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Suppose i have a attribute table as shown in the image below:

I have columns A and B with common FID. I would like to add a new column C, which contains the values of column A with the condition that if the value is equal to 25 in that column, then for those rows, the value should be assigned from column B.

The highlighted green color column C in the image is how the result should look like.

Is there a tool to do this or does an expression have to be used?

You can use Field Calculator using Python Parser:

in the Pre-Logic Script write the following code:

def getValue(f1,f2):

    if f1 == 25:

        return f2

    else:

        return f1

C=

getValue(!A! , !B!)

Python has ternary conditional operator:

!B! if !A! == 25 else !A!

You can use Field Calculator (see examples here) or the da.UpdateCursor like below. Change input and field names and execute in the python window of ArcMap.

import arcpy



fc = r'C:data.gdbfeature_class' #Change to match your data

fields = ['A','B','C'] #Change to match your data



with arcpy.da.UpdateCursor(fc,fields) as cursor:

    for row in cursor:

        if row[0] == 25:

            row[2] = row[1]

        else:

            row[2] = row[0]

        cursor.updateRow(row)

In the Field Calculator with a usage of Python parser, please type

Pre-Logic Script Code:

def CalcColumn(fieldA, fieldB):

  if fieldA == 25:

    return fieldB

  else: 

    return fieldA

C =

CalcColumn(!A!, !B!)

References:

• Basic If/Then in Python Parser of ArcGIS Field Calculator?


• Python script for if/elif condition in field calculator