Rust by Example

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
(&mut self, predicate: P) -> Option 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];

// `iter()` for vecs yields `&i32`.

let mut iter = vec1.iter();

// `into_iter()` for vecs yields `i32`.

let mut into_iter = vec2.into_iter();

// `iter()` for vecs yields `&i32`, and we want to reference one of its

// items, so we have to destructure `&&i32` to `i32`

println!("Find 2 in vec1: {:?}", iter     .find(|&&x| x == 2));

// `into_iter()` for vecs yields `i32`, and we want to reference one of

// its items, so we have to destructure `&i32` to `i32`

println!("Find 2 in vec2: {:?}", into_iter.find(| &x| x == 2));

let array1 = [1, 2, 3];

let array2 = [4, 5, 6];

// `iter()` for arrays yields `&i32`

println!("Find 2 in array1: {:?}", array1.iter()     .find(|&&x| x == 2));

// `into_iter()` for arrays unusually yields `&i32`

println!("Find 2 in array2: {:?}", array2.into_iter().find(|&&x| x == 2));

}

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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];

let index_of_first_even_number = vec.iter().position(|x| x % 2 == 0);

assert_eq!(index_of_first_even_number, Some(5));

let index_of_first_negative_number = vec.iter().position(|x| x < &0);

assert_eq!(index_of_first_negative_number, None);

}

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