[Qgis-developer] Create Multiple Polygons from Polygon and Polylines

[Matthew Denno]

Thank you everyone for your help.  I really appreciate it as I am trying to
learn the QGIS API and C++.  I think that I have a solid plan to achieve my
goal now.  However, I seem to be stuck on something that I think should be
simple, looping through the points of a QgsPolyline.  If I iterate through
the Polylines and print out the point coordinates using ->at(int).x() I can
print out x1, y1, x2, and y2 with out problems.  But, if I try to iterate
through the points of the polyline and print out the coordinates, I get x1,
y1 and then a bunch of junk values for the first 3 polylines and then
eventually it ends unexpectedly.  Below is the code I am trying to use.

 //now iterate through each polyline feature

while (mypPolylineLayer->nextFeature(currentPolylineFeat))

{

qDebug("feature = %i",f);

++f;


 //get point geometry for current feature

QgsGeometry * currentPolylineGeom = currentPolylineFeat.geometry();

QgsPolyline * currentPolyline;

currentPolyline = &currentPolylineGeom->asPolyline();

 //this part works

qDebug() << "x1 = " << currentPolyline->at(0).x();

qDebug() << "y1 = " << currentPolyline->at(0).y();

qDebug() << "x2 = " << currentPolyline->at(1).x();

qDebug() << "y2 = " << currentPolyline->at(1).y();


 QgsPolyline::const_iterator it = currentPolyline->constBegin();

for(; it != currentPolyline->constEnd(); ++it)

{

//this part doesn't

qDebug() << "x = " << (*it).x();

qDebug() << "y = " << (*it).y();

}

}


On Tue, Sep 15, 2009 at 5:48 AM, Carson Farmer <carson.farmer at gmail.com>wrote:

> Basically the polylines need to be "dissolved" first.  Is there an existing
>> function to acheive this?
>
>
> You will likely need to do this yourself by finding segments that share a
> node, and creating a single line from all the associated QgsPoints.
>
>
>> Also, is there an enexisting function to union polygons programaticlly?
>>
>
> Certainly, check out QgsGeometry.combine() in the docs.
>
> Carson
>
>
> --
> Carson Farmer
> National Centre for Geocomputation
> John Hume Building,
> National University of Ireland, Maynooth,
> Maynooth,
> Co. Kildare,
> Ireland.
> www.carsonfarmer.com
>
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