rust vector iterator in multi-layer loop

Question

I try to use iterator on vector slices, but it just doesn't work.

My code are as follows

  pub fn three_sum(nums: Vec<i32>) -> Vec<Vec<i32>> {
        let mut res: Vec<Vec<i32>> = Vec::new();

        for (n1, &i1) in nums.iter().enumerate() {
            for (n2, &i2) in nums[(n1 + 1)..].iter().enumerate() {
                for (n3, &i3) in nums[(n2 + 1)..].iter().enumerate() {
                    if i1 + i2 + i3 == 0 {
                        res.push(Vec::from([i1, i2, i3]));
                    }
                }
            }
        }
        return res;
    }

I expected the n2 loop in nums ranging n1 to the end, but it just loop from the beginning, regardless of what n1 is.

Same happened on n3.

Did I use iterators and slices correctly?

Answer 1

There is no way for enumerate to know where you want to start counting from so it always starts at 0. You can use zip with a range if you want to start counting from a different number.

pub fn three_sum(nums: Vec<i32>) -> Vec<[i32;3]> {
    let mut res = Vec::new();

    for (n1, &i1) in nums.iter().enumerate() {
        for (n2, &i2) in (n1+1..).zip(nums[(n1 + 1)..].iter()) {
            for (n3, &i3) in (n2+1..).zip(nums[(n2 + 1)..].iter()) {
                if i1 + i2 + i3 == 0 {
                    res.push([i1, i2, i3]);
                }
            }
        }
    }
    res
}

Answer 2

As you can see in the docs , enumerate just counts the number of iterations. If you want to skip the first n elements of an iterator, you should use the skip function instead, which also more clearly expresses your intent.

nums.iter().enumerate().skip(n)

Note the order of enumerate and skip however: this way you're first constructing an enumerated iterator, then skipping some elements, so your index will also "start from" n . The other way around you'll skip th elements first, then count them, starting from 0.