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

In math, factorial of n is just the gamma function of n+1
(see: http://en.wikipedia.org/wiki/Gamma_function)

Ruby has Math.gamma() so just use Math.gamma(n+1) and cast it back to an integer if desired.

Answer 2

Shamelessly cribbed from http://rosettacode.org/wiki/Factorial#Ruby, my personal favorite is

class Integer
  def fact
    (1..self).reduce(:*) || 1
  end
end

>> 400.fact
=> 64034522846623895262347970319503005850702583026002959458684445942802397169186831436278478647463264676294350575035856810848298162883517435228961988646802997937341654150838162426461942352307046244325015114448670890662773914918117331955996440709549671345290477020322434911210797593280795101545372667251627877890009349763765710326350331533965349868386831339352024373788157786791506311858702618270169819740062983025308591298346162272304558339520759611505302236086810433297255194852674432232438669948422404232599805551610635942376961399231917134063858996537970147827206606320217379472010321356624613809077942304597360699567595836096158715129913822286578579549361617654480453222007825818400848436415591229454275384803558374518022675900061399560145595206127211192918105032491008000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

This implementation also happens to be the fastest among the variants listed in Rosetta Code.

update #1

Added || 1 to handle the zero case.

update #2

With thanks and appreciation to Mark Thomas, here's a version that is a bit more efficient, elegant and obscure:

class Integer
  def fact
    (2..self).reduce(1,:*)
  end
end

Answer 3

Why would the standard library require a factorial method, when there is a built-in iterator for this exact purpose? It is called upto.

No you do not need to use recursion, like all these other answers show.

def fact(n)
  n == 0 ? 1 : n * fact(n - 1)
end  

Rather, the built-in iterator upto can be used to calculate factorials:

factorial = 1
1.upto(10) {|x| factorial *= x }
factorial
 => 3628800

Answer 4

Just call this function

def factorial(n=0)
  (1..n).inject(:*)
end

examples

factorial(3)
factorial(11)

Answer 5

It's not in the standard library but you can extend the Integer class.

class Integer
  def factorial_recursive
    self <= 1 ? 1 : self * (self - 1).factorial
  end
  def factorial_iterative
    f = 1; for i in 1..self; f *= i; end; f
  end
  alias :factorial :factorial_iterative
end

N.B. Iterative factorial is a better choice for obvious performance reasons.

Answer 6

class Integer
  def factorial
    return self < 0 ? false : self==0 ? 1 : self.downto(1).inject(:*)
    #Not sure what other libraries say, but my understanding is that factorial of 
    #anything less than 0 does not exist.
  end
end