src/fn/closures/closure_examples/iter_find.md

# Searching through iterators

`Iterator::find` is a function which iterates over an iterator and searches for the
first value which satisfies some condition. If none of the values satisfy the
condition, it returns `None`. Its signature:

```rust,ignore
pub trait Iterator {
    // The type being iterated over.
    type Item;

    // `find` takes `&mut self` meaning the caller may be borrowed
    // and modified, but not consumed.
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item> where
        // `FnMut` meaning any captured variable may at most be
        // modified, not consumed. `&Self::Item` states it takes
        // arguments to the closure by reference.
        P: FnMut(&Self::Item) -> bool;
}
```

```rust,editable
fn main() {
    let vec1 = vec![1, 2, 3];
    let vec2 = vec![4, 5, 6];

    // `vec1.iter()` yields `&i32`.
    let mut iter = vec1.iter();
    // `vec2.into_iter()` yields `i32`.
    let mut into_iter = vec2.into_iter();

    // `iter()` yields `&i32`, and `find` passes `&Item` to the predicate.
    // Since `Item = &i32`, the closure argument has type `&&i32`,
    // which we pattern-match to dereference down to `i32`.
    println!("Find 2 in vec1: {:?}", iter.find(|&&x| x == 2));
    
    // `into_iter()` yields `i32`, and `find` passes `&Item` to the predicate.
    // Since `Item = i32`, the closure argument has type `&i32`,
    // which we pattern-match to dereference down to `i32`.
    println!("Find 2 in vec2: {:?}", into_iter.find(|&x| x == 2));

    let array1 = [1, 2, 3];
    let array2 = [4, 5, 6];

    // `array1.iter()` yields `&i32`
    println!("Find 2 in array1: {:?}", array1.iter().find(|&&x| x == 2));
    // `array2.into_iter()` yields `i32`
    println!("Find 2 in array2: {:?}", array2.into_iter().find(|&x| x == 2));
}
```

`Iterator::find` gives you a reference to the item. But if you want the _index_ of the
item, use `Iterator::position`.

```rust,editable
fn main() {
    let vec = vec![1, 9, 3, 3, 13, 2];

    // `position` passes the iterator’s `Item` by value to the predicate.
    // `vec.iter()` yields `&i32`, so the predicate receives `&i32`,
    // which we pattern-match to dereference to `i32`.
    let index_of_first_even_number = vec.iter().position(|&x| x % 2 == 0);
    assert_eq!(index_of_first_even_number, Some(5));

    // `vec.into_iter()` yields `i32`, so the predicate receives `i32` directly.
    let index_of_first_negative_number = vec.into_iter().position(|x| x < 0);
    assert_eq!(index_of_first_negative_number, None);
}
```

### See also:

[`std::iter::Iterator::find`][find]

[`std::iter::Iterator::find_map`][find_map]

[`std::iter::Iterator::position`][position]

[`std::iter::Iterator::rposition`][rposition]

[find]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.find
[find_map]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.find_map
[position]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.position
[rposition]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.rposition