topic Python if then codeblock in field calculator in New to GIS Questions

Python if then codeblock in field calculator

KeithAddison1 — Mon, 14 Sep 2015 14:45:30 GMT

I need to write an if then code block for field calculator to see if the value in field A equals 0, and if it is to make the value in field B equal -1. Else do nothing. How can I do this? FIeld A is type float & field B is type interger.

Re: Python if then codeblock in field calculator

DanPatterson_Retired — Mon, 14 Sep 2015 15:04:49 GMT

They can be emulated using this syntax

simple comparison and reclass

" True condition " if !some_field! == "some_value" else " False condition "

ie . "Agricultural 10 Acres" if !Zone! == "A-10" else "False"

1 if !A! == 0 else -999

assuming you are calculating in the B field, using python and you can't do nothing...so I put in -999 and you can replace it with integer nothingness

Re: Python if then codeblock in field calculator

RyanClose — Mon, 14 Sep 2015 15:06:41 GMT

Assuming you are using ArcMap since you mentioned field calculator:
Right click the field you wish to change to -1 and calculate that field.

Switch the radio button to Python

In the code block enter:

def changefield(whatif,okthen):

if (whatif == 0):

okthen = -1

return okthen

Then in the text box below FIELDB =

Enter: changefield(!FIELDA!, !FIELDB!)

Re: Python if then codeblock in field calculator

JoshuaBixby — Mon, 14 Sep 2015 21:02:52 GMT

Python has conditional expressions, a.k.a., ternary operators, that can be used in situations like this one. Using a ternary operator eliminates the need for using a code block and defining a function.

-1 if !FieldA! == 0 else !FieldB!

Re: Python if then codeblock in field calculator

DanPatterson_Retired — Mon, 14 Sep 2015 21:21:00 GMT

hmmm my previous thoughts exactly

Re: Python if then codeblock in field calculator

RyanClose — Tue, 15 Sep 2015 15:00:01 GMT

Excellent. Nice to know!