Searching through iterators - Rust By Example
Description: Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries.

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# 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:

    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;
    }

    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`.

    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`

`std::iter::Iterator::find_map`

`std::iter::Iterator::position`

`std::iter::Iterator::rposition`