Is it possible to use variables stored in the workspace inside another function ?

Is it possible to use variables stored in the workspace inside another function?

[Johannes Hougaard]

Probably.

Look for the function 'global' to declare global variables

doc global % To see the help

If that doesn't suit your need you can use the evalin option inside your function

x_insidefunction = evalin('base','x');

[Stephen23]

@HN: global variables and evalin are two anti-pattern options that you should definitely avoid using, unless you really want to force yourself into writing slow, compplex, obfuscated, buggy code that is difficult to debug.

The MATLAB documentation clearly states the the best practice is to pass input/output arguments:

https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

[Johannes Hougaard]

agreed @Stephen - it's hideous and obscure...(but possible)

Answer 1

[Star Strider]

The best option is to pass them to the function as extra parameters. See: Passing Extra Parameters for an extended discussion.

[Johannes Hougaard]

agreed - that is a way better solution - it is possible (but you almost always end up with something incredibly messy) to use evalin or variables.

[Star Strider]

Not necessarily messy, although that depends on how the code is written. The extra parameters can be assigned to a structure, and the structure passed as an argument that can then be ‘unpacked’ in the function so the variables can be used individually as necessary.

[HN]

%Dear all, I've tried all the recommended techniques and faild :(. Can you please have a look the code? 
initialization
:
:
for i =1: lenth(t)
    :
    :
    wwx(i)=..;
    wwy(i)=..;
    wwz(i)=..;
    W=[wwx',wwy',wwz']; % 63x3
end
% function to be integrated over time 
function dq=fqdot(i,q)
       global W
       ww=W;       
       omg=[0 , -ww(1), ww(2), -ww(3);.....;..]; % ww=4 by 4 skew matrix changes with i. Total length is 63 x 4
       n=0.1;
       c=n*(1-q*q');
       dq=1/2*(omg*q')'+c*q;
end
% Integrator 
function yout=RK4(F,to,h,tFinal,y0)
        y=y0;
        yout=y;
        tt=t0:h:tfinal-h
        s1=F(tt,y);
        s2=F(t+h/2, y+h*s1/2);
        s3=F(t+h/2, y+h*s2/2);
        s4=F(t+h, y+h*s3);
        y=y + h*(s1 + 2*s2 + 2*s3 + s4)/6;
        y=y/norm(y);
        yout = [yout; y];        
end
% Now integrate fqdot using RK4
tmax=1
time=0:0.0164:tmax
q_init=[1,0,0,0];
qrk4=RK4(@fqdot,0,0.0164,tmax,q_init)
% The error I got is shown in the image I attached and couldn't figureout eventhough I set index. 

Thanks 

[Star Strider]

As always, my pleasure!

The error is straightforward, although impossible to detect in the code you posted. At some point, the indexing into ‘ww’ exceeds the row or column sizes of ‘ww’. That is as much as I can say about it.

Also, I would definitely not use global variables. Pass the other variables you need as extra arguments to your function, as I stated in my original post.

[HN]

I found it a bit harder to catch up the pass variable. Can you be a bit more specific please. Example.

When I run the for loop W is saved into the workspace. After finishing the for loop I wanted to use W inside fqdot function. How can I pass W into this funciton to compute omg?

Thank you so much for the help.

[Star Strider]

Please see the documentation on Passing Extra Parameters that I linked to in my original post.

I have no idea what you are doing in your code (other than it appears to be a Runge-Kutta integrator).