In mathematics and computing, we often use the verb "filter" to describe the process of selecting a collection of items from some bigger collection based on whether or not they match a given predicate. For example, we "filter out all the even numbers from this set of numbers", or we "filter in all the prime numbers from this set".

It's this last term, "filter in", that I feel uneasy with. "Filter in" applies when you are merging from one lane to another on a motorway. The usage in the last paragraph doesn't seem to be plain English. What would you use?

  • You could look at existing questions about the word filter. This might be relevant.
    – Stuart F
    Nov 30, 2022 at 14:11
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    Does this answer your question? Can 'filtered' be ambiguous? Nov 30, 2022 at 16:00
  • @EdwinAshworth, it doesn't: the other question was about the ambiguity of filter when used without either preposition; this one is about whether in can be used with it. The accepted answer to the other question summarily condemns such usage, but that was not the focus of either the question or the answer. Given that both the OP and one commentator point out that such usage is not uncommon, it deserves to be explicitly addressed.
    – jsw29
    Nov 30, 2022 at 16:19
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    Flying aeroplanes can be dangerous. Nov 30, 2022 at 17:19
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    There are over 20k Google hits in a search for ["then filter in the" -"a filter"] (trying to disallow the many false positives hereabouts). They all (as far as I've scan-checked) seem computer-related. Obviously, filtering off/out is the mechanical process, leaving a filtered residue. Nov 30, 2022 at 17:26

1 Answer 1


I do not find any examples of this usage of filter as a verb with the preposition ‘in’ in OED online (subscription needed) and am not even familiar with ‘filter in’ in relation to traffic, although it may be exclusively US usage. I would therefore avoid the term in the context indicated, and use a construction without the preposition, e.g.

The set was filtered to leave only the prime numbers…

or use a different verb with greater connotations of positivity, e.g.

Prime numbers were selected from the set by…

  • Yes, I'd tend to adopt select being the positive variation of the more negative filter. I might select information by starting with a superset, and then filter out all the things I don't want. If I know what things I want at the outset, then I can positively select them. Nov 30, 2022 at 14:58
  • I'm quite used to intransitive filter in for traffic (UK NW). Also to transitive filter out as in filtered off/out the precipitated sulphur (but usually in the passive). But here I'd use filtered ... to leave [all the primes in this set]. One filters the starting material/set, and filters off/out the insoluble etc material (eg sulphur) / 'insoluble' or disallowed members of the set (eg composite numbers). Nov 30, 2022 at 16:06
  • There's somewhere I drive fairly often - I think it might be in Halifax in Yorkshire - where two lanes go down to one, and there is a road sign "Filter in turn".
    – Colin Fine
    Nov 30, 2022 at 16:41
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    @Colin That's the Vintrans + PP [idiomatic], not a MWV + DO (arguably [V+Pp] + [Comp]) construction. A1,B1,A2,B2 .... [Filter] [taking turns to do so]. Nov 30, 2022 at 17:17

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