fsolve in for loop: failure in initial objective function evaluation

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Anastasia Gladkina 2017-3-3

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/327824-fsolve-in-for-loop-failure-in-initial-objective-function-evaluation

回答： Anastasia Gladkina 2017-3-6

Matlab_fNLSE_FSolve_Method.m

I'm trying to solve a system of nonlinear equations using fsolve, and I have to do it for many timesteps, nsteps. The initial condition (defined outside the loop, I'm solving a differential equation underneath) is u, which is used in the definition of the function func. The first time we go around the loop, u is defined as the initial condition; but after that, I want the solution given by fsolve to be assigned to u. Therefore, I have to keep the function definition inside the loop so that u is able to change.

g=1;
r=1j*dt/2;
nsteps=100;
nn=100;
for i=1:nsteps
    func=@(z) r*g*(norm(z)^2)*z+z-deriv*z+r*g*(norm(u)^2)*u-u-deriv*u;
    input=ones(nn,1)';
    u=fsolve(func,input);
end

I get the error in the fsolve(func,input) line that says "Failure to initial objective function evaluation. FSOLVE cannot continue." I read somewhere it could be because the function is defined in the loop and should be defined outside in a separate file, but I cannot do that fix because u has to be the previous solution that fsolve found. I am not saving these u's elsewhere.

deriv is an nn x nn matrix, z is the free vector that we're trying to solve for, dimension nn x 1, u is also nn x 1, r and g are constants.

edit: I also get an error "Error using *, Inner matrix dimensions must agree" for the function definition. Maybe Matlab's not recognizing that z is a vector instead of a variable, and then norm would be undefined?

edit 2: I posted all of my code.

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Answer 1

Anastasia Gladkina 2017-3-6

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https://ww2.mathworks.cn/matlabcentral/answers/327824-fsolve-in-for-loop-failure-in-initial-objective-function-evaluation#answer_257530

The problem was that I uncommented one of the lines that I needed to set up the initial condition. Whoops!

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Answer 2

Alan Weiss 2017-3-3

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在 MATLAB Online 中打开

Perhaps the problem is your statement

input=ones(nn,1)';

That makes input a 1-by-nn vector, when you want it to be nn-by-1. Change the line to

input=ones(nn,1);

and see if that fixes things.

Alan Weiss

MATLAB mathematical toolbox documentation

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Anastasia Gladkina 2017-3-6

If I change input to not be transposed, I get almost the same error. It says "Error using -, Matrix dimensions must agree", while before, the error was "Error using *, Matrix dimensions must agree".

I will post all of my code because I realize it's difficult to troubleshoot.

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