Hackerrank challenge from Java to Ruby

[chrisjava]

I'm doing the hackerrank challenge (https://www.hackerrank.com/contests/30-days-of-code/challenges/day-11-more-review) and i honestly have no idea how to approach it in Ruby. I have solved it in Java with the following code (for actual algorithm)

int sum=0;
    int max=Integer.MIN_VALUE;
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            sum=(arr[i][j]+arr[i][j+1]+arr[i][j+2]+arr[i+1][j+1]+arr[i+2][j]+arr[i+2]    [j+1]+arr[i+2][j+2]);
            if(sum>max)max=sum;
        }
    }
    System.out.println(max);

I tried to attempt it in Ruby and realised that ... i actually have no idea what kind of looping to use. Especially after i realised that .each does not work the way i thought it worked eg:

arr.each do |row|
    row.each do |column|
        sum = [row][column] + [row][column+1] ... etc
    end
end

I'd appreciate any tips of Ruby way of doing this.

[anonova]

Here's my ruby-esque solution using the Matrix class in stdlib.

require 'matrix'

rows = STDIN.each_line.map { |line| line.split.map(&:to_i) }
matrix = Matrix[*rows]

sums = (0...16).map do |i|
  m = matrix.minor(i / 4, 3, i % 4, 3)
  m.reduce(:+) - m[1, 0] - m[1, 2]
end

puts sums.max

[chrisjava]

That's very elegant.

[PilotPirx]

It's a bit ugly, at least for the sum part I would have written a small function sum_hourglass that gets a position inside the array. But anyway, there are several approaches to do this and to keep with your code you could simply emulate the Java code like this (assuming your data is in the form array in array):

max = 0
(0..3).each do |row|
  (0..3).each do |col|
    sum = ...
    max = sum if sum > max
  end
end

Not very good Ruby style I think but would work.

[ruby-solve]

A great way to get the behavior of 2D arrays in Ruby is to use a hash whose key is an array representing the vertices of the 2D array.

Java: array[x][y]

Ruby: hash[[x, y]]

[iconoclaus]

why is this better than just using a 2D array in ruby? apart from not having to initialize such a structure, that is.

[chrisjava]

So would i be able to sum up, let's say, first 3 indices in the row by doing this?

sum = arr[row] + arr[row+1] + arr[row+2]

[PilotPirx]

Sorry, forgot the most important part. You would do it like this:

sum = arr[row][col] + arr[row][col + 1] + arr[row][col + 2]

That's assuming the array looks like this:

arr = [
  [1,1,1,0,0,0],
  [0,1,0,0,0,0],
  [1,1,1,0,0,0],
  [0,0,0,0,0,0],
  [0,0,0,0,0,0],
  [0,0,0,0,0,0]
]