How would you combined sort-by and filter in Clojure?

Question

Let's say I have a collection of strings, and I want to return all strings over 4 characters long, sorted by shortest string first.

You could solve that with something along the lines of:

(def strings ["this" "is" "super" "cool" "everybody" "isn't" "clojure" "great!?!?"])
(sort-by count < (filter #(> (count %) 4) strings))
;; > ("super" "isn't" "clojure" "everybody" "great!?!?")

Notice we're using count twice. This is probably fine here, but what if count wasn't count? What if instead of count we called super-expensive-function that we'd really rather not run more than absolutely necessary?

So:

• We have a collection of things
• We want to return an ordered collection of things
• Filtered and sorted using the result of a computationally expensive function which should only be called once per thing

Is there an existing function that does this, or do I need to build my own?

Answer 1

The simplest solution would be to pair up each item together with its expensive-to-compute property, then filter and sort, then discard the parameter:

(->> strings
     (map (juxt identity count))
     (filter (fn [[_ c]] (> c 4)))
     (sort-by peek)
     (map first))

If computing the property in question is really all that expensive, the overhead of allocating the vectors should pretty much vanish.