I'm a programmer, I have programmed in C#, Java, but not with Python. And I'm not a geographic worker. How can I write a python script into Field Calculator to be able to check the other column value. If that other column value is "a certain string", the value in this column should be 0, otherwise it should be computed with an equation. The value I want to look is a Text type, and I want to count in a Double type column.

  • It will make it much easier if you can be more specific about the field types you are dealing with, and how/whether the equation involves these or any other fields.
    – PolyGeo
    Commented Oct 22, 2014 at 10:25
  • Thx. I have edited that.
    – greyline
    Commented Oct 22, 2014 at 10:29

2 Answers 2


You don't 'Dim' variables in python, you just declare and assign them. That said, your basic route is to set the parser to python and check Show Codeblock. In the Pre-Logic Script Code box, enter your function like this:

def DoThis(fld):
    val = 0
    if fld <> 'a certain string':
        val = # do your calculation here
    return val

In your 'field =' box, enter:


In python , # means a comment. Indentation counts. If your calculation depends on other fields, pass those in as well. That should be straightforward with your background. In calling the function, the field name is bracketed by exclaimation (!) marks.

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  • I have used this solution by dark fader: gis.stackexchange.com/questions/81817/… It works with unaccentuated strings, but there's a problem with accentuated strings. What to do?
    – greyline
    Commented Oct 22, 2014 at 12:23
  • @gregtom6 your question makes no mention of accentuated strings so based on what you did ask I think you should accept this answer and write a more detailed new one.
    – PolyGeo
    Commented Oct 22, 2014 at 12:32
  • @PolyGeo I have accepted.
    – greyline
    Commented Oct 22, 2014 at 12:35
  • This may be worth looking at gis.stackexchange.com/q/61598/115
    – PolyGeo
    Commented Oct 22, 2014 at 12:52

I'm a long time ArcGIS user but find that the QGIS field calculator is a lot more intuitive & provides options for dragging & dropping fieldnames, expressions & operators.

If you are a programmer, then you'll probably find it a lot easier and you'll find it builds your knowledge for using the expression builder in ArcGIS. QGIS Expression Builder

  • Thanks, but now it seems to be it would take hours to find out how to use that with ArcGIS, and I don't do this kind of work for months. I need only short scripts and that would be already completed if I would know the syntax of doing that. gis.stackexchange.com/questions/81817/… I read that and I think I need a double variable instead of Dim, but I'm not sure.
    – greyline
    Commented Oct 22, 2014 at 11:02
  • It seems to be it is possible with this method: gis.stackexchange.com/questions/81817/… But the problem is maybe with the accentuated letters I'm using in the string type field.
    – greyline
    Commented Oct 22, 2014 at 12:17
  • So you're saying the the value in the field has/can have an accented character, and the problem is with the comparison value ('a certain string') not being accented?
    – recurvata
    Commented Oct 22, 2014 at 12:37

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