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]; // `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)); }
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); }