Getting Clojure

Map

Another thing we might want to do with a collection is transform it by evaluating a function on each element. We might, for example, have a collection of numbers and want a collection of all the numbers doubled. Here Clojure provides you with a choice.

Behind door number one is map. The map function takes a function and a collection and gives you back a sequence cooked up by applying the function to each member of the original collection. So if we started with some numbers:

(def some-numbers [1, 53, 811])

and we wanted to double them, we could write this:

(def doubled (map #(* 2 %) some-numbers))

Or if we had our collection of book maps and we just wanted the titles, we could do this:

(map (fn [book] (:title book)) books)

which ...