Calculate field with formula and conditions

[BeckHag]

I am not a strong Python programmer, and do not know the proper way to tackle a problem I have run into.  I am attempting to replicate a process defined by others and cannot deviate from that process.  I am calculating values for a field in an attribute table (Field5) which is derived from other fields in the table using a given formula. 

Here is a simplified version of the formula:

Field5 = math.log(!Field1! / !Field2!) + math.log(!Field3! / !Field4!)

In cases where Field1 = 0 or Field3 =0 an error is produced, since the natural log of 0 is undefined. In these instances the dataset I am replicating has assigned a value of "0" to that term in the expression. For instance, if Field1 was equal to zero then math.log(!Field1! / !Field2!) was assigned a value of "0". Similarly, if Field3 had a value of 0, then math.log(!Field3! / !Field4!) was assigned a value of "0". 

Using the ArcGIS Pro Calculate Field tool, is there a Python script that will allow me to calculate values for this field using the correct formula and then specify that a value of "0" should be assigned in these instances? Something like "If math.log(!Field1! / !Field2!) is undefined, then assign a value of zero", or "If math.log(!Field1! / !Field2!) = math.log(0 / !Field2!), then assign a value of 0". Putting in the formula without such conditions results in the following: 

ERROR 000539: Traceback (most recent call last):
  File "<expression>", line 1, in <module>
ValueError: math domain error

 
I am using ArcGIS Pro 3.0.3. Thank you for your help!

[Kepa]

Hello @BeckHag, 

You can accomplish your goal using the code block:

import math
def calculate_field5(field1, field2, field3, field4):
    try:
        return math.log(field1 / field2) + math.log(field3 / field4)
    except ValueError:
        return 0

Regards,

[BeckHag]

Thank you, that almost gets me there. Is there a way to set it up so that the term rather than Field5 is given a value of zero? For instance, suppose the following values applied:
Field1 = 2, Field2 = 3, Field3 = 0, Field4 =3 
Applying these to the formula we get:
Field5 = math.log(!Field1! / !Field2!) + math.log(!Field3! / !Field4!)

Field5 = math.log(2 / 3) + math.log(0 / 3)

I want this to reduce to: Field5 = -0.40547 + 0 

Thank you again for your help with this. 

[Kepa · Accepted solution]

Then, I think you should check field1/2 and field3/4 separately this way:

import math
def calculate_field5(field1, field2, field3, field4):
    exp1 = field1 / field2
    exp2 = field3 / field4
    try:
        exp1 = math.log(exp1)
    except ValueError:
        exp1 = 0
    try:
        exp2 = math.log(exp2)
    except ValueError:
        exp2 = 0
    return exp1 + exp2

Hope that helps!