Mysteriously empty arrays in a recursive Ruby instance method - Ruby 1.8.7

Question

I am struggling with some strange variable behavior in a recursive instance method. Running Ruby 1.8.7. I'm doing this as a learning exercise and have hit a bit of a wall. Any advice would be appreciated.

This code tries to figure out all of the different ways you can make a row of bricks with a length of max_length using bricks of lengths 1 ft, 2 ft, 3 ft, and 4ft.

The instance method add_block_to_row([0],10) will recursively add bricks of different lengths until putting together all possible combinations that satisfy the total length criteria. I am trying to store all of the different combinations of bricks in an instance variable, @rows, which is an array of arrays each representing a series of bricks in a row.

Everything works fine, EXCEPT, when I look at the rows variable of the instance it's full of a bunch of [0] arrays. The strange thing is that when I change @rows.push(row) to @rows.push(row.to_s) everything works fine. What's going on here? Why do the row arrays become empty when I try to put them in @rows? Below is the code and sample output for the two scenarios described here.

class Array; def sum; inject( nil ) { |sum,x| sum ? sum+x : x }; end; end

class Brick
  attr_accessor :rows

  def initialize(rows)
    @rows = rows
  end

  def add_brick_to_row(row,max_length)
    (1..4).each do |n|                                                                                                                                         
      # Add a brick if needed                                                                                                                                      
      if row.sum <= max_length
        row.push(n)
        if row.sum == max_length
          # If the row is complete add it to the rows matrix                                                                                                       
          # BELOW IS THE PROBLEMATIC LINE
          @rows.push(row)
          # EVERYTHING SEEMS TO WORK FINE WHEN I CHANGE row TO A STRING (SEE NEXT LINE)
          # @rows.push(row.to_s)
          print "pushed row: "
          print row.to_s+"\n"
        elsif row.sum < max_length
          # Recurse if more bricks are needed                                                                                                                       
          # print "adding brick\n"                                                                                                                                 
          self.add_brick_to_row(row, max_length)
        end
      end
      row.pop
    end

  end

end

Here's the output running in IRB. Normal case (where I have @rows.push(row.to_s)):

>> l.add_brick_to_row([0],3)
>> l.rows
=> [0, "0111", "012", "021", "03"]

Pathological case (where I have @rows.push(row)):

>> l.add_brick_to_row([0],3)
>> l.rows
=> [[0], [0], [0], [0]]

brentvatne · Accepted Answer · 2011-12-06 05:04:27Z

2

The problem is that you are pushing multiple references to the same row array into rows

Replace the push line with the following:

@rows.push(Array.new(row))

This will create a new array instance for each row and should fix your problem.

answered Dec 6, 2011 at 5:04

brentvatne

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1 Comment

NAD Over a year ago

Thanks brentvatne! That makes perfect sense.

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