Re: [matplotlib-devel] axis / loglog / semilog issues

[Todd M. <jm...@st...>]

On Mon, 2004-04-05 at 18:32, John Hunter wrote:
> >>>>> "Todd" == Todd Miller <jm...@st...> writes:
> 
>     Todd> Confusingly, the explicit call to nonzero() is not what is
>     Todd> causing the numarray problem.  The problem is an implicit
>     Todd> call to NumArray.__nonzero__() resulting from the range
>     Todd> expression.  I found I could "fix" the numarray exception by
>     Todd> adding parens to the range expression:
> 
>     Todd> nonzero((ticklocs >= 10.0**ymin) * (ticklocs <= 10.0**ymax))
> 
> 
> I wasn't able to replicate your problem in ticklocs.py on my system
> (could be a version issue?).

Reproducing the exception requires using numarray-0.9.  Also, 
ticklocs.py is coded to demonstrate the unexpected Numeric results
rather than the numarray exception to there is a try/except block which
passes the numarray exception on to parenthesized code.

>  Ie, the following works fine for me; I
> assume it generated a RuntimeError for you based on your example code.
> 
> 
> ______________________________________________________________________
> In any case, in general I try to avoid using multiplication for
> logical arrays because it sometimes produces surprising results.  I
> think here we want
> 
>             ind = nonzero(logical_and(ticklocs>=10.0**vmin ,
>                                       ticklocs<=10.0**vmax))
> 

This is clearer and unambiguous.

> 
>     Todd> Lastly, assuming the parens are correct, there is still a
>     Todd> problem with the log functions which I haven't looked at
>     Todd> yet:
> 
>     >>>> loglog(arange(2,800))
> 
> Here's what's going on under the hood
> 
> You are using the single arg form of plot with log scaling for both
> axes.  The y values are arange(2,800) and the x values are implicit:
> arange(len(y)).  Since this starts with 0, when take the log of zero
> in loglog, you are in a world of pain.  The current version of
> matplotlib warns but then tries to go ahead, which generates the
> error.  Specifically, it took the positive parts of x but left y
> alone, resulting in the unequal sized array problem.
> 
> I changed the code to raise rather than warn.  You now get the more
> helpful error message
> 
>   ValueError: Cannot take log of negative data
> 

Makes sense.  Thanks for the explanation.

> Both changes in CVS.
> 
> JDH

Regards,
Todd

-- 
Todd Miller <jm...@st...>