Python Parser in ArcGIS – Using Nested If Statements in Field Calculator

arcgis-desktopfield-calculatorpython-parservbscript

The solution in VB for the base process is at If/then VBScript/Python code equivalent to SQL IN ('x','y',z') expression in ArcGIS Field Calculator? works great but I was trying to nest 'if' statements and can't.

So pre-logic code of

if [grid_code] = 1 then 
f2= 2
end if
if [grid_code] = 2 then
f2=3.5
end if

…etc. but this doesn't work.

What am I doing wrong?

I tried the second 'if' before the 'end if' and also tried 'elif' but these give script errors.

Also I am seeing a warning message about "The calculated value is invalid for the row with objectID=1" and when you click click 'yes' to continue it works fine.

Best Answer

For VB, something like this should work, I think:

Pre-logic script code:

Dim f2 As Double
If [grid_code] = 1 Then
  f2 = 2
ElseIf [grid_code] = 2 Then
  f2 = 3.5
End If

asp_code =

f2

In Python, I think it would be something like this (though I don't have ArcGIS 10 so I can't check):

Pre-logic script code:

def my_func(my_field):
    if my_field == 1:
        f2 = 2
    elif my_field == 2:
        f2 = 3.5
    return f2

asp_code =

my_func(!grid_code!)

The error message saying that the "calculated value is invalid" is a bit strange. Perhaps double-check that your asp_code field is of float or double data type (not integer), and that you have all of the options in the grid_code field covered by your if statements? Sorry if that's too obvious, but it's all I can think of at the moment. Good luck!

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if !myField! == 'NULL' will not test for a null value. That will test a for a string equal to 'NULL'. Use:

if !myField! is None:

To accomlish your task, define a python function in the pre-logic code block and use that function in the expression window. The following code sample pulls the first token from a '-' delimited string.

enter image description here

[GIS] Using Python in the field calculator – if/else over multiple fields

Try this (edited per @alpha-beta-soup comment):

def myFunction (inter, prop, patch, shp, nat, edge):
    myList = [inter, prop, patch, shp, nat, edge]
    myList.sort()
    myDict = {'Interconnectivity' : inter,
              'PropCover' : prop,
              'PatchSize' : patch,
              'ShapeIndex': shp,
              'Naturalness' : nat,
              'EdgeNat' : edge}

    myVal = 101   # using 101 as a max + 1 possible value; adjust if needed
    myReturn = ''
    for k, v in myDict.iteritems():
        if v < myVal:
            myVal = v
            myReturn = k

    return myReturn

Note that if you have two matching low values, there's nothing to select one over the other. That would require additional checking based on whatever your criteria is. Since a dictionary is unordered, you can't control it that way.

Here's another way without dictionaries, with an optional clause for ties. You may want different logic in case of ties.

def myFunction2 (inter, prop, patch, shp, nat, edge):
    # use tuples to ensure value order
    myScoreTuple = (prop, patch, shp, nat, edge)
    myTextTuple = ('PropCover',
                   'PatchSize',
                   'ShapeIndex',
                   'Naturalness',
                   'EdgeNat')

    # initialize values
    myLowValue = inter
    myReturn = 'Interconnectivity'

    # now check for any lower value
    for i in range(len(myScoreTuple)):
        if myScoreTuple[i] < myLowValue:
            myLowValue = myScoreTuple[i]
            myReturn = myTextTuple[i]

        # optional - add multiple text values in case of tie. Remove if unneeded
        elif myScoreTuple[i] == myLowValue:
            myReturn = ', '.join((myReturn, myTextTuple[i]))

    return myReturn

Best Answer

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