From help-request at octave dot org Thu Jan 12 09:34:05 2006
Subject: Re: Easy tutorial on how to make 3-dimensional graphics
From: Quentin Spencer <qspencer at ieee dot org>
To: "Tetsuji \"Maverick\" Rai" <maverick31337 at gmail dot com>
CC: help at octave dot org
Date: Thu, 12 Jan 2006 09:29:30 -0600

Tetsuji "Maverick" Rai wrote:

>Thank you for your help...
>
>I'm a newbie on octave.
>
>Now I've begun writing a small function to make a 3-d plot, but I'm
>stuck at linspace(,,).  Following function fails like
>
>octave:2> h100(1)
>error: linspace: npoints is 1, but x1 != x2
>error: evaluating assignment expression near line 6, column 9
>error: evaluating assignment expression near line 6, column 5
>error: called from `h100' in file
>`/home/tetsuji/physics/hydrogen-atom/h100.m'
>
>- ----h100.m----
>function h100(dn)
>
>  a0 = 1;
>  n = 1;
>
>  x = z = linspace (-10, 10, dn);
>  [xx, zz] = meshgrid (x,z);
>  [theta,r] = cart2pol (zz,xx);
>
>  r1 = 1 ./(n*a0);
>  y = r1 .^1.5 * exp(-r1*r);
>  y = y*y ./ pi;
>
>  mesh(x,z,y);
>endfunction
>- ---------------
>
>  
>

The last argument to the linspace function must be >1. Type help 
linspace to learn why.

>Apparently x and z has a problem (I think I just borrowed this line from
>sombrero(n)).  I must be doing something wrong, but don't know what's
>wrong..  Looking at sombrero(n), I see a single quote after
>linspace(-8,8,n) and a period before slash '/' or hat '^'..... What are
>they??  As for math, C++ is much easier to me... very unusual :(
>  
>

In octave, a*b represents a matrix multiplication of matrices a and b 
(which requires that they have the correct dimensions). That is 
different than element-wise multiplication of a and b, so that is 
represented with the symbol ".*". Since the operators / and ^ also have 
special meanings for matrix operations, if you want to divide all of the 
elements in one vector or matrix by all of the elements in another of 
the same dimensions, you use "./"; the same is true for exponents and 
the ".^" operator. The single quote represents the transpose of a matrix.

>In above function, x and z axes are on the horizontal plane, and y axis
>stands vertically.
>
>So are there any good online tutorial for octave (not document on
>www.octave.org)?
>  
>

Maybe, but I don't know where. Maybe someone else can respond with 
suggestions.

-Quentin



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