Collections and Iterators

let mut v = vec![1, 2];
v.push(3);
v  // [1, 2, 3]

use std::collections::HashMap;
let mut m = HashMap::new();
*m.entry("a").or_insert(0) += 1;
m["a"]  // 1

let v: Vec<_> = (1..=5).filter(|n| n % 2 == 0).map(|n| n * 10).collect();
v  // [20, 40]

let ok: Result<Vec<_>, _> = ["1", "2"].iter().map(|s| s.parse::<i32>()).collect();
ok.unwrap()  // [1, 2]

use std::collections::BTreeMap;
let mut m = BTreeMap::new();
m.insert(2, "b");
m.insert(1, "a");
m.keys().copied().collect::<Vec<_>>()  // [1, 2]

use std::collections::HashSet;
let mut s = HashSet::from([1, 2, 2]);
s.insert(3);
s.len()  // 3

use std::collections::VecDeque;
let mut q = VecDeque::from([1, 2]);
q.push_front(0);
q.pop_back()  // Some(2)

let v: Vec<_> = ["a", "b"].iter().enumerate().map(|(i, s)| (i, *s)).collect();
v  // [(0, "a"), (1, "b")]

(1..=4).fold(0, |a, b| a + b)  // 10

[1, 2, 3].iter().any(|n| *n == 2)  // true
[1, 2, 3].iter().all(|n| *n > 0)  // true

let v: Vec<_> = [[1, 2], [3]].into_iter().flatten().collect();
v  // [1, 2, 3]

let a = [1, 2, 3, 4];
a.chunks(2).map(|c| c.len()).collect::<Vec<_>>()  // [2, 2]

use std::collections::BinaryHeap;
let mut h = BinaryHeap::from([1, 5, 3]);
h.pop()  // Some(5)

let mut v = vec![1, 2, 3, 4];
v.retain(|n| n % 2 == 0);
v  // [2, 4]

let mut v = vec!["bb", "a", "ccc"];
v.sort_by_key(|s| s.len());
v  // ["a", "bb", "ccc"]