I can not solve non-linear equation using m.file only

Hello I am trying to run the following file attached however it give me error undefined variable X, I attached the m.file can you help with that ,

Please note if i call the function at command window with the following steps @myfun x0=[0 0] x=fsolve(@myfun,xo) it is working and giving me the answer however i need only to use the mfile to get the answer

Answer 1

[Hamoon]

Cut this part of the code from the mfile you atteched:

 x0=[0 0];
 options = optimoptions('fsolve','Display','iter'); % Option to display output
 x = fsolve(@myfun,x0,options);

then copy it to another script, and run that script. Make sure that two scripts are in a same path.

[Yahia Mounir]

Thanks it is working perfectly now . However can you have a look for the attachment equation I am Trying to solve it but always giving me error ? I attached the m.file + the equation Thanks very much in advance .

[Hamoon]

first you should put your variables inside the function block when you are defining a function, so myfun should be like this:

 function F = myfun(x)

 K1=1.912284191525973e+08;
 K2=1.003211416173691;
 K3=0.003211416173691;
 K4=43.627688688552304;

 F = [(x(1)^2*K1*x(2))-K4;
       K2*(1-K3*log((1+K2*x(1))^(1/x(1))))-x(2)];

 end

You've got another problem in your code too, through F evaluation you have this term:

 1/x(1)

and when algorithm tries to get the value of F for example for x=[0,0], the second term of F will be NaN (Not a Number) because 1/x(1) when x(1)=0 is undefined and you'll get an error. please accept the answer if you are happy with it and it's the answer of your question.

Answer 2

[Yahia Mounir]

Thanks it is very helpful

[Yahia Mounir]

I have one more question can i call these constants from workspace ? how can i do so?

[Walter Roberson]

Which workspace are they in?

[Hamoon]

It would be better if you asked this question as a new question, first because other people can help you on this and nobody open an answered question if they want to answer a question, they don't know there is an another question here (I didn't know too, I just saw this by accident) and also it can be someone else's question too, so they will find the answer easier if you ask another question as a new one.

For what you asked, there is several ways to do that.

1) You can pass the constants to your evaluation function like this:

 function F = myfun(x,K1,K2,K3,K4)

 F = [(x(1)^2*K1*x(2))-K4;
     K2*(1-K3)-x(2)];

 end

and then in the solver script you can use this code:

 x0=[0 0];
 K1=1.912284191525973e+08;
 K2=1.003211416173691;
 K3=0.003211416173691;
 K4=43.627688688552304;

 options = optimoptions('fsolve','Display','iter'); % Options
 x = fsolve(@(x) myfun(x,K1,K2,K3,K4),x0,options);

-----------------------------------------------------------------------

2) you can define global variables:

so your function script can be:

 function F = myfun(x)

 global K1 K2 K3 K4

 F = [(x(1)^2*K1*x(2))-K4;
     K2*(1-K3)-x(2)];
 end

and your solver script:

 x0=[0 0];

 global K1 K2 K3 K4

 K1=1.912284191525973e+08;
 K2=1.003211416173691;
 K3=0.003211416173691;
 K4=43.627688688552304;

options = optimoptions('fsolve','Display','iter'); % Options
fsolve(@myfun,x0,options);

I hope this helps you.