keys and vals could be any kind of collection. Calling seq will return either nil or a seq with at least one value. By forcing empty collections to nil allows the (and ks vs) to work.
That is if keys is [] and vals is [1], then: (and keys vals) is [1] (a truthy value) But (seq []) is nil and (seq [1]) is (1) and (and nil (1)) is nil (a falsey value). There are a lot of ways zipmap's performance can be improved (particularly if keys and vals are reducible) and there is a ticket where some ideas have been discussed: http://dev.clojure.org/jira/browse/CLJ-1005 On Tuesday, November 29, 2016 at 9:42:48 PM UTC-6, larry google groups wrote: > > I apologize for this question, because I think it has been asked before, > and yet I can not find the answer. > > In the definition of zipmap, what do these 2 lines do? > > ks (seq keys) > vs (seq vals) > > In particular, what is (seq) doing? Is this to ensure that ks is false if > "keys" is empty? And vs is false if "vals" is empty? Because an empty list > (or vector) is truthy but we want it to be falsey in this situation? > > Is there any other reason to use (seq), other than to set the > truthy/falsey values of ks and vs? > > > (defn zipmap > "Returns a map with the keys mapped to the corresponding vals." > {:added "1.0" > :static true} > [keys vals] > (loop [map {} > ks (seq keys) > vs (seq vals)] > (if (and ks vs) > (recur (assoc map (first ks) (first vs)) > (next ks) > (next vs)) > map))) > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
