On 19/01/2015 00:55, Steven D'Aprano wrote:
On Sun, Jan 18, 2015 at 02:20:55PM -0800, Danny Yoo wrote:
Just to add, log(0) is mathematically undefined.
https://en.m.wikipedia.org/wiki/Logarithm.
For the record, IEEE-754 specifies that log(0.0) should return -INF.
That's what Decimal does:
py> from decimal import Decimal
py> Decimal(0).log10()
Decimal('-Infinity')
Alas, Python's math module only has partial support for the IEEE-754
standard.
So depending on the problem's context, it might be worth asking why log is
being applied on this input. Is such input expected? Make sure the code
isn't trying to correct for input that shouldn't be there in the first
place.
Correct. Replacing too small values with 0.0 is the wrong solution.
Using -INF is better, or a very large negative number. Otherwise,
sticking 0 in the result is equivalent to replacing the negative values
with 1.
data = [0.5, 1.0, 0.0] # Good value, good value, bad value.
# This is the wrong way:
[0.0 if x == 0.0 else math.log(x) for x in data]
=> returns [-0.6931471805599453, 0.0, 0.0]
That is mathematically equivalent to x = 0.0 being replaced with x = 1.0
before taking the log, and that makes no sense.
Thanks to both Steven and Danny. I will have to examine my logic again.
Superficially, just setting the very occasional 0.0 value to 0 served,
but it is clearly inappropriate. I will try and find the correct answer
to my problem. I may return for advice, although it is not strictly python.
Many thanks to you both.
--
Sydney
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