Reverse Job Interview Problem

Yesterday on twitter, this tweet got a lot of attention in the developer world.

It states:

Most candidates cannot solve this interview problem:

Small orange diamond Input: "aaaabbbcca" Small orange diamond Output: [("a", 4), ("b", 3), ("c", 2), ("a", 1)]

Write a function that converts the input to the output

I ask it in the screening interview and give it 25 minutes

How would you solve it?

I noticed that Clojurian Gene Kim gave this response.

(->> (partition-by identity "aaaabbbcca")
     (map (fn [x]
            [(->> x first str) 
             (count x)])))

I'm a huge fan of finding a use for partition-by.

On Clojurians, Savo Djuric shared his solution, which he blogged here.

defn chop-chop [coll]
  (let [x (partition-by identity coll)]
    (map list (map (comp str first) x)
                (map count x))))
=> (chop-chop "aaaabbbcca")
=> (("a" 4) ("b" 3) ("c" 2) ("a" 1))

All of this conversation made me wonder, what if I did it backwards? I'd like to create a function with the input of

[("a", 4), ("b", 3), ("c", 2), ("a", 1)]

and output of the string

"aaaabbbcca".

So I started playing around with it. I gave myself the restriction that the input

[("a", 4), ("b", 3), ("c", 2), ("a", 1)]

is a vector of lists. Not a list of vecs like Kim's solution and and not list of lists, like Djuric's.

This is what I've come up with.

(def alpha-num-tuples ['("a" 4) '("b" 3) '("c" 2) '("a" 1)])

(defn tuple-to-chars [tuple] (repeat (last tuple) (first tuple)))

(clojure.string/join (mapcat tuple-to-chars alpha-num-tuples))
;; "aaaabbbcca"

The function tuple-to-chars takes ones of the tuples such as ("a" 4)

and returns ("a" "a" "a" "a").

The result of mapcat-ing all of the tuples-turned-chars is ("a" "a" "a" "a" "b" "b" "b" "c" "c" "a").

And then simply join the chars to a string!

"aaaabbbcca"

There is probably a more beautiful or idiomatic way. Let me keep playing with it.