Enter an integer and the program will return the factorial.

factorial.rb

#defining the method and argument
def factorial(number1)
  #string exception
  if number1.kind_of? String 
    "Enter a number, smartass."
  #array exception
  elsif number1.kind_of? Array
    "Just one number buddy."
  #negative numbers exception 
  else
    if number1 < 0
      "The factorial of negative numbers are undefined."
    #0 exception
    elsif number1 == 0
      1
      #Main calculation
    elsif number1.kind_of? Integer
      array1 = []
      array1 << number1
      array1 << number1 -= 1 until number1 <= 1
      array1.inject(1) {|x, y| x * y }
    #fraction exception
    else
      "Number must be an integer.  Think whole numbers."
    end
  end
end


#unit tests:
puts "'#{factorial('a')}' should be 'Enter a number, smartass.'"
puts "'#{factorial([1,2,3])}' should be 'Just one number buddy.'"
puts "'#{factorial(-5)}' should be 'The factorial of negative numbers are undefined.'"
puts "'#{factorial(0)}' should be '1'."
puts "'#{factorial(5)}' should be '120'."
puts "'#{factorial(12)}' should be '479001600'."
puts "'#{factorial(3.23)}' should be 'Number must be an integer.  Think whole numbers'."