Remove, vecElements can be added to a vector with push, but we often need to remove them later. With remove() and swap_remove() this is possible.
Using the swap_remove function is often much faster, but it often disturbs the ordering of elements in the vector. So avoid it if ordering matters.
To begin we create a vector of 4 elements with the "vec" macro. We then call remove() to eliminate the first element—the value 10 no longer is present in the vector.
swap_remove on the first element (index 0). This eliminates the 20, but it swaps the last element for the 20.fn main() {
let mut items1 = vec![10, 20, 30, 40];
println!("{:?}", items1);
// Use remove.
items1.remove(0);
println!("{:?}", items1);
// Use swap remove.
items1.swap_remove(0);
println!("{:?}", items1);
}[10, 20, 30, 40]
[20, 30, 40]
[40, 30]Moving elements forward on each removal can be slow—a lot of memory copying must occur. Thus with swap_remove() we can see a performance improvement.
swap_remove instead. This changes the ordering of the elements, but it avoids copying most of them.swap_remove function is much faster. In a real program, remove() could cause noticeable slowdowns, but swap_remove would be instant.use std::time::*;
fn main() {
if let Ok(max) = "100000".parse::<usize>() {
let mut items1 = vec![100; max];
let mut items2 = items1.clone();
// Version 1: use remove.
let t0 = Instant::now();
for _ in 0..max {
items1.remove(0);
}
println!("{}", t0.elapsed().as_millis());
// Version 2: use swap remove.
let t1 = Instant::now();
for _ in 0..max {
items2.swap_remove(0);
}
println!("{}", t1.elapsed().as_millis());
println!("{}/{}", items1.len(), items2.len());
}
}393 ms
0 ms (swap_remove)
0/0With swap_remove, we avoid shifting elements in a vector. It removes by index like the remove() function. It is important to carefully choose the best element removal function.