[Qgis-user] QgsVectorLayerDataProvider problem?

[Gary Smith]

Hello,

I am not sure whether this is the right forum but extensive Google
searches have not helped. I am writing a plugin to convert the segments
of a polyline to a layer of individual 2 vertex line features.

This is the code I used to do the job

        feat = QgsFeature()
        p = 0
        while p < cl.featureCount():
            cl.featureAtId(p, feat, True, True)
            geom0 = QgsGeometry(feat.geometry())
            vl = QgsVectorLayer("LineString", "Mylines", "memory")
            provider = vl.dataProvider()
            provider.addAttributes( [ QgsField( "id" , QVariant.String),
QgsField( "agl" , QVariant.String) ])
            pnt10 = QgsPoint( geom0.vertexAt(0) )
            segfeat = QgsFeature()
            itr = 1
            while ( QgsPoint( geom0.vertexAt(itr) ) <> QgsPoint(0,0) ):
                pnt10 = QgsPoint( geom0.vertexAt(itr -1) )
                pnt11 = QgsPoint( geom0.vertexAt(itr)    )
                newGeom = QgsGeometry.fromPolyline( [ pnt10, pnt11 ] )
                segfeat.setGeometry( newGeom )
                segfeat.setAttributeMap( { 0 : QVariant( str(itr) ), 1 :
QVariant( str(agl) ) } )
                provider.addFeatures( [segfeat] )
                itr += 1
            p += 1
        vl.commitChanges()
        QgsMapLayerRegistry.instance().addMapLayer(vl)

It appears to work fine until I need to make use of the data in the
attributes.

To do this I use the following code:

...
        p = 0
        while p < self.cl.featureCount():
          feat = QgsFeature()
          self.cl.featureAtId(p, feat, True, True)

          attmap = feat.attributeMap()
          attid = attmap[0].toString()
          agl = attmap[1].toString()
          geom0 = QgsGeometry(feat.geometry())
          pnt10 = QgsPoint(geom0.vertexAt(0))

...
This code works fine on features that I create manually through the QGIS
user interface. But I get an error on the line with layers which I have
created above
The error is raised by the line 

attmap = feat.attributeMap()
The error appears to be because the featureAtId function is returning
null objects and I think it is because the way I am using the provider
does not create feature id's in an orderly fashion.

Please help.
Thanks 
Gary
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