CODEWITHSUNDEEP
rust
njaard
njaard

Reputation: 549

How does one have a for loop borrow the iterator?

How does one have a for ... in loop borrow the iterator it is operating over? For example:

let x = vec![1, 2, 3, 4];
let i = x.iter();
for a1 in i { break; } // iterate over just one "i"
for a2 in i { break; } // continue iterating through "i" here

You can't simply give &i to the for, because then it can't convert the &Iterator to an Iterator object.

Upvotes: 3

Views: 2358

Answers (3)

Shepmaster
Shepmaster

Reputation: 432139

You can take a mutable reference to the iterator (&mut i):

let x = vec![1, 2, 3, 4];
let mut i = x.iter();
for a1 in &mut i { break; }
for a2 in &mut i { break; }

This is the same as what Iterator::by_ref does internally.

Upvotes: 1

malbarbo
malbarbo

Reputation: 11197

You can use Iterator::by_ref to borrow the iterator and continue to use it after the borrow ends:

fn main() {
    let x = vec![1, 2, 3, 4];
    let mut i = x.iter();
    for _ in i.by_ref() { break; } // iterate over just one "i"
    for _ in i.by_ref() { break; } // continue iterating through "i" here
    assert_eq!(Some(&3), i.next())
}

Upvotes: 4

AndrewBrinker
AndrewBrinker

Reputation: 51

All iterators have a next() function that advances the iterator and returns an Option<Self::Item> (that is, they return either None or a Some containing a value of whatever type you're iterating over. You can call this function yourself to manually increment the iterator however many times you want, which sounds like it would solve your problem in this case.

Upvotes: 1

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