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Divide by zero in a Calculate Field does not throw an error?!

GerryGabrisch

You learn something new every day in GIS.

If I open a shapefile attribute table, select an attribute, and use the field calculator to populate the field with an expression such as...

attributeX / 0

I get an error all calculated values were invalid, no rows were updated.

But

attributeX / attributeY returns no error even if values in attributeY have zero values, In fact, it populates those values with zero values.

This is disturbing.

If I do this same test in a geodatabase I get null values for the zero division rows.

I understand that there are data structure limitations. Shapefile fields are not nullable... but the shapefile example ought to give some warning!

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TedKowal

That is one hell of a spinning back kick ...

Still goes to my point (my laziness) that this should be embedded within the language and not have to be manually accounted for (and thought of).

I believe that I have been spoiled by other 4th generation languages and have the expection that python honors certain expected rules. When it does not intrinsically, I am left with no/lesser confidence. Again I am beating a dead horse... either jump on the train or get left behind.

def Live_Long_and_Prosper(vulcan)

return "Needs of the Many outweigh the Needs of the few"

print Live_Long_and_Prosper

DanPatterson_Retired

I was always an Arnold fan

def arnold(a,b):
    import numpy as np
    return (max(np.inf,a*b/1.))
print("I'll be back in... {!r:}".format(arnold(5,0)))

ChrisSmith7

Having trouble with Python when dividing by zero?

Traceback (most recent call last):

File "<pyshell#0>", line 1, in <module>

5/0

ZeroDivisionError: integer division or modulo by zero

Better get Chuck Norris to help:

def chuck_norris(a, b):
    try:
        value = a/b
    except ZeroDivisionError:
        if (a == 0):
            value = float('nan')
        elif (a > 0):
            value = float("inf")
        else:
            value = float("-inf")
    return value

DanPatterson_Retired

Gerry, maybe you can change this to a Discussion since there is obviously no answer that suits all situations.

DanPatterson_Retired

Yes.. it would be useful to implement the concept of a mask in database design, effectively excluding any operation from including records that are masked, unless you want to deal with the masked records explicitly

TedKowal

Dan I agree with your statements -- It is more of Laziness on my part. I would rather use a function isNull rather than a def for trapping all possibilities of nullness. In the same hand, knowing a division by zero will halt my program/script in its tracks. Pure Laziness!

However, I use python on databases outside of GIS/No shapefiles and still I have problems with Nulls! Yes they are Pesky! One day we will avoid Null in database design and this issue will go away (and replaced by with some other pesky issue) --- Keeping us in business.

DanPatterson_Retired

actually it is not python that can't catch nulls...that is totally wrong. The problem lies in the fact that shapefiles aren't nullable which means that they have to have a value in the field. Python wouldn't have emerged as one of the preferred languages of choice for scientific computing if it couldn't catch division by zero.

ADDENDUM

If you want to read up on null and related issues in doing field calculations, have a read through

Before I forget ... # 18 ... Those pesky <null> things...

TedKowal

Good Catch ....

This is why I like to stick with C or VB. I have confidence it will catch errors or inconsistencies such as you noted. In fact when I have to deal with Null values, I avoid python like a plague. I still have no confidence that it can trap and deal with null values. I still find situations where Python did not catch or trap (or more correctly I did not envision all the possibilities where a null value is described). I can'd give up isNull() ..... old, lazy and set my ways and now I have to worry about dividing by zero!

Good catch -- learned something else today.

GerryGabrisch

Tarek,

You could do the math using an arcpy cursor. Python would throw an error that you could catch. Still though, you would need to assign some value to the record in the shapefile that had a zero division error.

Another reason to use a geodatabase, I guess. At least you end up with a null value.

TarekKandakji

So can't an "if" statement do the task somehow?

DanPatterson_Retired

not in the field calculator, it needs to return a value to the field

TarekKandakji

I am not a python expert, but I think you can add an "if" statement. So if the value of "attribute Y" was zero it will print "Error" in the field.

DanPatterson_Retired

also, confirm that the field does actually contain 0.0 and just doesn't look like zero because of field decimal point settings.

DanPatterson_Retired

Yes it is unfortunate that you have to keep track of the math you are performing to assess appropriate values. You can use a field name to reflect the situation ... AdivBor_0 ( col A divided by B or 0 if possible) or use a code block block and specify number as the output like by comparing 0.0 to a small number to yield an improbably large value

>>> a = 1
>>> b = 0.0
>>> small= 9.9e-99
>>> a/(max(b,small))
1.0101010101010101e+98

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