[Numpy-discussion] log of negative real numbers -> RuntimeWarning: invalid value encountered in log
Why does numpy produce a runtime warning (invalid value encountered in log) when taking the log of a negative number? I noticed that if you coerce the argument to complex by adding 0j to the negative number, the expected result is produced (i.e. ln(-1) = pi*i). I was surprised I couldn't find a discussion on this, as I would have expected others to have come across this before. Packages like Matlab handle negative numbers automatically by doing the complex conversion. ___ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] log of negative real numbers -> RuntimeWarning: invalid value encountered in log
Thanks. That is for performance and memory which of course is valid for most use cases. Would it really be much different than doing type/size checking of all the np.array arguments to a function to ensure the appropriate final np.array sizes are allocated? I'm not trying to question current practice, just trying to understand as performance will eventually be important to me. Would a "complex default" mode ever make it into numpy, to behave more like Matlab and other packages with respect to complex number handling? Sure it would make it marginally slower if enabled, but it might open the door to better compatibility when porting code to Python. On Mon, May 25, 2020 at 9:49 AM Eric Wieser wrote: > One explanation for this behavior is that doing otherwise would be slow. > > Consider an array like > > arr = np.array([1]*10**6 + [-1]) > ret = np.log(arr) > > Today, what happens is: > >- The output array is allocated as np.double >- The input array is iterated over, and log evaluated on each element >in turn > > For what you describe to happen, the behavior would have to be either: > >- The output array is allocated as np.double >- > >The input array is iterated over, and log evaluated on each element in >turn >- > >If any negative element is encountered, allocate a new array as >np.cdouble, copy all the data over, then continue. This results in the >whole array being promoted. > > or: > >- The input array is iterated over, and checked to see if all the >values are positive >- > >The output array is allocated as np.double or np.cdouble based on this >result >- > >The input array is iterated over, and log evaluated on each element in >turn > > In either case, you’ve converted a 1-pass iteration to a 2-pass one. > > There are static-typing-based explanations for this behavior too, but I’ll > let someone else present one of those. > > Eric > > On Mon, 25 May 2020 at 14:33, Brian Racey wrote: > >> Why does numpy produce a runtime warning (invalid value encountered in >> log) when taking the log of a negative number? I noticed that if you coerce >> the argument to complex by adding 0j to the negative number, the expected >> result is produced (i.e. ln(-1) = pi*i). >> >> I was surprised I couldn't find a discussion on this, as I would have >> expected others to have come across this before. Packages like Matlab >> handle negative numbers automatically by doing the complex conversion. >> ___ >> NumPy-Discussion mailing list >> NumPy-Discussion@python.org >> https://mail.python.org/mailman/listinfo/numpy-discussion >> > ___ > NumPy-Discussion mailing list > NumPy-Discussion@python.org > https://mail.python.org/mailman/listinfo/numpy-discussion > ___ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] log of negative real numbers -> RuntimeWarning: invalid value encountered in log
Being relatively new to numpy, I'm learning as I go. Your typing suggestion helped me come to the conclusion that I need to type and coerce my input arguments as I'm doing complex math. My test arguments were real, and thus numpy was producing real results. Doing .astype(np.complex128) to my input values that are generally complex resolved my issue. On Mon, May 25, 2020 at 10:09 AM Brian Racey wrote: > Thanks. That is for performance and memory which of course is valid for > most use cases. Would it really be much different than doing type/size > checking of all the np.array arguments to a function to ensure the > appropriate final np.array sizes are allocated? I'm not trying to question > current practice, just trying to understand as performance will eventually > be important to me. > > Would a "complex default" mode ever make it into numpy, to behave more > like Matlab and other packages with