Use `.map()` when you want to turn each item from an iterator into a new item. Give it a closure—a small piece of code that receives one item, such as `|num| num * 2`. The result is another iterator that will produce the transformed items. section 8.2 section 12.1
fn main() {
let num_vec = vec![2, 4, 6];
let double_vec: Vec<i32> = num_vec #1
.iter() #2
.map(|num| num * 2) #3
.collect(); #4
println!("{:?}", double_vec);
}
Starting with `[2, 4, 6]`, the closure doubles each number. `.collect()` puts the results into a new `Vec<i32>`, which prints as `[4, 8, 12]`. section 8.3.2
Why collect? Iterator `.map()` is lazy: it prepares the transformation but does not run it until the iterator is used. Leaving a mapped iterator unused produces a compiler warning, not a panic. `.collect()` is one way to use it and make a vector. section 8.3.2
Choose how items enter the transformation:
- `.iter()` supplies references. Mapping them into a new vector leaves the original vector available. section 8.2
- `.into_iter()` supplies values. After it takes a vector by value, that original vector is no longer available. section 8.2
- For a different job—changing existing items in place—use `.iter_mut()` with `.for_each()` rather than mapping into a new vector. section 8.2 section 8.3.1
Arrays also have `.map()`. It returns an array of the same length, possibly with a different item type, without a separate `.collect()` call. For example, `[1, 5, 9]` can become an array of strings by mapping each number with `|i| i.to_string()`. section 20.1.2
