Reduce / Inject

my_inject.rb

class Array
  def my_inject(*args)
    acc = args.length >= 1 ? args[0] : self.shift
    self.each {|el| acc = yield acc, el}
    acc
  end
end

def assert_equal(actual, expected)
  puts "You expected #{expected} and got #{actual}" unless actual == expected
end 

assert_equal([1,2,3,4].my_inject(0) { |sum, el| sum + el },  10) # w/ a initial
assert_equal([1,2,3,4].my_inject { |sum, el| sum + el },  10) # w/o a initial

assert_equal([1,2,3,4].my_inject(0) { |sum, el| sum * el },  0) # w/ a initial
assert_equal([1,2,3,4].my_inject { |sum, el| sum * el },  24) # w/o a initial