no sign() function ?

Pierre-Alain Dorange pdorange at pas-de-pub-merci.mac.com
Mon Dec 22 14:51:32 CET 2008


Stephen Thorne <stephen at thorne.id.au> wrote:

> > def sign(x):
> >     if x==0.0:
> >         return 0.0
> >     elif x>0.0:
> >         return 1.0
> >     else:
> >         return -1.0
> 
> Isn't this approximately this? ::
> 
>     def sign(x):
>         return float(cmp(x, 0))

Yes cmp() is probably the closest function to sign.

I'm new to python and here i discover at least 4 methods, i just make a
small script for timing those methods (100 000 times each on a set of 10
values).
I do not use timeit, i can't make it work easyly as it need a standalone
env for each test.

---- the test script --------------------
#!/usr/bin/env python

from math import atan2
import time


def sign_0(x):
        if x==0.0:
                return 0.0
        elif x>0.0:
                return 1.0
        else:
                return -1.0
                
def sign_1(x):
    if x > 0 or (x == 0 and atan2(x, -1.) > 0.):
        return 1
    else:
        return -1
                
def sign_2(x):
        return cmp(x, 0)

sign_3 = lambda x:+(x > 0) or -(x < 0)

def main():
        candidates=[1.1,0.0,-0.0,-1.2,2.4,5.6,-8.2,74.1,-23.4,7945.481]
        
        startTime = time.clock()
        for i in range(100000):
                for value in candidates:
                        s=sign_0(value)
        print "sign_0 : ",time.clock() - startTime
        
        startTime = time.clock()
        for i in range(100000):
                for value in candidates:
                        s=sign_1(value)
        print "sign_1 : ",time.clock() - startTime
        
        startTime = time.clock()
        for i in range(100000):
                for value in candidates:
                        s=sign_2(value)
        print "sign_2 : ",time.clock() - startTime
        
        startTime = time.clock()
        for i in range(100000):
                for value in candidates:
                        s=sign_3(value)
        print "sign_3 : ",time.clock() - startTime
        
                
if __name__ == '__main__' :
    main()

---- the results -----------------------------
My config :
        iMac (2,66 GHz intel dual core 2 duo)
        MacOS X 10.5.5
        Python 2.5.1

sign_0 = 0.4156 second (0%)
sign_1 = 0.5316 second (+28%)
sign_2 = 0.6515 second (+57%)
sign_3 = 0.5244 second (+26%)

---- conclusions -------------------------------

1/ python is fast
2/ method (0) is the fastest
3/ cmp method (2) is the slowest
4/ the precise one (IEEE 754) is also fast (1)

-- 
Pierre-Alain Dorange        <http://microwar.sourceforge.net/>

Ce message est sous licence Creative Commons "by-nc-sa-2.0"
        <http://creativecommons.org/licenses/by-nc-sa/2.0/fr/>



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