Ace the Rust Interview

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intermediate · order 182

Custom Countdown iterator

iter-custom-01Related book chapters: 9 (optional — this page is complete on its own)iteratorcustomstate-machine

Slug: iter-custom-01 · Paired reading: Chapter 9

Prompt

Implement struct Countdown { remaining: u32 } with Countdown::new(n) and impl Iterator<Item = u32> that yields n, n-1, …, 1 then None. Yield nothing when n == 0.

Contract

  • After exhaustion, further next calls return None (fused-by-construction)
  • Store progress in the struct fields — do not hide the state machine in a range adapter inside next

Predict first

Where does the current value live between next calls?

Rust angle

Custom iterators are state machines: fields remember progress; next advances them.