ValueError: math domain error

[fairuz17]

Hi all,
I'm trying to calculate Information Gain of a term/word using this
code:

IG = (-float(N_i)/N_all * math.log(float(N_i)/N_all)) + (float(A)/
N_all * (float(A)/(A+B) * math.log(float(A)/(A+B)))) + (float(C)/N_all
* (float(C)/(C+D) * math.log(float(C)/(C+D))))

with this numbers: A=7, B=23, C=0, D=9, N_i=7, N_all=39

but got this error message:
Traceback (most recent call last):
File "/home/fairosilmi/Documents/IG_selftest1.py", line 50, in
<module>
ig_words[word] = infogain(word)
File "/home/fairosilmi/Documents/IG_selftest1.py", line 38, in
infogain
IG = (-float(N_i)/N_all * math.log(float(N_i)/N_all)) + (float(A)/
N_all * (float(A)/(A+B) * math.log(float(A)/(A+B)))) + (float(C)/N_all
* (float(C)/(C+D) * math.log(float(C)/(C+D))))
ValueError: math domain error

I realize that the problem comes from math.log(C) as C=0. So how can I
avoid this error?

Thanks and best regards.

[Matthew Berends]

You can either explicitly test for 0, e.g.

result = 0
if x > 0:
result = math.log(x)

Or you can wrap the computation in a try/except:

try:
result = math.log(x)
except ZeroDivisionError as e:
x = 0
# report the error if appropriate
print e

Hope this helps,

Matt

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[fairuz17]

ok matt,thanks a lot..

On May 22, 9:27 pm, Matthew Berends <matthewbere...@gmail.com> wrote:
> You can either explicitly test for 0, e.g.
>
> result = 0
> if x > 0:
>     result = math.log(x)
>
> Or you can wrap the computation in a try/except:
>
> try:
>     result = math.log(x)
> except ZeroDivisionError as e:
>     x = 0
>     # report the error if appropriate
>     print e
>
> Hope this helps,
>
> Matt
>