respect to complex number handling? > Sure it would make it marginally slower if enabled, but it might open the > door to better compatibility when porting code to Python. > > On Mon, May 25, 2020 at 9:49 AM Eric Wieser > wrote: > >> One explanation for this behavior is that doing otherwise would be slow. >> >> Consider an array like >> >> arr = np.array([1]*10**6 + [-1]) >> ret = np.log(arr) >> >> Today, what happens is: >> >>- The output array is allocated as np.double >>- The input array is iterated over, and log evaluated on each element >>in turn >> >> For what you describe to happen, the behavior would have to be either: >> >>- The output array is allocated as np.double >>- >> >>The input array is iterated over, and log evaluated on each element >>in turn >>- >> >>If any negative element is encountered, allocate a new array as >>np.cdouble, copy all the data over, then continue. This results in >>the whole array being promoted. >> >> or: >> >>- The input array is iterated over, and checked to see if all the >>values are positive >>- >> >>The output array is allocated as np.double or np.cdouble based on >>this result >>- >> >>The input array is iterated over, and log evaluated on each element >>in turn >> >> In either case, you’ve converted a 1-pass iteration to a 2-pass one. >> >> There are static-typing-based explanations for this behavior too, but >> I’ll let someone else present one of those. >> >> Eric >> >> On Mon, 25 May 2020 at 14:33, Brian Racey wrote: >> >>> Why does numpy produce a runtime warning (invalid value encountered in >>> log) when taking the log of a negative number? I noticed that if you coerce >>> the argument to complex by adding 0j to the negative number, the expected >>> result is produced (i.e. ln(-1) = pi*i). >>> >>> I was surprised I couldn't find a discussion on this, as I would have >>> expected others to have come across this before. Packages like Matlab >>> handle negative numbers automatically by doing the complex conversion. >>> ___ >>> NumPy-Discussion mailing list >>> NumPy-Discussion@python.org >>> https://mail.python.org/mailman/listinfo/numpy-discussion >>> >> ___ >> NumPy-Discussion mailing list >> NumPy-Discussion@python.org >> https://mail.python.org/mailman/listinfo/numpy-discussion >> > ___ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] log of negative real numbers -> RuntimeWarning: invalid value encountered in log
My Googling revealed none of this. I don't have anyone to talk to about using numpy, so Google (and this group) is all I have. This was extremely useful: https://numpy.org/devdocs/user/numpy-for-matlab-users.html so perhaps it could be extended to point out those modules and entry points if it isn't documented elsewhere. On Mon, May 25, 2020 at 11:17 AM Sebastian Berg wrote: > On Mon, 2020-05-25 at 11:10 -0400, Robert Kern wrote: > > On Mon, May 25, 2020 at 10:36 AM Sebastian Berg < > > sebast...@sipsolutions.net> > > wrote: > > > > > On Mon, 2020-05-25 at 10:09 -0400, Brian Racey wrote: > > > > Would a "complex default" mode ever make it into numpy, to behave > > > > more like > > > > Matlab and other packages with respect to complex number > > > > handling? > > > > Sure it > > > > would make it marginally slower if enabled, but it might open the > > > > door to > > > > better compatibility when porting code to Python. > > > > > > > > > > I think the SciPy versions may have such a default, or there is > > > such a > > > functionality hidden somewhere (maybe even the switching > > > behaviour). > > > I am not sure anyone actually uses those, so it may not be a good > > > idea > > > to use them to be honest. > > > > > > > The versions in `np.lib.scimath` behave like this. Of course, people > > do use > > them when they want to deal with real numbers as subsets of the > > complex > > numbers. > > > > True, I guess I just used complex numbers too rarely in programs (i.e. > never central to any programming project). > > It seems this is actually also exposed as `np.emath`, which is maybe a > better entry point? And I guess the scipy namespace uses them. > > - Sebastian > > > ___ > > NumPy-Discussion mailing list > > NumPy-Discussion@python.org > > https://mail.python.org/mailman/listinfo/numpy-discussion > > > ___ > NumPy-Discussion mailing list > NumPy-Discussion@python.org > https://mail.python.org/mailman/listinfo/numpy-discussion > ___ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion
