How to use global variable which is changing in every time step?

I have a particular code which looks like this:

    function H = newmain 

    global b1 b2 b3 

    b1=1.0;
    b2=2.0;
    b3=0.33;

    options=odeset('InitialStep', 0.01, 'MaxStep', 0.01, 'RelTol', 10., 'AbsTol',10.);
    [t2,y2]=ode45(@equation,[0:0.05:0.1],[1 2 0], options);

    H = [t2 y2]

    X=[b1 b2 b3]

    end

    function dy=equation(t,y)

    global b1 b2 b3 

    dy=zeros(3,1);

    b1=((b1+(1.-exp(-b3))))
    b2=b1-y(1)
    b3=b1+b2+y(2)

    dy(1)=-b1*y(1);
    dy(2)=b3*y(1)+b2*y(2);
    dy(3)=sqrt(b1)+y(1)+y(3);

    end

I have given initial values for b1,b2 and b3 which are my global variables. I want to understand how the solver uses this code to return a new value of b1,b2 and b3.

Can global be used to keep a value constant throughout the code?

I am solving 12 algebraic and 6 odes which look like the above code. b1,b2,b3 are synonymous with my algebraic variables and dy(1).... are my odes. When I solve that code I get an error saying "NaN". So I am unable to find out loophole in my code.

Thanks!

Answer 1

[Azzi Abdelmalek]

use break points to debug your code

[Urvi]

yes I will try that. thanks!

[Azzi Abdelmalek]

In your equation function insert

global b1 b2 b3 b

% at the end of equation function add

b=[b ;b1 b2 b3]

% don't forget in your main program to set

 global b1 b2 b3 b

[Urvi]

Hey thanks a lot! i got the solution for the above problem!!! But i have used the same logic for my actual code and it isn't working! How can i pass initial values? Thanks again!

[Azzi Abdelmalek]

what is your actual problem?

[Urvi]

Can you please check this link : http://www.mathworks.in/matlabcentral/answers/50185-i-have-a-problem-while-using-global-variable

I have tried to explain it there. Thanks once again!

Answer 2

[Walter Roberson]

One thing you need to keep in mind is that ode45() does not promise to evaluate in strictly increasing time steps.

[Urvi]

Okay. But I am confused about the role of "global variable" . Will the solver return a new value for the same in every time step?

[Urvi]

The solver isn't returning a new value for my code. It is using the initial values for some variables and a changing value for some global variables. What could be the problem?