Question

Can someone explain this?

def digit_block(size = 1)
  col = 2 + 1*size
  row = 1 + 2*size
  r = []
  for i in 0...col
    r.push ' '
  end
  a = []
  for i in 0...row
    a.push r
  end
  a
end

block = digit_block
puts block.inspect
block[1][2] = 'x'
puts block.inspect

outputs:

[[" ", " ", " "], [" ", " ", " "], [" ", " ", " "]]
[[" ", " ", "x"], [" ", " ", "x"], [" ", " ", "x"]]

My understanding is block[1][2] only changes the cell at row 1 column 2, but why it changes all the cell in column 2?

Answer 1

  for i in 0...row
    # you are pushing the same array object to an array
    a.push r
  end

So every element in block is the same object.

block[0] === block[1]  # true
block[1] === block[2]  # true

Update:

You need to make a new array for every element, your code could be rewrote as below:

def digit_block(size = 1)
  Array.new(1 + 2*size){Array.new(2 + size){' '}}
end