ArcMap Python Field Calculator does not work the same as Python

Question

I am trying to use the python field calculator in ArcMap to do a simple calculation, where:

• there are two input variables, the numerator and the denominator
• if the numerator is zero a zero is returned
• else, the numerator is divided by the denominator and multiplied by 100

The code I tried:

def getScore(num, den):
    if num == 0:
        return 0
    else:
        return (num/den) * 100

when I run the script i get no errors yet the 'else' statement is not getting returned.

there will be no zeros in the denominator, so div/0 will not be an issue. The input fields are all 'long' integers and the target field is a 'double.'

I've attached a couple of images showing a test in python where the same exact code works perfectly, as well as the the field calculator where it does not work.

Answer 1

The way ArcGIS Desktop divides integers will result in an integer, so percentages will always end up as zero (e.g. 1 / 10 = 0) -- annoying, and counterintuitive, but that's what it's doing.

ArcGIS Pro uses Python 3 and in ArcGIS Desktop, it uses Python 2. Python 2 uses integer math, meaning that dividing two integer values will always produce an integer value (3 / 2 = 1). In Python 3, dividing two integer values will produce a float (3 / 2 = 1.5).

You can instead explicitly cast the variables as float, which will then do float division and give you a float result (1 / 10 = 0.1). The following works for me:

def getScore(num, den):
    if num == 0:
        return 0
    else:
        return (float(num)/float(den)) * 100

---

One observation: your conditional is basically saying "if the numerator is zero, don't bother dividing, just return zero" -- however, that's what 0 divided by anything is going to be anyway. You can skip the conditional, and the entire codeblock, and just directly calculate -- but still remember to cast the values as float first, or you'll still get a bunch of zeros :)

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