class Array

def in_groups_of(number, fill_with = nil, &block)

["5"]
["3", "4"]
["1", "2"]
%w(1 2 3 4 5).in_groups_of(2, false) {|group| p group}

["5", " "]
["3", "4"]
["1", "2"]
%w(1 2 3 4 5).in_groups_of(2, ' ') {|group| p group}

["10", nil, nil]
["7", "8", "9"]
["4", "5", "6"]
["1", "2", "3"]
%w(1 2 3 4 5 6 7 8 9 10).in_groups_of(3) {|group| p group}

padding any remaining slots with +fill_with+ unless it is +false+.
Splits or iterates over the array in groups of size +number+, 
def in_groups_of(number, fill_with = nil, &block)
  if number.to_i <= 0
    raise ArgumentError,
      "Group size must be a positive integer, was #{number.inspect}"
  end
  if fill_with == false
    collection = self
  else
    # size % number gives how many extra we have;
    # subtracting from number gives how many to add;
    # modulo number ensures we don't add group of just fill.
    padding = (number - size % number) % number
    collection = dup.concat(Array.new(padding, fill_with))
  end
  if block_given?
    collection.each_slice(number, &block)
  else
    collection.each_slice(number).to_a
  end
end