How to write function to plot?

[Ben Abbott]

On Apr 25, 2009, at 10:52 AM, John B. Thoo wrote:

> Hi, everyone.  I have a question about writing a function for
> plotting some data saved to a file.
>
> I have some data saved to a file "heat_q.mat" for the variables k, x,
> t, u.  Here's a snippet.
>
> %%%%%%%%%% begin snippet %%%%%%%%%%%
> # Created by Octave 3.0.5, Fri Apr 24 13:11:38 2009 PDT <jbthoo at John-
> Thoos-Computer.local>
> # name: k
> # type: scalar
> 3
> # name: x
> # type: matrix
> # rows: 1
> # columns: 21
>  0 0.3141592653589793 0.6283185307179586 0.9424777960769379
> 1.256637061435917
> ...
> # name: t
> # type: range
> # base, limit, increment
> 0 1 0.1
> # name: u
> # type: matrix
> # rows: 21
> # columns: 11
>  0 0 0 0 0 0 0 0 0 0 0
>  0.8090169943749475 0.3323236694342593 0.136510137653631
> 0.05607490346365374
> ...
> %%%%%%%%%% end snippet %%%%%%%%%%%
>
> If I load the file and then enter on the command line
>
>> l = 3;
>> plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');
>
> then I get a plot.  However, if I use the following function
>
> %%%%%%%%%%%%%%%%%%%%%%%
> function plotheat_q (l)
>
> % plot numerical solution and exact solution at time  t0+l*dt;
> plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');
>
> endfunction
> %%%%%%%%%%%%%%%%%%%%%%%
>
> then I get the following error:
>
> octave-3.0.5:2> load heat_q.mat k x t u
> octave-3.0.5:3> plotheat_q (3)
> error: `x' undefined near line 4 column 7
> error: evaluating argument list element number 1
> error: called from `plotheat_q' in file `/Users/jbthoo/Desktop/
> NumMethPractice/plotheat_q.m'
> octave-3.0.5:3>
>
> What am I doing wrongly?
>
> Thanks.
>
> ---John.

The function has its own private variable space. You can fix this is  
two ways

First, pass the variables to the function

> %%%%%%%%%%%%%%%%%%%%%%%
> function plotheat_q (l, k, t, x, u)
>
> % plot numerical solution and exact solution at time  t0+l*dt;
> plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');
>
> endfunction
> %%%%%%%%%%%%%%%%%%%%%%%

then

> octave-3.0.5:2> load heat_q.mat k x t u
> octave-3.0.5:3> plotheat_q (3, k, x, t, u)

Second, load the file inside the function

> %%%%%%%%%%%%%%%%%%%%%%%
> function plotheat_q (l)
>
> load heat_q.mat k x t u
>
> % plot numerical solution and exact solution at time  t0+l*dt;
> plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');
>
> endfunction
> %%%%%%%%%%%%%%%%%%%%%%%

and then

> octave-3.0.5:3> plotheat_q (3)


Ben