clojure：根据其他键/值对在映射中计算新的键/值对

Question

I have a vector of maps which represents a DataFrame. I want to compute a new column (key/val pairs for all maps in the vector). Need some help to do this in the most efficient and idiomatic way.

(def dt [{:foo 10 :bar 20 :cat "A"}
         {:foo 15 :bar 10 :cat "B"}
         {:foo 12 :bar 15 :cat "C"}
         {:foo 16 :bar 22 :cat "A"}
         {:foo 13 :bar 11 :cat "B"}
         {:foo 10 :bar 19 :cat "C"}])

What I want to do is define :baz = :foo + :bar. I can get the computation done with a map operation as follows.

(map #(+ (:foo %) (:bar %)) dt)

I would prefer to write a function where the vector is maps is the last argument, so that I can chain operations with the thread-last macro

Something like this:

(->> dt
     (filter #(= (:foo %) 10))
     (compute-new-column #(...)))

I am beginner in both Clojure and FP, so any additional insights would be appreciated as well.

Answer 1

If I understand your question correctly you want to add a new key value pair to each map in your collection based on :foo and :bar .

(defn compute-new-column [row]
  (assoc row :baz (+ (:foo row) (:bar row))))

Then you can map this function in your thread macro. This is easily done with an anonymous function too as you are doing in your filter.

(->> dt
     (filter #(= (:foo %) 10))
     (map compute-new-column))


({:foo 10, :bar 20, :cat "A", :baz 30} {:foo 10, :bar 19, :cat "C", :baz 29})