CODEWITHSUNDEEP
matlabequationequation-solving
Mardor
Mardor

Reputation: 11

Solve equation {exponential *prod exponential(ln)}

I want to solve a equation in MATLAB with fsolve

I'm trying to do that:

function F = root2d(P); 

lambda = 2*10^-4;
th = -40:-1:-106;
PL1 = 10471285.480509; % (mw)
p1 = 10;
p2 = 6 ; 
p3 = 8 ; 
al = 2.5;
T = 10.^(th./10);
r = (p1*PL1^(-1)./T).^(1/al);
R = (p2*PL1^(-1)./T).^(1/al);
syms P
c = (lambda.*pi.*(R.^2));
j = 1:3;
D = zeros(3,67);
for k = 1:numel(j)
    F(1) = (prod(D(k,:)==exp(P(1).*c.*(log(P(1)).^k/factorial(k))))).*exp(-(lambda*P(1).*pi.*(r.^2)))-P;
end
fun = @root2d;
P0 = 0;
P = fsolve(fun,P0)

Do you have an idea?

Upvotes: 1

Views: 144

Answers (1)

Nicky Mattsson
Nicky Mattsson

Reputation: 3052

There are a couple of problems in this code. I will help you with the most tricky.

It is identified using rubberduck debugging. Let me exemplify it by explaining your code to you.

Starting from the top you define a function called root2d, which returns some object F, then in this function you define alot of constants. With these cosntants defined, you can define your symbolic function F, which we earlier saw was the output of the function. Continuing in this function, root2d, you now create a function handle fun to root2d, and then in the last line you call fsolve to solve fun.

Did you see the error?

fsolve calls fun which is a function handle to root2d. Thus fsolve calls root2d. Running root2d MATLAB comes across a new fsolve which calls root2d. I.e. root2d will be pushed unto the stack over and over again.

This is solved by taking the last three lines out of the function, and then running them from another script or from the terminal.

Doing so results in a new error:

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

Error in sym/privsubsasgn (line 1085)
L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/subsasgn (line 922)
C = privsubsasgn(L,R,inds{:});

Error in SO (line 18)
F(1) = (prod(D(k,:)==exp(P(1).*c.*(log(P(1)).^k/factorial(k))))).*exp(-(lambda*P(1).*pi.*(r.^2)))-P;

Error in fsolve (line 242)
fuser = feval(funfcn{3},x,varargin{:});

Caused by:
Failure in initial objective function evaluation. FSOLVE cannot continue.

I.e. you have to ensure that your objective function, F(1) =... is correctly written.

Upvotes: 1

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