Calculations in if statement

Question

I want to perform a simple calculation with the Field Calculator using this piece of script (note that all the elif statements are attempts for another syntax and non of the worked):

def change(OldFor_Com):
    if (OldFor_Com) == "OHWH":
        return shape.area@Hectares / 40
    elif (OldFor_Com) == "OHWH":
        return !shape.area@Hectares / 30!
    elif (OldFor_Com) == "OHWH":
        return (!shape.area@Hectares / 30!)
    elif (OldFor_Com) == "OHWH":
        return !shape.area@Hectares! / 30
    elif (OldFor_Com) == "OHWH":
        return (!shape.area@Hectares! / 30)
    if (OldFor_Com == "OTHH"):
        return !shape.area@hectares! / 40

If the field is populated with OTHH I want another field to be populated with the area of the field, divided by 40, or 30, or... and so on. I modified my syntax a couple of times (as you can see) but it still says Invalid Syntax.

Is a calculation possible with these if statement or do I need a workaround?

Note that the shape.area@Hectares is an expression to calculate the area of the field, and not another field in my attributetable!

EDIT: 7/15/2014 around 11pm

I modified the script. I added a field to the table and calculate the area of the shape first (with !shape.area@Hectares!) and then I try to run this

def change(OldFor_Com):
    if OldFor_Com == "OTHH":
        return Area_Ha / 40
    if OldFor_Com == "OHWH":
        return Area_Ha / 30
    if OldFor_Com == "OSFH":
        return Area_Ha / 375
    if OldFor_Com == "PINE":
        return Area_Ha / 10
    if OldFor_Com == "OOFH":
        return Area_Ha / 375
    if OldFor_Com == "None":
        return Area_Ha
    else:
        return 0

It bounces back with the following:

ERROR 000539: Error running expression: change(u"OTHH", 7) 
Traceback (most recent call last):
File "<expression>", line 1, in <module>
TypeError: change() takes exactly 1 argument (2 given)

Failed to execute (Calculate Field (10)).

Is it not possible to do any calculations in the if statement?

EDIT: 7/15/14 12:30pm

This how the attributestable looks like before I add the field SizeScore

After I add the field SizeScore (FLOAT).

And the following screen comes up if I use this code:

def change(OldFor_Com, Area_Ha):
    if OldFor_Com == "OTHH":
        x = Area_Ha / 40
        return x
    if OldFor_Com == "OHWH":
        x =  Area_Ha / 30
        return x
    if OldFor_Com == "OOFH":
        x = Area_Ha / 375
        return x
    if OldFor_Com == "OSFH":
        x = Area_Ha / 375
        return x
    if OldFor_Com == "PINE":
        x =  Area_Ha / 10
        return x
    if OldFor_Com == "None":
        return Area_Ha
    else:
        return 0

It only populates the rows where it does not need to calculate. The only statement that is working is that one.

if OldFor_Com == "None":
     return Area_Ha

But for the others are no error messages coming back!

SOLVED, SEE ACCEPTED ANSWERE Thanks a lot to everybody who went through this with me!

Answer 1

You need to send your area field into your calculation as a parameter.

Expression: change(!OldFor_Com!, !shape.area@hectares!)

Code Block:

def change(OldFor_Com, area):
    if OldFor_Com == "OTHH":
        return area / 40
    elif OldFor_Com == "OHWH":
        return area / 30

Answer 2

After your newest updates to the question, this should work:

def change(OldFor_Com, Area_Ha):
    if OldFor_Com == "OTHH":
        return Area_Ha / 40.0
    elif OldFor_Com == "OHWH":
        return Area_Ha / 30.0
    elif OldFor_Com == "OOFH":
        return Area_Ha / 375.0
    elif OldFor_Com == "OSFH":
        return Area_Ha / 375.0
    elif OldFor_Com == "PINE":
        return Area_Ha / 10.0
    elif OldFor_Com == "None":
        return Area_Ha
    else:
        return 0

The reason you were seeing all 0s (except for the "None" case) was not that the function wasn't working, but rather that you were dividing integers by integers. The results were always less than 1, and Python always rounds down for integer math. Adding .0 to each of the denominators forces decimal math, and the results will be decimals.

(Alternatively -- and this is probably better -- is to make the "Area_Ha" field a float/double instead of an integer. Then your function would work exactly as you've written it.)

Answer 3

Your exclamation marks need to be wrapped around the field name:

def change(OldFor_Com):
    if OldFor_Com == "OTHH":
       return !shape.area@Hectares! / 40
    elif OldFor_Com == "OHWH":
       return !shape.area@Hectares! / 30

Also, make sure Python is the defined Parser.

Answer 4

Going off of your latest edit and ew_GIS try this:

Expression: change(!OldFor_Com!, !Area_Ha!)

Code Block:

def change(OldFor_Com, area):
  if OldFor_Com == "OTHH":
     return area / 40
  elif OldFor_Com == "OHWH":
     return area / 30
  elif OldFor_Com in ("OOFH", "OSFH", "OOFH"):
    return area / 375
  elif OldFor_Com == "PINE":
    return  area / 10
  elif OldFor_Com == "None":
    return area
  else:
    return 0