Using if/elif/else statements in ArcGIS Field Calculator?

Question

I have no experience with Field Calculator in ArcGIS 10.

I have a very simple calculation I would like to perform that I could do easily via pure SQL but just can't seem to get my head around it in the Field Calculator.

I have a table of 5000 weeds of which there are around 10 types. I want to update one field in the table (called CLASS) to be either A, B, or C depending on another field: WEED_NAME

So, if WEED_NAME = 'Hyptis' then Class = 'A' else if WEED_NAME = 'Parkinsonia' then Class = 'B' etc

Answer 1

It's quite similar to Using conditional statements in Field Calculator, but this code should do the trick:

Pre-logic block:

def ifBlock(weedName):
 if weedName== 'Hyptis':
  return "A"
 elif weedName== 'Parkinsonia':
  return "B"
 elif weedName== 'xxxx':
  return "C"

Then in the actual code:

ifBlock(!WEED_NAME!)

Make sure you select Python as the script engine

Answer 2

As a modification of Stev_k's code you may use dictionaries.

def weedClassFromName(weedName):
    # create dictionary
    weedDic = {'Hyptis':'A', 'Parkinsonia':'B'}
    # process weedName in dictionary; default is used for undefined
    return weedDic.get(weedName,'undefined weed')

On the other hand, as you have loads of weeds in just a few classes, it may be more convenient to use lists.

def weedClassFromName(weedName):
    if weedName in ['Hyptis', 'another Hyptis', 'yet another Hyptis']:
        return 'A'
    elif weedName in ['Parkinsonia', 'another Parkinsonia', 'yet another Parkinsonia']:
        return 'B'
    else:
        return 'undefined weed'

In both cases you need the actual code

weedClassFromName(!WEED_NAME!)

It should be fairly easy to produce those lists out of your tables using advanced search and replace.

Answer 3

You could probably do this as a conditional in the field calculator, but I think it would be quicker to do the following:

In the table window:

Select by Attributes: WEED_NAME = 'Hyptis' Right click and open Field Calculator on "Class" field. In the value window, enter 'A' and press OK.

Repeat this for each combination of WEED_NAME and 'B', then 'C'.

I find sometimes I spend more time trying to find an easier solution to the problem, than I would by doing it the "Hard" way.

Hope this helps!