Depending on your iterator implementation (or, lackthere of), the functional boils down to your first example. For example, Rust iterators are lazily evaluated

Depending on your iterator implementation (or, lackthere of), the functional boils down to your first example. For example, Rust iterators are lazily evaluated with early-exits (when filtering data), thus it's your first form but as optimized as possible. OTOH python's map/filter/etc may very well return a full list each time, like with your intermediate. [EDIT] python returns generators, so it's sane. I would say that any sane language allowing functional-style data manipulation will have them as fast as manual for-loops. (that's why Rust bugs you with .iter()/.collect())