Change equation in fsolve inside a loop

Hi All,

I am having some trouble with the following problem:

I create some symbolic equations from a complex mathematical model (the following equations are just to show my problem) as follows:

syms x1 x2 x3 

EqsToSolve= [x1+x2*x2+7*x3 ;log(x1)+exp(x2)+x3 ;3*x1+x2*x3+2]

what I do now is copying this to a M-File that looks like

function y=myfun(x)
x1=x(1)
x2=x(2)
x3=x(3)

y=[x1+x2*x2+7*x3 ;log(x1)+exp(x2)+x3 ;3*x1+x2*x3+2]
end

and I solve it using fsolve as usual.

With the solution of this system I update my model and create another set of equations such as:

EqsToSolve2= [54*x1^2+x2+x3 ;(x2)+x3+32 ;3*x1+x2*x3+2+log(x3)]

and I copy that to myfun.m and so on till convergence. The problem is that convergence can arrive after 1000 iterations and this procedure is slow.

IS there any way to solve this problem, that is, to achieve something like:

while not_converged
create_updated_model
solve new_equation
end

or how to change the equation in the myfun.m inside a while o for loop.

Thank you very much!! Any help will be appreciated!!

Answer 1

[Conrad]

Try using anonymous functions:

f1 = @(x) [x(1)+x(2)*x(2)+7*x(3) ;log(x(1))+exp(x(2))+x(3);3*x(1)+x(2)*x(3)+2];
f2 = @(x) [54*x(1)^2+x(2)+x(3) ;(x(2))+x(3)+32 ;3*x(1)+x(2)*x(3)+2+log(x(3))];

Answer 2

[esbolico]

Hi Conrad,

thank you for your answer. I'll check it and keep you informed!!!

Answer 3

[esbolico]

I still do not know how to make it work. The thing is I get my model as a function of x1,x2,x3 or x,y,z. How can I change it to get x(1),x(2),x(3)??

I'll try to explain it better:

After running my code I get a symbolic variable such as

EqsToSolve= [x1+x2*x2+7*x3 ;log(x1)+exp(x2)+x3 ;3*x1+x2*x3+2]

as far as I understand you are telling me to change manually x1-->x(1), x2-->x(2) and so on

[Conrad]

f1 = @(x1,x2,x3) [x1+x2*x2+7*x3 ;log(x1)+exp(x2)+x3;3*x1+x2*x3+2];

[Walter Roberson]

See also my call_on function http://www.mathworks.co.uk/matlabcentral/answers/48928-converting-a-vector-to-multiple-variables-required-as-input-for-matlabfunction