I have a Problem while using global variable.

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Urvi 2012-10-8

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I have a system of equations as follows:

dy1/dt = f(y1,a,b,c,d,y2...) . . . . dy6/dt = f(y1,a,b,c,d,y2...)

These are my ordinary differential equations. a,b,c are changing with time and I have 12 equations for 12 such variables. All of these equations are interdependent on each other.

Now, in the beginning of my main function I have declared a,b,c as global and I have specified their initial values.

Now, I have written the equations for a,b,c.... followed by my odes, i.e. algebraic equations followed by my ordinary differential equations. I am using ode45. When I run the code, I get results but wrong because of the following reasons :

The code is not calculating new values for some global variables in each time step. The first 9 algebraic equations have the same value throughout whereas the last 3 values are changing. I cannot figure out how to pass new values for these 9 variables. As a result of this, the results for my odes are coming out be extremely absurd.

Any help/suggestion will be appreciated. Thanks.

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Urvi 2012-10-9

I have posted it in one of the comments below. Thanks!

swaraj 2012-10-9

if u dont mind can you pls ping to me mail.here i dont have acceses to take

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回答（1 个）

Sean de Wolski 2012-10-8

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This is a good reason to not use globals. Can you show us a snippet of your code?

Have you seen:

How to Pass extra parameters?

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Urvi 2012-10-9

在 MATLAB Online 中打开

Pass those in as in? Here is a similar code and i'll explain what is happening with my actual code using the following example:

M FILE (I intend to run the code directly from the editor)

    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

b1 b2 b3 are my variables changing with time and I have declared them as global. dy(1),dy(2) and dy(3) are my odes. This code is working fine. The code takes a new value for b1 b2 b3 with every time step. But, for my actual code, b1 b2 b3........b9 are not changing whereas b10 b11 b12 are changing with time. (I have 12 variables hence b1.....b12) Why is this happening? I have used the exact same logic as the code above. It doesn't seem to work.

Thanks!!

Urvi 2012-10-9

在 MATLAB Online 中打开

    function F=Main
    global a b c d      % these are my constants
    input_parameters()             % file which contains the above constants
    global b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12;           
 %these are the variables changing with time. they have their equations. Values need to be calculated at evry time step
    %Initial values of the global variables declared above
    b1=0;
    b2=400;
    b3=0;
    b4=0;
    b5=0;
    b6=0;
    b7=0;
    b8=0;
    b9=0;
    b10=0.00056;
    b11=0.0.0008;
    b12=0.009;
     % calling the function that has the equations
    options=odeset('InitialStep', 1, 'MaxStep',0.1, 'RelTol', 0.0010, 'AbsTol',0.0010);
    [t1,x1]=ode15s(@newfunction,[0:60:1500],[200 200 200 200 1 0.5], options);
    Z = [b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12]
    F = [t1 x1] 
    end

% this function has the system of equations that need to be solved

    function f=newfunction(t,x)
    f=zeros(6,1);
    global a b c d ......z     % these are my constants
    global b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12;   
    input_parameters()
    y1=x(1);
    y2=x(2);
    y3=x(3);
    y4=x(4);
    y5=x(5);
    y6=x(6);
    % ALGEBRAIC EQUATIONS:
    %equation 1
    if y5<=1
        b1=0
    else
        b1=a*(y5-1)
    end
    % equation 2
    b2=273-230.170+3855.70/(16.3872-log(100*y5))
    %equation 3
    if y1<=400
        b3=0
    else
        b3=(0.8*c*d*(y1-400)/l)
    end
    %equation 4
    b4=b6/[(b5/(1-b4))-0.2]
    %equation 5
    y5=y9/(p*q)
    %equation 6
    b6=b3/(r*q)
    %equation 7
    b7=(6*b10*b4)/0.003
    %equation 8
    b8=sqrt(2*y6*b4*j/k)
    %equation 9
    b9=p*q*b8
    %equation 10
    b10=m*n^2*y6
    %equation 11
    b11=m*(0^2-n^2)*y6
    %equation 12
    b12=m*0^2*[h-y6]
    % ORDINARY DIFFERENTIAL EQUATIONS:
    % ODE1
    dy1dt=(1/p*q*b10)*[b3*y1-b4*y2];
    f(1)=dy1dt;
    % ODE2
    dy2dt=(1/p*q*b11)*[b3*y1-b4*y2+b9*y3+b1*y4];
    f(2)=dy2dt;
    %  ODE3
    dy3dt=(1/p*q*b12)*[b3*y2-b4*y1+b9*y3+b1*y3];
    f(3)=dy3dt;
    % ODE4
    dy4dt=(1/p*q*b12)*[b1*y1-b1*y4];
    f(4)=dy4dt;
    % ODE5
    dy5dt=a*b/y5 + dy4dt
    f(5)=dy5dt;
    % ODE6
    dy6dt=s-b3/p*b4+q*y6
    f(6)=dy6dt;
    end

Walter Roberson 2012-10-10

You initialize some of them once in your main routine when you call input_parameters() there, and then you re-initialize some of them in your other routine when you call input_parameters() again there

Urvi 2012-10-10

编辑：Urvi 2012-10-10

But they are constants. Their value isn't changing at all. What I need to change with every time step is : global b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12; input_parameters does not contain b1 b2 b3 b4......it contains a..z(constants). I am initializing b1 b2 b3...in the present code itself.

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