Why is my bubblesort "if" statement not working?

Question

I'm trying to get a bubble-sort method working. The problem occurs in the if statement where I have to compare a number and the number on the next index. Here's the code:

numbers = [4, 2, 3, 1, 9]

def bubble_sort(arr)

  arr.each do |i|

    arr.each_index do |j|

      if arr[j] > arr[j+1]
        puts "works"
      end

    end     
  end
end #end method 

bubble_sort(numbers)

And this is the error I'm getting:

sorting.rb:11:in `>': comparison of Fixnum with nil failed (ArgumentError)
    from sorting.rb:11:in `block (2 levels) in bubble_sort'
    from sorting.rb:9:in `each'
    from sorting.rb:9:in `block in bubble_sort'
    from sorting.rb:7:in `each_index'
    from sorting.rb:7:in `bubble_sort'
    from sorting.rb:19:in `<main>'

By looking at the error message it seems I get an error because I compare to nil, but I don't see why.

Answer 1

One solution is to use an enumerator to make iteration efficient. See Enumerator. Here we use Array#each_index to extract an enumerator from the array. The solution is based from the Bubble sort described in Wikipedia.

#!/usr/bin/env ruby

numbers = [4, 2, 3, 1, 9]

def bubble_sort(arr)
  return unless arr.size > 1
  indices = arr.each_index
  begin
    swapped = false
    i = indices.next
    indices.each do |j|
      a = arr[i]
      b = arr[j]
      if a > b
        arr[i] = b
        arr[j] = a
        swapped = true
      end
      i = j
    end
    indices.rewind
  end while swapped
end

bubble_sort(numbers)

puts numbers.inspect

Output:

[1, 2, 3, 4, 9]