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I would like to iterate over N unknown attributes in the QGIS Graphical Modeler. For example dividing all attributes in an input layer by its area to create an area weighting.

I realise it could be done with Python, but I'm wondering if there is some clever way to accomplish this with out-of-the-box tools.

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  • You can do that by using the Field Calculator. – geoSAM Oct 17 '19 at 7:12
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    @geoSAM Okey, interesting, could you write an example as an answer? – hexamon Oct 17 '19 at 7:18
  • One of the inputs is called "field mapper," which sounds promising. I can't find any documentation or figure out how it works though. – csk Oct 17 '19 at 16:08
  • Or you could add as many vector field inputs as the maximum possible number of fields, then manually select them when you run the model. – csk Oct 17 '19 at 16:08
  • see if this helps you gis.stackexchange.com/questions/292324/… – Herbert Santos Oct 18 '19 at 14:36
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You can do that by using the sum function from the Field Calculator. Specifically for your case let's assume that you have a polygon vector with an attribute "AREA" representing the area of each polygon. In Field Calculator you create a new field with expression : "AREA"/sum("AREA")

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  • Ah, the key word in my question is unknown however. I want to iterate over all attributes in a layer without knowing their names and data types. So basically a for each loop where I do something with each attribute in a layer, and where the number of attributes and their names are unknown. – hexamon Oct 18 '19 at 7:18
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I ended up writing a Python script after all. It's available here:

https://github.com/hexamon/qgispythonprocessing/blob/master/math_on_all_fields.py

If someone answers with a non-Python solution that works I will accept that instead.

Here is the relevant logic in the script:

    op = self.parameterAsEnum(parameters, self.OPERATION,context) #Math operation type fetched from enumerated list parameter
    col = self.parameterAsFields(parameters, self.DIVCOL,context)[0] #Column for operation

    for current, feature in enumerate(features):

        # Stop the algorithm if cancel button has been clicked
        if feedback.isCanceled():
            break

        #Iterate over all fields
        for current, attr in enumerate(feature.fields()):

            #Don't process the column used for the operation
            if current != feature.fieldNameIndex(col):

                #Current attribute in iteration
                c = feature.attribute(feature.fieldNameIndex(attr.name()))
                #Column to use for operation
                d = feature.attribute(feature.fieldNameIndex(col))

                if(attr.isNumeric() == TRUE):

                    if(op == 0):
                        newVal = c * d
                    elif(op == 1):
                        newVal = c / d
                    elif(op == 2):
                        newVal = c + d
                    elif(op == 3):
                        newVal = c - d

                    feature.setAttribute(current, newVal)
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