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FxDart 入门 › 章节 4 · 过滤 & 去重 › compress

compress

Keeps the elements of an iterable wherever a parallel list of booleans is true.

Lecture

compress is a positional mask: element i of iterable survives only if selectors[i] is true. It's built directly out of two functions you already know — map((r) => r.$2, filter((r) => r.$1, zip(selectors, iterable))) — zip the mask with the data, filter to the true pairs, then unwrap. That also means it inherits zip's behavior on mismatched lengths: iteration stops as soon as the shorter of selectors or iterable runs out, so a short selector list silently truncates the result. Reach for it when you already have (or can cheaply compute) a boolean mask up front — for example, "which quiz answers were correct" — rather than recomputing a predicate per element the way filter would.

There's no chain method; call the data-first function or its async counterpart directly, or wrap the result with fx(...) / fxAsync(...) to keep chaining.

Demo 1 · Basics & length mismatch

Demo 2 · Async, with concurrency

Try it yourself

Exercise: use compress to keep only the correct answers.