Iteration for Factorials

[mensanator at aol.com]

On Oct 22, 3:22 pm, Tim Chase <python.l... at tim.thechases.com> wrote:
> > def fact(x):
> >     if x == 0 or x == 1:
> >         return 1
> >     else:
> >         return reduce(operator.mul, xrange(1,x+1))
>
> If obscurity is the name of the game,
>
>    >>> from operator import mul
>    >>> fact = lambda i: i > 1 and reduce(mul, xrange(1,i+1)) or i
>  >= 0 and 1 or None
>    >>> for i in xrange(-2,10): print '%i! = %s' % (i, fact(i))
>
> My eyes hurt after reading that...as the order of operations is
> left to Python to discern (a few judiciously placed parens might
> improve things...though that may be like polishing coprolite)

Indeed. Particularly since it doesn't work:

-2! = -2
-1! = -1
0! = 0
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880

0! is defined to be 1. You'll have a hard time calculating
combinations if 0! returns 0. Also, fact() should raise an
exception when x is negative.

It not only has to work correctly, it has to fail correctly also.

>
> I haven't yet seen an implementation in C (using the python/C
> interface) or anybody handing off a python AST/opcode-list to an
> appropriate function :)
>
> -tkc