ruby_array_insert_every.rb

class Array
  def insert_every!(skip, str)
    orig_ary_length = self.length
    new_ary_length = orig_ary_length + ((orig_ary_length-1) / skip)
    i = skip
    while(i<new_ary_length) do self.insert(i,str); i+=(skip+1) end
    self
  end
end

# a recursive version which is a little cleaner/more elegant:
class Array
  def insert_every!(skip, str, from=0, to=self.length)
    insert_every!(skip, str, from+skip, to) if from < to
    insert(from,str) unless from==0 || from==to
    self
  end
end

p %w[ 1 2 3 4 5 6 7 8 9 10 11 12 ].insert_every!(3, ',')
#=> ["1", "2", "3", ",", "4", "5", "6", ",", "7", "8", "9", ",", "10", "11", "12"]

# Spooner's version using range stepping
class Array
  def insert_every!(skip, str)
    (skip...size).step(skip).with_index {|n, i| insert(n + i, str) } 
    self
  end
end

p %w[ 1 2 3 4 5 6 7 8 9 10 11 12 ].insert_every!(3, ',')
#=> ["1", "2", "3", ",", "4", "5", "6", ",", "7", "8", "9", ",", "10", "11", "12"]