[Pyrex] Basic Pyrex question

[Matt Hammond]

pyrex compiles down to C and the integer data type in C is probably 32bit  
on your platform. That means it can hold a values over the range +/-  
2000000000 roughly. The result of the calculation you issue is  
substantially larger than that, so the maths fails as the value probably  
simply wraps back round when it reaches the limit.

regards


Matt

On Fri, 16 May 2008 17:12:43 +0100, Rick Muller <rpmuller at gmail.com> wrote:

> I'm just learning Pyrex, and am making a mistake somewhere. I wanted to  
> play
> with the simple function:
>
> def fastsumpairs(l):
>     s = 0
>     for i in xrange(len(l)):
>         for j in xrange(i):
>             s += l[i]*l[j]
>     return s
>
> The first time through, the python and pyrex versions of this routine  
> gave
> the same result, and the pyrex version was about 30% faster. Sweet.
>
> timeit(sumpairs(range(1,1000)): 0.202557 seconds
> 124583708250
> timeit(fastsumpairs(range(1,1000)): 0.155977 seconds
> 124583708250
>
> Now I tried to modify the code to tell it that s,i,j were ints:
>
> def fastsumpairs(l):
>     cdef int s, i, j
>     s = 0
>     for i in xrange(len(l)):
>         for j in xrange(i):
>             s += l[i]*l[j]
>     return s
>
>
> timeit(sumpairs(range(1,1000)): 0.202557 seconds
> 124583708250
> timeit(fastsumpairs(range(1,1000)): 0.069682 seconds
> 29656666
>
> So the function is now much faster, but it gives the wrong results. Any
> hints as to why?
>



-- 
| Matt Hammond
| Research Engineer, FM&T, BBC, Kingswood Warren, Tadworth, Surrey, UK
| http://www.bbc.co.uk/rd/