Iterator adaptors: map, filter, collect and lazy evaluation

Developers write explicit for loops when iterator adaptor chains would be more concise, and are surprised that iterator chains do nothing until consumed.

Chain iterator adaptors and collect into the desired collection type:

let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8];

// Filter evens, double them, collect into Vec
let result: Vec<i32> = numbers.iter()
    .filter(|&&x| x % 2 == 0)
    .map(|&x| x * 2)
    .collect();
println!("{:?}", result); // [4, 8, 12, 16]

// Collect into a HashMap
use std::collections::HashMap;
let map: HashMap<&str, usize> = vec!["apple", "banana", "cherry"]
    .iter()
    .map(|&s| (s, s.len()))
    .collect();

// fold for custom aggregation
let sum: i32 = (1..=100).fold(0, |acc, x| acc + x);

// flat_map to flatten nested iterators
let words: Vec<&str> = vec!["hello world", "foo bar"]
    .iter()
    .flat_map(|s| s.split_whitespace())
    .collect();

Iterators in Rust are lazy — adaptors like map and filter return new iterator structs that do no work. The computation only happens when a consumer (collect, for_each, sum, fold) drives the iterator.