`sum`

Adds up every number a lazy pipeline produces.

num sum(Iterable<num> iterable) Future<num> sumAsync(FxAsyncIterable<num> iterable) num Fx<num>.sum() // chain (sync) Future<num> FxAsync<num>.sum() // chain (async)

Lecture

sum is a terminal operator, and one of the simplest special-cased folds in the library: it's literally fold(0, (a, b) => a + b, iterable) underneath. Like every terminal, calling it pulls the entire lazy pipeline upstream of it — so you can build an elaborate chain of map/filter steps and only pay for the values sum actually needs, which is all of them.

Because the seed is 0, summing an empty iterable gives 0 — no error, no NaN. That's different from its neighbors min, max, and average, which each have their own (surprising!) empty-input behavior worth knowing about.

The chain form is available as an extension method, Fx<num>.sum() / FxAsync<num>.sum() — it only shows up once your chain's element type is known to be num (or int/double, thanks to generic covariance).

Demo 1 · Basics

Demo 2 · Async

Try it yourself

Exercise: sum only the even numbers in the list.

Related: average — sum divided by count · fold — the general form sum specializes · min · max — the other numeric terminals