Restart autoincremental numbering by unique values

Question

I'm using the graphic modeler and 'Add unique value index field' algorithm to assign IDs to unique records in a vector layer which works great, but then I want to add a 2nd field that will autoincrement for each unique value created. I can use the 'Add autoincremental field' algorithm, but then I get 1 to n for my IDs, where I'd like it to re-start for each unique id.

Example: rather than 10-0, 10-1, 10-2, 11-3, 11-4 I'd like to see 10-0, 10-1, 10-2, 11-0, 11-1.

Bonus: If there's a way to start the autoincremental number at 10 instead of 0, that would be even better!

Answer 1

You could create a custom script which increments the value index you created and saves these in a new field. You can create one from:

Processing Toolbox > Scripts > Tools > Create new script

Then use something like the following:

##Example=name
##Layer=vector
##Field_to_group_increment=field Layer
##New_index_field=string index
##Output=output vector

from qgis.core import QgsField, QgsVectorFileWriter, edit, QgsVectorLayer
from PyQt4.QtCore import QVariant

layer = processing.getObject(Layer)
writer = QgsVectorFileWriter.writeAsVectorFormat(layer, Output, "utf-8", None, "ESRI Shapefile")
output = QgsVectorLayer(Output, 'any_name', 'ogr')

idx = output.fieldNameIndex(Field_to_group_increment)
values = output.uniqueValues(idx)

with edit(output):
    output.addAttribute(QgsField(New_index_field, QVariant.Int))
    for feat in values:
        x = 10
        for feature in output.getFeatures():
            if feat == feature[Field_to_group_increment]:
                feature[New_index_field] = x
                output.updateFeature(feature)
                x = x + 1

del writer

---

Make sure to save the script into your /.qgis2/processing/scripts directory. You can then run the script from the Processing Toolbox or include it in your model.

---

Example:

The interface when you run the script:

The result:

Answer 2

If you download the next nightly of the QGIS 3.0 pre-release, it's now supported by the "Add autoincremental field" algorithm:

Answer 3

With QGIS 3.x you can use the aggregate expression function "minimum" in a virtual field to autoincrement your numbers:

$id - minimum( $id,group_by:="group_field") + 1

If you want to start autoincremental number at 10 you have to replace "+ 1" with "+ 10".