First post on this blog after a long hiatus – hopefully, this will stick. I was cleaning up my browser tabs, and found a Project Euler window open and decided to attempt some problems in Haskell. I’ve been writing Haskell for a couple of months now, and it was a great way to test myself. I picked one of the problems I attempted, a simple one – Problem 8: Largest product in a series.

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
...

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

Approach

Initially, I approached this with an imperative mindset – splitting the string, iterating over possible windows, using index-based iterations, etc. However, I stuck to a functional way:

Implementation

First, I did a little trick to retrieve just the string digits. I opened the browser terminal and ran the following:

'...'.trim().replace(/\s+/g, '')

Then, in Haskell, I initialised a variable to hold a list of digits:

import Data.Char (digitToInt)
import Data.List (tails)

let digitsM = map digitToInt "731671765313...428252483600823257530420752963450"
-- [7,3,1,6,7,1,7,6,5,3,1,3,3,0,6,2,4,9,1,9...]

Next step is to compute the greatest product by generating all contiguous substrings matching the expected length (13), and tracking the maxium from all computed products.

maximum $ map product [take 13 xs | xs <- tails digitsM, length xs >= 13]
-- => 23514624000

For a more idiomatic solution, we could also write it as:

largestProductInSeries :: Int -> String -> Int
largestProductInSeries n digits = 
    maximum $ map (product . take n) $ filter (\xs -> length xs >= n) $ tails $ map digitToInt digits

largestProductInSeries 13 "73167176531330624919225119674426574742355349194934..."
-- => 23514624000

Conclusion

I plan to keep tackling more Project Euler problems and writing about Haskell and functional programming here. Let’s see if I can keep up the habit—should be fun.