4 ms·

> … to call them Select and Where. While map is a great name, I always struggle to remember if ‘filter’ keeps elements that match the condition or removes them

• seanw444 18d ago
  I've never run into a generic "filter" function which keeps only the non-matching elements.
  • jdougan 18d ago
    Smalltalk has #reject: which does that. You could, of course, just wrap a not around the test in the closure, but sometimes reject with a well-named predicate is easier to read. bsnpApproved := tvShows reject: [ :eachShow | eachShow hasNaughtyContent ].
  • dreamcompiler 18d ago
    Common Lisp's filter is remove-if which works this way. (It also has remove-if-not but that's deprecated and if you use it your code smells.)
    • shawn_w 18d ago
      `remove-if-not` was deprecated before the Common Lisp standard was approved and yet it remained (and will never be removed because the standard will never be updated). That's not deprecated for any practical purpose. And it's more convenient than using `(remove-if (complement #'some-predicate) sequence)` Scheme has `filter` and `filter-not` in the SRFI-1 list library. Both of which can easily be written using a fold to bring this vaguely on topic.
    • kazinator 16d ago
      Nope! Common Lisp's filter is in fact remove-if-not, which is exactly the same thing as "keep if": keep all items which match the predicate (removing those that do not). I suspect the reason the function was deprecated was its naming, nothing more; had it been called retain-if or keep-if, it would not have attracted deprecating attention. The smell added to your code is just the double-negative name of that function, not what it's doing for you. The name filter smells even more. Is that filtering for items that match? Or filtering out? In physical filters, sometimes the filtrate is considered the payload output (that which passes through the filter) and sometimes the retentate (that which is caught in the filter). keep-if is readable.
      • dreamcompiler 15d ago
        I agree that keep-if is a better name. But you reiterated my point: remove-if works in the counterintuitive way the original comment noted, i.e. not like the common connotation of 'filter.' As for deprecation, IIRC the '-if-not' functions were deprecated because the committee felt the 'complement' function accomplished that task better. Edit: My IIRC seems largely correct. More detail at https://www.lispworks.com/documentation/HyperSpec/Issues/iss345_w.htm https://www.lispworks.com/documentation/HyperSpec/Issues/iss...
        • kazinator 14d ago
          Nonetheless, they were fooled by that function, because it's just keep-if by a funny name that includes "not" suggesting that it contains a complement that might be factored out. If you want keep-if, you don't want to use a different function, which forces you to complement your predicate. If you want (keep-if #'redp jellybean-list) to keep the red jelly beans, you don't want to write (remove-if (complement #'not-red-p) jellybean-list). If keep-if has a silly name remoe-if-not, you might nonetheless prefer (remove-if-not #'redp jelly-bean-list). Shims like complements are ugly, and compilers won't optimize through them for arbitrary function definitions (whose source code is not even in scope), so it is good to have both keepers and removers. Heck, it's useful to have a function which does both in one pass returning two values: the filtrate and the retentate.
• pavlov 18d ago
  Maybe those two could be filter_for (the “where” case) and filter_out.
  • listenallyall 18d ago
    Kotlin has filter and filterNot (it also has separate "reduce" and "fold" functions, dependingon whether you want to specify an initial accumulator value or not)
• dcminter 18d ago
  If you're making tea with a colander something is very wrong ;)
  • NooneAtAll3 18d ago
    depends on the size of the sieve, but sometimes one does cook a whole stewpot of tea at once (f.e. in canteen)
    • dcminter 17d ago
      A colander would allow the tea leaves through to no purpose. A colander is a type of sieve but a sieve is not necessarily a colander.
  • SamBam 18d ago
    I was thinking an apt analogy might be making stock -- you filter out all the solid food you don't want to keep in the liquid. And it's a doubly-good analogy, because I have occasionally gotten that confused in real-life as well. Twice in the past ten years I've had a stock boil away for three hours, and then set a colander in the sink and poured it through, only to watch my beautiful stock swirl down the drain because motor-memory made me forget that I wasn't draining pasta but should have put the colander in a bowl...
    • dcminter 17d ago
      This is painfully relatable.
• reddit_clone 18d ago
  Talking about un-guessable, misleading function names, C++ std::remove. I would never have guessed what it does exactly. (It moves elements that match the filter to the front, and moves the end-marker forward. Leaves all the elements in the collection. You need to erase them yourself. )
  • mitxela 18d ago
    It returns the new end marker
  • anitil 18d ago
    Oh it's a bit like unordered-delete when using an arena. I guess I would have expected an ordered-delete instead
• andrekandre 18d ago
  > I always struggle to remember if ‘filter’ keeps elements that match the condition or removes them if you had parameter names maybe it might help? `filter(where:)` like in swift...?
  • mikebenfield 18d ago
    Doesn't seem to help the ambiguity to me.
    • andrekandre 17d ago
      maybe its the verb? what about `exclude()` or `keep()`? in those cases its less ambiguous (to me anyways) that returning true means 'yes' to 'keep' or 'exclude', whereas saying yes or no to filter is like 'filter to exclude or include?' thats my take anyway
• quaverquaver 18d ago
  select/reject (Ruby)
• deleted 18d ago
  [deleted]
• saghm 18d ago
  There's always the Ruby strategy of just making all the names work. `select` and `filter` are buddies and you can use whichever you want or even go back and forth. Not a fan of `reduce`? That's fine, `inject` has got your back. Miss getting to type `collect` from Java or Rust? Don't worry, just use it instead of `map`, it's the same thing.
  • brabel 18d ago
    I believe Ruby uses names inspired by SmallTalk.
    • arnsholt 18d ago
      Yeah, Smalltalk has select, reject (inverted filter), collect (map) and inject.
    • saghm 17d ago
      Right, I'm just saying that it has a lot of aliases, which is fairly uncommon in most mainstream languages
• rmunn 18d ago
  The filter keeps the tea... it's just that you then lift the filter out of the cup, carrying the tea with it. Flip your brain around to see it from that direction and it might help you with the mnemonics.
• thaumasiotes 18d ago
  > While map is a great name, I always struggle to remember if ‘filter’ keeps elements that match the condition or removes them. In Common Lisp both functions exist, under the names `remove-if` and `remove-if-not`.
• ihumanable 17d ago
  In elixir we have Enum.filter (run a predicate over the enumerable keeping the things that match the predicate) and Enum.reject (run a predicate over the enumerable removing the thing that match the predicate) I think since we have a pair of them and reject is so obvious it helps me remember which way filter works. I think Enum.keep and Enum.reject might be a better pair, but I've used them enough to internalize it now