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

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Yahia Mounir
Yahia Mounir on 14 Sep 2015
Commented: Hamoon on 15 Sep 2015
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

Accepted Answer

Hamoon
Hamoon on 14 Sep 2015
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.
  2 Comments
Hamoon
Hamoon on 15 Sep 2015
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.

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More Answers (1)

Yahia Mounir
Yahia Mounir on 15 Sep 2015
Thanks it is very helpful
  3 Comments
Hamoon
Hamoon on 15 Sep 2015
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.

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