Is there a vector `insert_slice_at` or equivalent?

1

Searching around I found this question from 4 years ago; "inserting multiple items into a vec efficiently" which also links a very small discussion from 9 years ago; "add vec insert slice at...", and another post on reddit, also from 4 years ago, "vec prepend insert from slice".

I would like to ask the following:

  • Since I can't find any other discussions since, has there been any developments on such a method?
  • If I'm to implement this functionality myself, am I safe to assume that the (unsafe) code from the 9 year old post "add vec insert slice at...", linked here is sound, and a good implementation (by your interpretation of "good")?
2

This is Vec::splice – it’s a lot more general than just “insert_slice_at” but that use case should be optimal according to the doc.

2 Likes
3

Another way without the iterator dance is to use extend_from_slice() and rotate_right(): Playground

fn insert_slice<T: Clone>(vec: &mut Vec<T>, index: usize, slice: &[T]) {
    assert!(index <= vec.len());
    vec.extend_from_slice(slice);
    vec[index..].rotate_right(slice.len());
}
1 Like
4

Unfortuantly this replaces the prior elements rather than shifting them to the positions after the insert length. My expected output would be something like the following:

let mut v = vec![1, 2, 3, 7, 8, 9];
let new = [4, 5, 6];
let u: Vec<_> = v.insert_slice(1, new).collect();
assert_eq!(v, &[1, 2, 3, 4, 5, 6, 7, 8, 9]);

Or the equivalent but mutating the vector v.

5

Splice does that. You provide an empty range, so no element will be removed.

6

No, you simply use a zero-length range as the insert location: playground

7

My mistake. Thanks for the clarification. I will mark your answer as the correct one as it is the std approach, however I'm definitely going to benchmark it against paramagnetics approach.

8

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.