Ruby factorial function

Question

I\'m going crazy: Where is the Ruby function for factorial? No, I don\'t need tutorial implementations, I just want the function from the library. It\'s not in Math!

Answer 1

Just another way to do it, although it really isn't necessary.

class Factorial
   attr_reader :num
   def initialize(num)
      @num = num
   end

   def find_factorial
      (1..num).inject(:*) || 1
   end
end

number = Factorial.new(8).find_factorial
puts number

Answer 2

You will probably find a Ruby feature request useful. It contains a nontrivial patch that includes a demo Bash script. The speed difference between a naive loop and the solution presented in the batch can be literally 100x (hundred fold). Written all in pure Ruby.

Answer 3

Using Math.gamma.floor is an easy way to produce an approximation and then round it back down to the correct integer result. Should work for all Integers, include an input check if necessary.

Answer 4

Just one more way to do it:

# fact(n) => Computes the Factorial of "n" = n!

def fact(n) (1..n).inject(1) {|r,i| r*i }end

fact(6) => 720

Answer 5

class Integer
  def !
    (1..self).inject(:*)
  end
end

examples

!3  # => 6
!4  # => 24

Answer 6

I just wrote my own:

def fact(n)
  if n<= 1
    1
  else
    n * fact( n - 1 )
  end
end

Also, you can define a falling factorial:

def fall_fact(n,k)
  if k <= 0
    1
  else
    n*fall_fact(n - 1, k - 1)
  end
end