[gdal-dev] A question of eficiency

[Abel Pau]

Hello!

I’ve been trying to use the OGRLayer::GetFeatureCount() when I am reading another format inside the ICreateFeature() but I think it’s impossible. At this point I don’t have access to the layer info, so to the GetFeatureCount().
In fact I don’t mind where to call this function but I need to call it in order to write our format efficiently (at least in most of cases).

So the question is, in my driver, when I read a format I get some information about the layer but I can’t acces to the count of the features of the layer. I am right? If not, how I do that?

Thanks but before sending this mail I’ve been like hours trying to find it out debbuging and consulting the call stack from all points and nothing seems to indicate I can do that. But perhaps I am wrong.

It would be very helpful knowing that information. I not my writting strategy with change radically. Thankssssss!

De: gdal-dev <gdal-dev-bounces at lists.osgeo.org> En nombre de Abel Pau
Enviado el: divendres, 7 de juliol de 2023 12:44
Para: Even Rouault <even.rouault at spatialys.com>; gdal-dev at lists.osgeo.org
Asunto: Re: [gdal-dev] A question of eficiency

Hi again,

I can totally give this number in GetFeatureCount(). No problem from my driver to other.
Then I understand that in one way (efficient) or in another (no efficient) I can always get the number of features with the GetFeatureCount(). True?

About the other question imagine a multiline shape with two lines of two parts every line. I need to know the total count of individual lines (cause my driver doesn’t have multiline and I have to convert every multiline to N lines). It is 4 (2+2). Can I get this number without asking every feature how many parts it have? I doubt that, but...

Thanks for your kind anwser!

De: Even Rouault <even.rouault at spatialys.com<mailto:even.rouault at spatialys.com>>
Enviado el: divendres, 7 de juliol de 2023 12:32
Para: Abel Pau <a.pau at creaf.uab.cat<mailto:a.pau at creaf.uab.cat>>; gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>
Asunto: Re: [gdal-dev] A question of eficiency


Abel,

At the minimum for the reading side of a vector driver, you need to implement GetNextFeature().

If there is an efficient way of knowing the number of features without iterating over the whole file (typically a field in a header giving that number), you may implement GetFeatureCount(). If you don't implement it, and code using your driver needs the feature count, the generic implementation of OGRLayer::GetFeatureCount() will call GetNextFeature() repeatdly until it returns NULL.

Not totally sure to understand your question about multielements. If you have an instance of OGRGeometryCollection or any deriving class, there's a getNumGeometries() method to get the number of parts

Even
Le 07/07/2023 à 11:33, Abel Pau a écrit :
Hi,
I would like to know if when we use the code of GDAL to program a driver (or whatever) in vectors (so, ogr) we can assume that we can know the number of elements we are going to import in any way or it would be possible not knowing that until NextFeature is empty (or something like that). So, there is any way to obtain this number of elements without doing a while? And in case of multielements its possible to know how many parts are there in total?
I need that to write my format (I don’t explain the details cause I don’t get you bored :) )

If anyone know that I’d appreciate an answer. Thanks in advance.

Abel.


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