Undefined function or variable 'filename'.

Hi,

I've written some functions to numerically integrate the equations of motion of a double pendulum. It is a first order differential equation (like ydot = f(t,y)). I am writing my own numerical integration (as part of an assignment). Now here's the thing:

The matrices used to solve for ydot (the derivative of the state) are created with the symbolic toolbox. Because I do not want to use 'subs' each time before I solve, because it is very slow, I have used 'diary' to create an m-file frmatrix.m that simply states: fr = [blabla]; So when I run it, with the variables in my workspace, it simply creates the matrix fr. I do this for an Mr and an fr because I want to calculate Mr\fr. The lines are below eachother but I get an error for fr: Undefined function or variable 'frmatrix'. It has done exactly the same for 'Mrmatrix' which doesn't fail. I have checked and all.. the name is correct and frmatrix.m exists. Now here's the thing: if I restart matlab, it works fine. If i put a keyboard in the loop so I have to press f5 on every iteration, it works. But it will work only once after restarting matlab. if i run it again, i get the error again. even if i delete frmatrix.m

I really have no clue why this is happening so I hope anyone can help me out?

Kind regards, Iris

ps. below is a minimum working example, I hope it is clear. It consists of 2 separate m-files.

function tmt(par)

syms m I l g
syms p1 p2 p1d p2d

%%%%%%%Symbolic Mr and fr are created here, I left it out for simplicity %%%%%%%

% variables m,I,l and g are given a value
m = par.m;
I = par.I;
l = par.l;
g = par.g;

% The symbols of m, I, l and g in Mr and fr are subsituted with the values
Mr = subs(Mr);
fr = subs(fr);

% the files Mrmatrix.m and frmatrix.m are created.
if exist('Mrmatrix.m', 'file')
    ! del Mrmatrix.m
end
diary Mrmatrix.m
    disp('Mr = ['), disp(Mr), disp('];');
diary off

if exist('frmatrix.m', 'file')
    ! del frmatrix.m
end
diary frmatrix.m
    disp('fr = ['), disp(fr), disp('];');
diary off

function statederivative = eom(t,state,par)

p1 = state(1);
p2 = state(2);
p1d = state(3);
p2d = state(4);

Mrmatrix;
frmatrix;    % HERE IT FAILS!!!

qdd = double(Mr\fr);
statederivative = [state(3); state(4); qdd];

[Star Strider]

Is there some reason you must use the Symbolic Math Toolbox here rather than using ode45?

[Iris]

The symbolic toolbox is used here to derive the equations of motion which is not the same as numerically integrating them.

Answer 1

[Jan]

It is cruel to check the existence of a file by exist(name, 'file') without using an absolute path. It is cruel to delete a file by !del name, especially when no absolute path name is used. Creating a new file by disp and diary is even more cruel. Better:

folder = cd;  % for example the current folder, better define it explicitly
name   = 'fmatrix.m';
fid = fopen(fullfile(folder, name), 'w');
if fid == -1, error('Cannot open file for writing.'); end
fprintf(fid, 'fr = [%s];\n', fr);
fclose(fid);

This is much more direct.

If the created cannot be found, try clear fmatrix and / or rehash .

Prefer to create the m-file in a folder, which is contained in Matlab's path.

[Iris]

wow, you guys respond fast. thank you. I did not know that my methods are so cruel. I copied it from an example I had. rehash seems to work, although it makes the whole process very slow again. because time is important here, I will look into the other stuff you described tomorrow. Thank you for your answer so far!

[Iris]

I've tried to use your method to avoid the diary and disp and del.. but fprintf does not work for symbolics.. do you know of any other way to create this m-file that says fr = {contents of fr} ? I prefer not to use the rehash because it makes my program significantly slower.. Thanx in advance for your help

[Jan]

Please post your code. Otherwise it is much harder to suggest a modification. You can convert a symbolic expression to a string by the char command.

[Iris]

It has been a while but I now know that matlabFunction solves my problem. Thanx for your answer though, it certainly helped me understand things better. I'll accept the answer. It may be a solution for others and it includes my solution in this comment.

Answer 2

[Walter Roberson]

You need to

rehash

if you create a .m on the fly and want to execute it.

[Walter Roberson]

Faster than a general rehash would be to

clear frmatrix

[Iris]

Thanx for your reply. It has been a while but unfortunately this did not do the trick although I do not remember why exactly