Ruby factorial function

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.

Like this is better

(1..n).inject(:*) || 1

There is no factorial function in the standard library.

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