One-line factorial function in Python

November 3, 2006

def fact(x): return (1 if x==0 else x * fact(x-1))

7 Comments

1. Peter Hosey | December 25, 2006 at 7:24 pm

Here’s an alternate solution, unless you count the import operator statement…

def fact(x):
	return reduce(operator.mul, xrange(2, x+1))

# If you want to be really strict about it, you can do this:

def fact(x):
	return reduce(lambda x, y: x*y, xrange(2, x+1))

but it probably won’t be as fast, since you’re now using a Python function in place of a built-in.

As for exactly how these two functions stack up against yours, here’s what I get on my Mac Pro:

> >>> time_ptfact(500, 5000)
> 0.00046299901008605955
> >>> time_reducefact(500, 5000)
> 0.00028626961708068849
> >>> time_reducefact_lambda(500, 5000)
> 0.0003613057613372803

Shows once again the value of doing as much as possible in C code (reduce, operator.mul, and xrange all being built-ins).

The timing functions are defined as:

> def time_FUNC(x, count=500):
> import time
> start = time.time()
> for i in xrange(count):
> FUNC(x) and None
> end = time.time()
> return (end – start) / count

for all three values of FUNC.

2. Aditya | June 3, 2008 at 6:44 am

Thanks for the information Peter, but can’t you just give the command ‘time’ while calculating the factorial? I mean, give the command,

time python programName.py

it will output the time. Built-in time function for all *nix systems.

3. Michael Hartley | June 25, 2008 at 7:23 am

err.. pardon my ignorance of large non-venomous snakes, but how do I actually use this? Here’s what happened when I tried :

`[mikeh@pud21 ~]$ python
Python 2.4.3 (#1, Mar 14 2007, 19:01:42)
[GCC 4.1.1 20070105 (Red Hat 4.1.1-52)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def fact(x): return (1 if x==0 else x * fact(x-1))
File "", line 1
def fact(x): return (1 if x==0 else x * fact(x-1))
^
SyntaxError: invalid syntax
>>>`

4. Jameson | June 26, 2008 at 7:50 am

python needs a carriage returns followed by an indentation after defining a function… do it like this:

>>> def fact(x):
… return (1 if x==0 else x * fact(x-1))
…
>>> fact(1)
1
>>> fact(10)
3628800

5. David Borsheim | August 18, 2008 at 9:24 pm

I realize that this is a very old post, but I couldn’t resist throwing my 2 cents in…Here is an actual 1-line factorial:

seed =
	reduce(lambda x, y: x * y, xrange(1, seed+1))

# or as a literal
factorial_10 = reduce(lambda x, y: x * y, xrange(1, 11))
# 10 + 1

I use xrange() because range() chokes on very large numbers. Even so, if you send if off to return the factorial of 1,000,000, you will wait a while. By way of example, the factorial of

1,000 == “402387260077093773543702433923003985719374864210714632543799910429938512398629020592044208486969404800479988610197196058631666872994808558901323829669944590997424504087073759918823627727188732519779505950995276120874975462497043601418278094646496291056393887437886487337119181045825783647849977012476632889835955735432513185323958463075557409114262417474349347553428646576611667797396668820291207379143853719588249808126867838374559731746136085379534524221586593201928090878297308431392844403281231558611036976801357304216168747609675871348312025478589320767169132448426236131412508780208000261683151027341827977704784635868170164365024153691398281264810213092761244896359928705114964975419909342221566832572080821333186116811553615836546984046708975602900950537616475847728421889679646244945160765353408198901385442487984959953319101723355556602139450399736280750137837615307127761926849034352625200015888535147331611702103968175921510907788019393178114194545257223865541461062892187960223838971476088506276862967146674697562911234082439208160153780889893964518263243671616762179168909779911903754031274622289988005195444414282012187361745992642956581746628302955570299024324153181617210465832036786906117260158783520751516284225540265170483304226143974286933061690897968482590125458327168226458066526769958652682272807075781391858178889652208164348344825993266043367660176999612831860788386150279465955131156552036093988180612138558600301435694527224206344631797460594682573103790084024432438465657245014402821885252470935190620929023136493273497565513958720559654228749774011413346962715422845862377387538230483865688976461927383814900140767310446640259899490222221765904339901886018566526485061799702356193897017860040811889729918311021171229845901641921068884387121855646124960798722908519296819372388642614839657382291123125024186649353143970137428531926649875337218940694281434118520158014123344828015051399694290153483077644569099073152433278288269864602789864321139083506217095002597389863554277196742822248757586765752344220207573630569498825087968928162753848863396909959826280956121450994871701244516461260379029309120889086942028510640182154399457156805941872748998094254742173582401063677404595741785160829230135358081840096996372524230560855903700624271243416909004153690105933983835777939410970027753472000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L”

Best regards,

Borsheim

7. Tristam MacDonald | October 21, 2008 at 9:21 pm

Unfortunately, neither Peter or David actually defined a factorial function – in particular, neither handles factorial(1) or factorial(0).