Lazy evaluation
- Rust course
- Intermediate
- Lesson 52/115
- 4 min
Learn the lazy nature of iterators.
Lazy evaluation
Adapters are lazy: nothing is computed until a consuming method is called. That's why the compiler warns "iterators are lazy and do nothing unless consumed" for a map whose result isn't used:
let it = [1, 2, 3].iter().map(|x| {
println!("işleniyor {x}");
x * 10
});
println!("henüz yok"); // nothing processed yet
let ilk: Vec<i32> = it.take(2).collect(); // only 1 and 2 are processedThanks to laziness, even infinite iterators are used safely: (1..).map(|n| n * n).take(3).
Why laziness helps
Chained adapters don't create intermediate collections; each element passes through the whole pipeline one by one. The compiler usually turns such a chain into code as fast as a hand-written loop (a zero-cost abstraction):
// no intermediate Vec is created
let t: u64 = (1..=1_000_000u64)
.filter(|n| n % 2 == 0)
.map(|n| n * 3)
.sum();inspect(|x| ..) is handy in debugging for seeing values in the middle of the pipeline.
Practice Lazy pipeline
Write the example above: create a map printing işleniyor N for each element, print henüz yok first, then collect with take(2) and print the vector.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let veri = [1, 2, 3];
let it = veri.iter().map(|x| {
println!("işleniyor {}", x);
x * 10
});
println!("henüz yok");
let ilk: Vec<i32> = it.take(2).collect();
println!("{:?}", ilk);
}Expected output
henüz yok işleniyor 1 işleniyor 2 [10, 20]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How much work does <code>(1..).filter(|n| n % 7 == 0).next()</code> do?
- It loops forever
- It lists all numbers
- It doesn't compile
- It only tries 1 through 7 and returns Some(7) Correct answer
Explanation: next asks for only one element; the chain stops at the first match.
2 Take the first few
let ilk: Vec<i32> = it.____(2).collect();let ilk: Vec<i32> = it.take(2).collect();Explanation: take asks only for the first n elements; the rest is never processed.
3 Expression and behaviour
| Term | Meaning |
|---|---|
v.iter().map(f); |
Does nothing |
v.iter().map(f).count() |
f runs on each element |
(1..).take(3) |
Safe slice of infinity |
inspect(|x| ..) |
Observe in between |