Project Euler Problem 8

Ivar Thorson bio photo By Ivar Thorson

The definition of Problem 8 suggests some flexibility on how you treat the 1000-digit number: as a number or as a string. I decided that it would be easier to understand as the latter – using mod to extract digits seemed a little redundant and not really the point of the problem, which treats each digit as a distinct object. Why make things more complex than they need to be?

Because performance clearly isn’t an issue for this type of problem, I chose a slightly verbose style to see how self-documenting I could make it.

(defn euler-8 [string]
  "Finds the greatest product of five consecutive digits in string."
  (letfn [(by-fives   [s] (partition 5 1 s))
          (chars2ints [v] (map #(Integer/parseInt (str %)) v))
          (multiply   [v] (reduce * v))
          (max-of     [v] (reduce max v))]
    (max-of (map multiply (map chars2ints (by-fives string))))))
(euler-8 (concat "73167176531330624919225119674426574742355349194934"

The choice of strings and concat to represent the 1000-digit number is just to help me display it properly on the internet; clearly it could be written without line breaks, as a vector, or in several other forms.

Looking at other people’s solutions, I noticed that I was using the wrong function to convert characters to integers:

;; Original
(chars2ints [v] (map #(Integer/parseInt (str %)) v))

;; Better
(chars2ints [v] (map #(Character/getNumericValue %) v))

That’s good enough for government work, as they say. I’m not interested in nitpicking this uninteresting problem any further.