Correction: The result in the example should be [1 2 3] of course. On Wednesday, June 24, 2015 at 6:34:20 PM UTC+2, Leon Grapenthin wrote: > > Using a set as filter predicate is idiomatic Clojure. > > If you don't know whether your filter coll is a set, you can write your > filter-contains as > > (def filter-contains (comp filter set)) > > E. g. use as > > (into [] (filter-contains [1 2 3]) [1 2 3 4]) > ;-> [4] > > I am not up to date about optimizations in the PersistentHashSet world, > but I'd assume for very small filter colls you can gain speed with a linear > scan instead of contains?. > > If you are ok with input, output and filter being a set, I'd recommend > looking at clojure.set/intersection for that task. It optimizes by reducing > the smaller set. Depending on the sizes of the collections you are dealing > with, the set conversion cost may be worth the speed up you can gain. > > > On Wednesday, June 24, 2015 at 12:07:06 AM UTC+2, Sam Raker wrote: >> >> Let's say that, as part of an xf, I want to filter out everything in a >> sequence that's also in some other sequence. Here are some ways of doing >> that: >> >> (defn filter-contains1 [edn-file] (remove (partial contains? (set (read- >> edn-file edn-file))))) >> >> (defn filter-contains2 [coll] (remove (partial contains? (set coll)))) >> >> (def filter-contains3 [coll] (let [coll-as-set (set coll)] (remove ( >> partial contains? (set coll))))) >> >> I have the strong suspicion that `filter-contains3` is the best of the 3, >> and `filter-contains1` the worst. The internal mechanics of transduce are a >> bit of a mystery to me, however: if `filter-contains2` were to be used on a >> collection of, say, a million items, would `coll` be cast to a set a >> million times, or is Clojure/the JVM smarter than that? I'm also wondering >> if anyone has any "best practices" (or whatever) they can share relating to >> this kind of intersection of transducers/xfs and closures. It seems to me, >> for example, that something like >> >> (defn my-thing [coll & stuff] >> (let [s (set coll)] >> ... >> (comp >> ... >> (map foo) >> (filter bar) >> (remove (partial contains? s)) >> ... >> >> is awkward, but that a lot of limited-use transducer factory functions >> (like the ones above) aren't exactly optimal, either. >> >
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