How do I concatenate two slices in Rust?

Question

I want to take the x first and last elements from a vector and concatenate them. I have the following code:

fn main() {
    let v = (0u64 .. 10).collect::

        

Answer 1

Ok, first of all, your initial sequence definition is wrong. You say you want 1, 2, 3, 8, 9, 10 as output, so it should look like:

    let v = (1u64 .. 11).collect::>();

Next, you say you want to concatenate slices, so let's actually use slices:

    let head = &v[..3];
    let tail = &v[l-3..];

At this point, it's really down to which approach you like the most. You can turn those slices into iterators, chain, then collect...

    let v2: Vec<_> = head.iter().chain(tail.iter()).collect();

...or make a vec and extend it with the slices directly...

    let mut v3 = vec![];
    v3.extend_from_slice(head);
    v3.extend_from_slice(tail);

...or extend using more general iterators (which will become equivalent in the future with specialisation, but I don't believe it's as efficient just yet)...

    let mut v4: Vec = vec![];
    v4.extend(head);
    v4.extend(tail);

...or you could use Vec::with_capacity and push in a loop, or do the chained iterator thing, but using extend... but I have to stop at some point.

Full example code:

fn main() {
    let v = (1u64 .. 11).collect::>();
    let l = v.len();

    let head = &v[..3];
    let tail = &v[l-3..];

    println!("head: {:?}", head);
    println!("tail: {:?}", tail);

    let v2: Vec<_> = head.iter().chain(tail.iter()).collect();
    println!("v2: {:?}", v2);

    let mut v3 = vec![];
    v3.extend_from_slice(head);
    v3.extend_from_slice(tail);
    println!("v3: {:?}", v3);

    // Explicit type to help inference.
    let mut v4: Vec = vec![];
    v4.extend(head);
    v4.extend(tail);
    println!("v4: {:?}", v4);
}