tail = nums.split_off(final_length);

If you want access to the elements but do not want to create a whole new vector, you can use Vec::drain:

for i in nums.drain(final_length..) {
    println!("{}", i)
}