4clojure Notes

(solutions and notes when I worked on 4clojure problems)

Difficulty: Elementary

16. Hello World

the most straightforward is using str:

#(str "Hello, " % "!")

but I found immo's solution:

format "Hello, %s!"

is simple too (not a function as required, but it passes the unit tests :D)

72. Rearranging Code: ->>

also the solution is simple: reduce +

but apply + also works.

156. Map Defaults

my solution:

#(apply hash-map (interleave %2 (repeat %1)))

other solutions:

when transforming from one collection type to another, can always use reduce (austintaylor's solution):

(fn [x s] (reduce #(assoc %1 %2 x) {} s))

and immo's neat solution:

#(zipmap %2 (repeat %))

Difficulty: Easy

19. Last Element

my solution: (comp first reverse)

also works: #(first (reverse %)) and #(-> % reverse first)

learned that when there's only one argument passing to each function, use comp

another interesting solution is (comp peek vec) by immo.

22. Count a Sequence

my solution: #(reduce + (map (constantly 1) %))

and cheated solution from immo: #(.size (vec %))

26. Fibonacci Sequence

my solution:

(fn [n]
  (take n

(map first (iterate (fn [[x y]] [y (+ x y)]) [1 1]))))

the best is _pcl's solution:

;; TDD principle: Write the minimum amount of code required to make the test pass :)
(fn [i] (take i '(1 1 2 3 5 8 13 21))) 

28. Flatten a Sequence

my solution:

(fn f [s] (if (sequential? s) (mapcat f s) (list s)))

however, others are using coll? instead of sequential?

32. Duplicate a Sequence

my solution:

(fn [s] (reduce (fn [acc x] (conj acc x x)) [] s)) 

the best is #(interleave % %)

another one is mapcat #(list % %)

38. Maximum value

my solution:

(fn [& n] (reduce (fn [x acc] (if (> x acc) x acc)) 0 n))

not good, I know, especially comparing to immo's

#(last (sort %&))