I have a set of string values in a shapefile's attribute column and want to assign a number to them in a way that each unique string gets an integer number, like this:

strings        numbers
-------        -------
Cloud          1
Water          2
Water          2
Vegetation     3
Water          2
Cloud          1
Cloud          1
Vegetation     3

The order does not matter (but of course, if it's possible, alphabetically would be perfect).

Is there a way to achieve this without knowing the actual strings or the count of unique strings (assuming I have 20-30 different strings)?

I am using Python in QGIS 3.

Using the suggestion of BERA, I was able to run the Python script from the console. Then I tried to implement it as an expression:

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def numberUniqueStrings(layer_name, src_column, dst_column, feature, parent):
    layer_list = QgsProject.instance().layerTreeRoot().children()
    layer = [lyr.layer() for lyr in layer_list if lyr.name() == layer_name][0]
    idx = layer.getFeature(1).fieldNameIndex(dst_column)
    unique = []
    with edit(layer):
        for feature in layer.getFeatures():
            value = str(feature[src_column]).strip()
            if value not in unique:
            counter = unique.index(value) + 1
            feature.setAttribute(idx, counter)

and called it using the "Field Calculator" (created the field "numbers" as integer before):

numberUniqueStrings(@layer_name, "strings", "numbers")

Unfortunately nothing happens, all the fields stay empty.

  • in reply to your edit: that sounds like you're iterating over -and editing - the whole layer once for each feature. I think you want to call this as a function from the console, but only once, rather than making it into an expression function – Steven Kay Aug 16 '18 at 9:57
  • Actually, I don't want to call it from the console every time I need to do this. Instead, I'd rather like to use it either as an expression or a script tool (where I did not dive into yet). – s6hebern Aug 16 '18 at 10:56

Select your layer in the layer tree and:

layer = iface.activeLayer() #Select layer in tree
unique = []
with edit(layer):
    for feature in layer.getFeatures():
        if feature['strings'] not in unique:
        new_value = unique.index(feature['strings'])
        feature.setAttribute(feature.fieldNameIndex('numbers'), new_value)
  • This is great, but it only seems to work in the console and not as an expression. See the edit on how I implemented and called it. – s6hebern Aug 16 '18 at 9:48
  • @s6hebern sorry cant answer that – BERA Aug 16 '18 at 10:01
  • I will leave the question unanswered for now hoping that someone else has an idea, although your suggested code works perfectly from the console. Thanks, @BERA – s6hebern Aug 16 '18 at 15:05

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