How do I convert a Vec to Vec<&str>?

Question

I can convert Vec to Vec<&str> this way:

let mut items = Vec::<&str>::new();
for item in &ano         


        

Answer 1

All of the answers idiomatically use iterators and collecting instead of a loop, but do not explain why this is better.

In your loop, you first create an empty vector and then push into it. Rust makes no guarantees about the strategy it uses for growing factors, but I believe the current strategy is that whenever the capacity is exceeded, the vector capacity is doubled. If the original vector had a length of 20, that would be one allocation, and 5 reallocations.

Iterating from a vector produces an iterator that has a "size hint". In this case, the iterator implements ExactSizeIterator so it knows exactly how many elements it will return. map retains this and collect takes advantage of this by allocating enough space in one go for an ExactSizeIterator.

You can also manually do this with:

let mut items = Vec::<&str>::with_capacity(another_items.len());
for item in &another_items {
    items.push(item);
}

Heap allocations and reallocations are probably the most expensive part of this entire thing by far; far more expensive than taking references or writing or pushing to a vector when no new heap allocation is involved. It wouldn't surprise me if pushing a thousand elements onto a vector allocated for that length in one go were faster than pushing 5 elements that required 2 reallocations and one allocation in the process.

Another unsung advantage is that using the methods with collect do not store in a mutable variable which one should not use if it's unneeded.