By the end of this lesson
Build a readable transformation pipeline and consume it.
Before you start
- Rust 1.90.0 or later, a terminal and a plain-text editor. Examples were tested with Rust 1.90.0, Edition 2024 on Linux.
- Complete lesson 19, or be comfortable explaining and running its examples.
Three words to know
- Iterator
- An object that produces a sequence of items.
- Closure
- A small function-like value that can capture its environment.
- Consumer
- An operation such as sum or collect that drives iteration.
1. Understand the idea
Iterator adaptors such as filter and map describe work but usually do not perform it until a consumer is used. A closure uses vertical bars to declare its parameters. Keep each stage simple enough to explain in plain language. Iterator syntax is not automatically clearer than a loop; use the version that makes validation and limits easiest to see.
2. Save, run and observe
Save this complete program as lesson.rs in a fresh practice folder. It replaces that lesson file only; keep earlier examples separately. Compile it with rustc, then run the executable. Rust source goes in the editor; rustc and cargo commands go in the terminal.
fn main() {
let values = [1, 2, 3, 4];
let total: i32 = values.into_iter().filter(|n| n % 2 == 0).map(|n| n * 10).sum();
println!("total={total}");
}
What you should see · example output
total=60
rustc --edition=2024 lesson.rs -o lesson
./lesson3. Explain the result
filter retains 2 and 4. map produces 20 and 40. sum consumes those results and returns 60. The type annotation tells sum which result type to build. For a pipeline reading user data, preserve and handle errors rather than silently filtering failed parses away.
YOUR TURN
A small check before you continue
What result follows if the filter keeps odd numbers instead?
Show a suggested answer
40: the values 1 and 3 become 10 and 30.
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.