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);
}