Re: Help Plotting Symbolic Vector Fields with Substitutions

[Colin Macdonald]

On 21/04/17 12:38 AM, WARDEVIL_UFO wrote:
> I have had trouble posting on Nabble's Octave Forum the past 2 days.  I had
> this response written Wednesday, but haven't had any luck until now.
>
>
> Colin Macdonald-2 wrote
> > I filed a bug to do that: https://github.com/cbm755/octsympy/issues/779.
> >    Since this is fresh on your mind, perhaps you could suggest a short
> > paragraph for "help subs" perhaps with a simple example?
> >
> > Another option: we could overload @sym/quiver3.
> >
> > thanks,
> > Colin
>
> I am new to Matlab/Octave.  I don't know what overloading is.  Also, to date
> I have never successfully called subs in Octave.  The code you guys saw was
> my first attempt to do just that.  I know next to nothing about the approved
> applications for subs.  In the above case I attempted copying code I saw
> from a MatLab help example.  As far as a simple example, I would be honored
> to help!!!!

Cool, maybe we can take that discussion to 
https://github.com/cbm755/octsympy/issues/779

re, your example below, my first thought was a scalar or 1D 
example---but I'm not sure.  It is nice to show at least partial output 
(our examples are also part of our test suite).

You could try editing `/some/path/to/octave_packages/@sym/subs.m` 
directly and see how it looks/reads?

> The following code will produce the same vector field plot as Figure 1.14
> from Example 1.6 (pg. 39) from A Student's Guide to Maxwell's Equations by
> Dr. Daniel Fleisch.  Beware that this code is somewhat custom tailored to
> this particular example.
> ________________________________________________________________________________________________
> pkg load symbolic;
>
> syms x y
>
> vector = [sin(pi*y ./ 2); -sin(pi*x ./ 2)];
>
> iComponent = function_handle(vector(1,1));
> jComponent = function_handle(vector(2,1));
>
> [X,Y] = meshgrid([-.5:.05:.5]);
>
> figure
>
> quiver(X,Y,iComponent(Y),jComponent(X))
> ________________________________________________________________________________________________

This example (with fixes per my previous email) would be very good for 
the wiki.

http://wiki.octave.org/Symbolic_package

Could you post it there?  Maybe after the first example in "I'm trying 
to substitute a double value into an expression, how can I avoid the 
rats warning?".

Colin