Drain, what is it for

1

What is a drain method on iterator used for?
Thanks

2

It is used to remove a range of elements from a vector, optionally giving you the elements in the range.

1 Like
3

Why clear couldn't be used?

4

Clear removes all values in the vector, but drain lets you remove only a sub-range.

let mut vec = vec![1, 2, 3, 4, 5];
vec.drain(1..=3);
println!("{:?}", vec);

[1, 5]

Additionally, the drain function returns an Iterator, which means you can see the elements as they get removed, which clear also does not let you do.

let mut vec = vec![1, 2, 3, 4, 5];
for num in vec.drain(1..=3) {
    println!("{} was removed.", num);
}
println!("{:?}", vec);

2 was removed.
3 was removed.
4 was removed.
[1, 5]
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5

I don't see it mentioned much, but splice is another handy method in the same family.

You can consider drain to be a special case of splice where the input (replace_with) iterator is empty.

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6

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