Elixir - Handling binaries, strings, and charlists (project euler 8)

Question

I was trying to solve some project Euler problems with Elixir but I stumbled upon a hard one I can't solve, and I think it's largely due to the fact that I still don't understand strings, chars and binaries.

Even after reading the docs on bin, strings and chars, I still don't get how to use them.

This is the problem I'm trying to solve.

This is my solution in Ruby (which I think is pretty simple and readable).

input = "
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450"
.gsub(/\s+/, '')

puts input.chars
          .map(&:to_i)
          .each_cons(13)
          .map { |seq| seq.reduce(:*) }
          .max

I tried to do sort of the same thing with Elixir:

#!/usr/bin/env elixir
defmodule Problem008 do
  def solve do
    "73167176531330624919225119674426574742355349194934
    96983520312774506326239578318016984801869478851843
    85861560789112949495459501737958331952853208805511
    12540698747158523863050715693290963295227443043557
    66896648950445244523161731856403098711121722383113
    62229893423380308135336276614282806444486645238749
    30358907296290491560440772390713810515859307960866
    70172427121883998797908792274921901699720888093776
    65727333001053367881220235421809751254540594752243
    52584907711670556013604839586446706324415722155397
    53697817977846174064955149290862569321978468622482
    83972241375657056057490261407972968652414535100474
    82166370484403199890008895243450658541227588666881
    16427171479924442928230863465674813919123162824586
    17866458359124566529476545682848912883142607690042
    24219022671055626321111109370544217506941658960408
    07198403850962455444362981230987879927244284909188
    84580156166097919133875499200524063689912560717606
    05886116467109405077541002256983155200055935729725
    71636269561882670428252483600823257530420752963450"
    |> Enum.chunk(13, 1)
    |> Enum.map(fn(subsequence) -> Enum.reduce(subsequence, &*/2) end)
    |> Enum.max
  end
end

IO.puts Problem008.solve

I know there's definitely something wrong about how I'm dealing with the input string.

score 3 · Accepted Answer · answered Sep 13 '17 at 09:13

3

The below is more-or-less one-to-one translation of your ruby code to Elixir:

input
|> String.codepoints()
|> Enum.filter(& String.trim(&1) != "")
|> Enum.map(&String.to_integer/1)
|> Enum.chunk(13, 1)
|> Enum.map(&Enum.reduce(&1, &Kernel.*/2))
|> Enum.max
#⇒ 23514624000

More performant version would be to reduce manually instead of producing a huge in-memory array of mapped integers:

input
|> String.codepoints()
|> Enum.filter(& String.trim(&1) != "")
|> Enum.chunk(13, 1)
|> Enum.reduce(0, fn e, acc ->              
     value = e
             |> Enum.map(&String.to_integer/1)
             |> Enum.reduce(&Kernel.*/2)
     if value > acc, do: value, else: acc
   end)
#⇒ 23514624000

answered Sep 13 '17 at 09:13

Aleksei Matiushkin

119,336
10
100
160

Thank you, @mudasobwa, that was great! Just one question, why should I use `&Kernel.*/2` instead of just `&*/2`? – Frank Kair Sep 15 '17 at 02:26
You should not. `Kernel` is imported, hence `&*/2` works perfectly. I just always put original modules names for clarity: to simplify code understanding for future myself. It’s a matter of a good habit for me. – Aleksei Matiushkin Sep 15 '17 at 04:50

score 2 · Answer 2 · answered Sep 13 '17 at 03:44

To convert a binary string to a list of integer digits, you can first convert it to a character (integer) list, then subtract off the character code for '0':

"7316717653133062491922511967442"
|> String.to_charlist()
|> Enum.map(fn x -> x - ?0 end)

[7, 3, 1, 6, 7, 1, 7, 6, 5, 3, 1, 3, 3, 0, 6, 2, 4, 9, 1, 9, 2, 2, 5, 1, 1, 9,
 6, 7, 4, 4, 2]

Elixir - Handling binaries, strings, and charlists (project euler 8)

2 Answers2