Sort a Clojure map by two or more keys

Wed, 21 Oct 2009 02:23:30 -0700

Note to self. Given this:

user> (def x [{:foo 2 :bar 11}
              {:bar 99 :foo 1}
              {:bar 55 :foo 2}
              {:foo 1 :bar 77}])

#'user/x

How do you sort by :foo, and where :foo is equal, sort by :bar?

Obviously not like this:

user> (sort x)
; Evaluation aborted.
;; java.lang.ClassCastException: clojure.lang.PersistentArrayMap cannot be cast to java.lang.Comparable

Also not like this:

user> (sort-by #(map % [:foo :bar]) x)
; Evaluation aborted.
;; java.lang.RuntimeException: java.lang.ClassCastException: clojure.lang.LazySeq cannot be cast to java.lang.Comparable

What's a guy got to do to find something Comparable? Well, vectors are Comparable, so we can do this:

user> (sort-by #(vec (map % [:foo :bar])) x)
({:foo 1, :bar 77} {:bar 99, :foo 1} {:foo 2, :bar 11} {:bar 55, :foo 2})

briancarper.net (λ) (Tag: Sorting)

Sort a Clojure map by two or more keys