Create array of empty subarrays in Ruby

All the proposed method to get this array [[], [], [],...] works fine:

Array.new(n, []) # the best, 20 times faster
Array.new(n){ [] }
n.times.map { [] }

The first is the fastest, so the best, but works strangely (see next).

it created an array with all the sub arrays being the same object

If i get the point, you mean that whit the methods described happens what follows:

a = Array.new(5, [])
p a # => [[], [], [], [], [], [], [], [], [], []]
p a.map { |e| e.object_id} # => [70189488740180, 70189488740180, 70189488740180, 70189488740180, 70189488740180]]

The object is the same, so if you try to fill subarrays with values, all of the subarrays assume the same value (replication):

a[0][0] = 10
p a # => [[10], [10], [10], [10], [10]]

To avoid this don't pass a default value, but map to empty array instead:

a = Array.new(5).map{ |e| [] }
p a # => [[], [], [], [], []]

Or pass the block

a = Array.new(5){ [] }
a = 5.times.map { [] }

Now each subarray is an independent object:

p a.map { |e| e.object_id} # => [70253023825640, 70253023825620, 70253023825600, 70253023825580, 70253023825560]

And if you insert some values there is no replication:

a[0][0] = 10
a[1][0] = 20
p a # => [[10], [20], [], [], []]

Please try this. It work for me.

  n = 15
  arr = Array.new(n)
  arr.map!{ |x| x = [] }

Now, putting value in the array.

arr[0][0] = 10 
arr[5][0] = 50
p arr # => [[10], [], [], [], [], [50], [], [], [], [], [], [], [], [], []]