Strange result with timeit execution time measurment

Dave Angel davea at davea.name
Sat Nov 15 19:08:28 CET 2014


"ast" <nomail at invalid.com> Wrote in message:
> Hi
> 
> I needed a function f(x) which looks like sinus(2pi.x) but faster.
> I wrote this one:
> 
> --------------------------
> from math import floor
> 
> def sinusLite(x):
>     x = x - floor(x)
>     return -16*(x-0.25)**2 + 1 if x < 0.5 else 16*(x-0.75)**2 - 1
> --------------------------
> 
> then i used module timeit to compare its execution time with math.sin()
> I put the sinusLite() function in a module named test.
> 
> then:
> 
>>>> import timeit
>>>> t1 = timeit.Timer("y=test.sinusLite(0.7)", "import test")
>>>> t2 = timeit.Timer("y=math.sin(4.39)", "import math")        ## 4.39 = 2*pi*0.7
> 
>>>>  t1.repeat(3, 1000000)
> [1.9994622221539373, 1.9020670224846867, 1.9191573230675942]
> 
>>>> t2.repeat(3, 1000000)
> [0.2913627989031511, 0.2755561810230347, 0.2755186762562971]
> 
> so the genuine sinus is much faster than my so simple sinLite() !
> Amazing isnt it ? Do you have an explanation ?
> 
> Thx
> 

Sure, the library function probably used the trig logic in the
 processor. 
Perhaps if you timed things on a processor without a
 "math coprocessor" things could be different.  But even there,
 you'd probably be comparing C to python. Library code is
 optimized where it's deemed helpful. 

-- 
DaveA




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