Question

Why both pieces of code are not printing the same thing. I was intending the first piece to produce the output of the second

a=Array.new(5,Array.new(3))
for i in (0...a[0].length)
  a[0][i]=2
end
p a

# this prints [[2, 2, 2], [2, 2, 2], [2, 2, 2], [2, 2, 2], [2, 2, 2]]*

a=Array.new(5).map{|d|d=Array.new(3)}
for i in (0...a[0].length)
  a[0][i]=2
end
p a

# this prints [[2, 2, 2], [nil, nil, nil], [nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

Answer 1

This one

a=Array.new(5,Array.new(3))

Creates an array that contains the same array object within it five times. It’s kinda like doing this:

a = []
b = a
a[0] = 123
puts b[0] #=> 123

Where this one:

a=Array.new(5).map{ Array.new(3) }

Creates a new 3 item array for each item in the parent array. So when you alter the first item it doesn’t touch the others.

This is also why you shouldn’t really use the Array and Hash constructor default arguments, as they don’t always work they way you might expect.