Two-dimensional array in Ruby

Question

I have the two-dimensional array

a = [[1,'a'],[1,'b'],[2,'c'],[2,'d'],[3,'e'],[3,'f'],[4,'g'],[4,'h']] 

and I want to separate the letters in another array b so that it looks like this:

[["a","b"], ["c", "d"], ["e", "f"], ["g", "h"]] 

Where content each index of the new array(Ex: ["a", "b"] matches the first item of the array to (a[0][0] and a[1][0]...etc), I did this to try to resolve the issue:

• first created the two-dimensional array

  for i in (1..4)
    b[i] = Array.new
  end
  

• then the code

  c = 0
  x = 0
  a.each do |k|
    if  k[0] == a[x+1][0]
      b[k[0]][c] = k[1]
      c += 1
      x += 1
    else      
      b[k[0]][c] = k[1]
      c = 0
    end
  end
  

The problem is the index 'c' from the third index of array b, is going to index 2 and the next progressing 4 is coming out like this...

[nil, ["a", "b"], ["c", "d"], [nil, nil, "e", "f"], [nil, nil, nil, nil, "g", "h"]]

I think it might be a simple thing on this index, but I’m already mixing things up.

Answer 1

Try to use less index manipulation, which makes code confusing. Ruby usually allows this.

The code below creates elements in b for each index found in the elements of a. Then add to this element in b the elements found in a for that index.

a = [[1,'a'],[1,'b'],[2,'c'],[2,'d'],[3,'e'],[3,'f'],[4,'g'],[4,'h']] 
b = Array.new
a.each do |k|
  b[k[0]] ||= Array.new
  b[k[0]] << k[1]
end
b # => [nil, ["a", "b"], ["c", "d"], ["e", "f"], ["g", "h"]]

Answer 2

The beautiful thing about Ruby is that almost all these types of processing can be written in one line, without using loops directly. It’s like working with a functional language. Note:

a = [[1,'a'],[1,'b'],[2,'c'],[2,'d'],[3,'e'],[3,'f'],[4,'g'],[4,'h']]
result = a.inject([]) {|r, k| r[k[0]] ||= []; r[k[0]] << k[1]; r }

Inject serves to build a value iteratively. You pass the value as argument and, at each iteration, receive the current value and an argument and return the next value. A little more explained:

result = a.inject([]) do |temp, element|
   index, value = element
   temp[index] ||= []
   temp[index] << value
   temp
end

You can use the inject! also to apply the modification inplace.