Ruby block_given and yield example

ruby_block_given

class MyArray
  attr_reader :array

  def initialize(array)
    @array = array
  end

  def sum(initial_value = 0)
    return array.inject(:+) + initial_value unless block_given?
    sum = initial_value
    array.each {|n| sum += yield(n) }
    sum
  end
end

Problem Statement
Given a custom class MyArray,
write a 'sum' method that takes a block parameter.

Example:
my_array = MyArray.new([1, 2, 3])
my_array.sum gives 6
my_array.sum(10) gives 16
my_array.sum(0) {|n| n ** 2 } gives 14