Writing If-Then Statement in ArcGIS Field Calculator using Python Parser?

Question

Scenario:

I have four fields: A, B, C and D

Field A is text-based and contains values from '000' up to some number 'xxx' Field B contains a set of values (also text, but represented by numbers) Field C contains a set of values (also text, but represented by numbers) Field D is empty and waiting to be populated

I would like to construct an if-then statement in Field Calculator that does something like this:

If Field A = 000 then Field D = Field B, 
If Field A =/= 000 then Field D = Field C

I have spent the better part of my day trying to find the proper 'language' to ask Field Calculator, but I'm stuck. I have 0 programming experience also.

Could anybody show me how this should be set up either in Python?

I'm using ArcGIS 10.0.

Answer 1

Using Python, the calculation would look like this:

For the Pre-Logic Script:

def Calc(a,b,c):
 if a == "000":
 return b
 else:
 return c

For the expression:

Calc( !A! , !B! , !C! )

• user24272: Hi Thanks for the reply! I was not able to get this script to work....should it be used in Python or Python_9.3? Also when I go to insert the real names of my fields, they always have the exclaimation points before/after. Should they also appear that way in the Pre-logic script?
• nmpeterson: In the pre-logic, the variable names are simply placeholders and can be called whatever you like.
• user24272: Wow - ok thanks nmpeterson for pointing that out. Totally worked! Many, many thanks!

Answer 2

Pythonic inline if is working in Field Calculator expression

res=(on_false, on_true)[condition]

arcpy.CalculateField_management("fc" , "field" ,"(!C!, !B!)[!A!=='000']" , "PYTHON_9.3")

• user24272: Thank you for the response! I was able to -briefly- get your post to work, but when I tried to integrate the script into a larger model it broke down and keeps giving me this error: ERROR 000539: Runtime error <class 'arcgisscripting.ExecuteError'>: ERROR 000732: Input Table: Dataset PARCEL does not exist or is not supported Failed to execute (Calculate Field). Any ideas what might be happening?
• geogeek: Input Table: Dataset PARCEL does not exist or is not supported Failed to execute (Calculate Field) this error means , i think that the feature class has not been found.