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

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)
}

Answer 2

You should take a look at the Vec::truncate function from the standard library, that can do this for you.

(playground)

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;
    let new_len = nums.len() - n;
    nums.truncate(new_len);

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

Answer 3

An alternate approach would be to use Vec::drain instead. This gives you an iterator so you can actually use the elements that are removed.

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;
    let new_len = nums.len() - n;

    for removed_element in nums.drain(new_len..) {
        println!("removed: {}", removed_element);
    }

    for retained_element in nums {
        println!("retained: {}", retained_element);
    }
}

The drain method accepts a RangeArgument in the form of <start-inclusive>..<end-exclusive>. Both start and end may be omitted to default to the beginning/end of the vector. So above, we're really just saying start at new_len and drain to the end.