Iterators in Depth

fn main() {
    let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    // Core operations
    let sum: i32 = v.iter().sum();
    let product: i32 = v.iter().product();
    let max = v.iter().max();
    let count = v.iter().count();
    println!("sum={} product={} max={:?} count={}", sum, product, max, count);

    // map: transform each element
    let doubled: Vec = v.iter().map(|&x| x * 2).collect();

    // filter: keep matching elements
    let evens: Vec<&i32> = v.iter().filter(|&&x| x % 2 == 0).collect();

    // filter_map: filter AND transform in one step
    let strs = vec!["1", "two", "3", "four", "5"];
    let nums: Vec = strs.iter().filter_map(|s| s.parse().ok()).collect();
    println!("{:?}", nums); // [1, 3, 5]

    // fold: reduce to a single value
    let sum2 = v.iter().fold(0, |acc, &x| acc + x);

    // any / all
    println!("{}", v.iter().any(|&x| x > 9));   // true
    println!("{}", v.iter().all(|&x| x > 0));   // true

    // find / position
    println!("{:?}", v.iter().find(|&&x| x > 5));      // Some(&6)
    println!("{:?}", v.iter().position(|&x| x == 5));  // Some(4)

    // Chain multiple operations (zero-cost at runtime!)
    let result: Vec = (1..=20)
        .filter(|x| x % 3 == 0)
        .map(|x| x * x)
        .take(4)
        .map(|x| format!("{}", x))
        .collect();
    println!("{:?}", result); // ["9", "36", "81", "144"]
}