MATLAB: Error of binary operator ‘/’ not implemented for ‘scalar’ by ‘function handle’ operations

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So I have been experimenting with function handles and generally trying to vectorise my code to make it run nice and fast. I have been trying to solve the forced KdV equation using a Newton-Raphson scheme and I have the basic code working with a simple example but I am having trouble with the odd error message in the title of this cry for help.

Anyone know what I should do?

I use matlab at work and Octave at home.

Best Answer

The weakly_nonlinear function includes these lines:

h=@eta;
while (sol_error>converge_limit)
F=-feval(h,x,sol,p,F,h,B,choice)';

This calls the eta function with @eta as the fifth input argument. Let's look at a chunk of eta:

function y=eta(x,f,p,F,h,B,choice)
N=length(x);
y=zeros(1,N+1);
rho=1;
g=9.81;
a_1=1-f(N+1);
a_2=0.75./h;

The last line is the one throwing the error. When computing a_2 you're dividing by @eta. That doesn't look right. Usually in this type of problem a variable named h represents the spacing between elements in x or something similar. If that's the case, you probably want to use diff(x) or something along those lines.

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MATLAB: Read inputs from GUI into a second master script

To put the summary of all this in an answer.

Think what you are doing before just always putting any or all of the following in a script:

clear
clear all
close all

etc.

If you are expecting to have variables defined in a GUI then clearing them all at the start of your script is not very useful.

If you used a function instead of a script you wouldn't feel the need to do this at all as each function has its own workspace and doesn't retain all the detritus from whatever workspace it was called from (nor does it pollute said workspace with its own variables once it has completed!)

MATLAB: How to keep a handle and use FINDOBJ to get a handle again

What Matt Fig said, plus: are you actually copying the number in (rather than saving it in a variable and passing the variable into the function)? Because that won't work for anything other than a figure handle. Figure handles are integers, but everything else is actually a hex reference (that appears as a double because, as the old joke goes, you can have any data type you want in MATLAB as long as it's a double -- no longer true, of course, but the remnants of that are seen in places, like HG). Anyway, the key point being that the handle isn't 123.0045 -- it's 123.0045blahblahblah. Any attempt to copy/paste the actual number will invariably get the floating point representation slightly off, and will therefore result in an incorrect hex reference, and that error message you love so much.

Bottom line, then: save handles as variables.

As Matt F alluded to in his answer, you can store handles as outputs from 99% of graphics function calls:

hf = figure;
ha = axes;
hp = plot(x,y,x,z);

etc. In this case, hf and ha will be scalar handles, hp will be a vector of 2 handles (one to each line object on the plot).

Use findobj to locate handles that you don't already have:

h = findobj('type','line')

And use the parent and children properties to navigate the hierarchy. For example, the above example could also be done:

hp = plot(x,y,x,z)
ha = get(hp(1),'parent')  % parent of a line is the axes
hf = get(ha,'parent')     % parent of the axes is the figure

Best Answer

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MATLAB: Read inputs from GUI into a second master script

MATLAB: How to keep a handle and use FINDOBJ to get a handle again

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