How to use Iterator scan and inspect

Use inspect to observe values during iteration without changing them, and scan to maintain state across iterations by returning a new value for each step.

let numbers = vec![1, 2, 3, 4];

// inspect: prints each item without modifying the stream
numbers.iter().inspect(|n| println!("Inspecting: {}", n)).count();

// scan: accumulates a running total starting from 0
let sum: Vec<i32> = numbers.iter()
    .scan(0, |state, &n| {
        *state += n;
        Some(*state)
    })
    .collect();

println!("Running sums: {:?}", sum); // [1, 3, 6, 10]

Use inspect to peek at items in a list as you process them, like checking a conveyor belt without stopping it. Use scan to carry a value forward through the list, like keeping a running total on a calculator as you add numbers one by one.