Speed-up for loops

Terry Reedy tjreedy at udel.edu
Fri Sep 3 00:20:27 CEST 2010


On 9/2/2010 8:55 AM, Tim Wintle wrote:
> On Thu, 2010-09-02 at 12:02 +0200, Michael Kreim wrote:
>> Hi,
>>
>> I was comparing the speed of a simple loop program between Matlab and
>> Python.
>
>> Unfortunately my Python Code was much slower and I do not understand why.
>
> The main reason is that, under the hood, cpython does something like
> this (in psudo-code)
>
> itterator = xrange(imax)
> while 1:
>    next_attribute = itterator.next
>    try:
>      i = next_attribute()
>    except:
>      break
>    a = a + 10
>
> where C (and I'm assuming matlab) does this:
>
> while 1:
>    i = i + 1
>    if (i>  imax):
>      break
>    a = a + 10

Which is to say, 'for' in python is not the same as 'for' in C/matlab 
and the latter is what Michael should use in a fair comparison. 
Otherwise, apples and oranges.


-- 
Terry Jan Reedy




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