[C++-sig] Iterating over small objects

[Babak Khataee]

Okay cool thanks your input.

For any one else has a similar problem - writing a simple iterator function
in python (which is a generator) and assigning that to the __iter__ member
works well as an alternative. For me this turned out to be considerably
faster than using a c++ iterator/getitem iteration.

On 19 January 2012 19:37, Jim Bosch <talljimbo at gmail.com> wrote:

> On 01/19/2012 02:08 PM, babak wrote:
>
>> Hi guys,
>>
>> Has anyone encountered (what feels like) quite slow iteration when looping
>> over small containers ? I'm in a situation where I'm iterating over
>> 100000+
>> 3d vectors and its taking much longer than I'd expect. The vector type is
>> bound via boost::python and has an iter method which uses range(being,
>> end)
>> for the iteration.
>>
>> I've attached a simple example binding and python test to illustrate what
>> I
>> mean more clearly. I'm under the impression that in a situation like this
>> the overhead of creating the iterator is greater than the actual
>> iteration,
>> but thats just a guess. Alternatively my perspective on what the iteration
>> speed should be might be completely skewed, so any help in clarifying
>> what's
>> going on here would be greatly appreciated.
>>
>>
> My sense is that this is something that really can't be expected to be
> efficient across the C++/Python boundary.  There is some overhead
> associated with creating the exposures, and there's also some overhead in
> converting the types, especially if those vectors aren't builtin numeric
> types.
>
> If you can redesign the C++/Python boundary to convert your vectors
> directly into Python lists or tuples instead of wrapped C++ vectors, I
> expect you'll see an improvement, because you'll be able to take advantage
> of whatever optimization the Python developers have put into iteration over
> those classes.  Of course, it's best by far to do whatever iteration you
> can in C++ if performance is important, but I'm guessing that's not an
> option here.
>
> Jim
>
>
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