loop intial guesses in fsolve

Question

Joon Jeon 2012-4-3

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https://ww2.mathworks.cn/matlabcentral/answers/34401-loop-intial-guesses-in-fsolve

采纳的回答： Matt Tearle

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I am trying to solve some equations by using solve.

Actually I did it. But this time, I am trying to vary some parameters.

Let's say I have one parameters which is 1:1:9.

And I code it like,

a=1:1:9
for i=1:9
y=@(x) x*exp(a(i))-x/2;
f=@(x) a(i)*x^2+1/2*y(x)-300;
x0=200:-10:110;
p(i)=fsolve(f, x0(i));
end

But it gives me an error message that index matrix is not matching. Is it possible to loop initial guesses in fsolve? (above coding is just an example.)

Since I only use one set of initial guesses in my real model, fsolve keep giving me the same solution.

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Answer 1

Matt Tearle 2012-4-3

1
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https://ww2.mathworks.cn/matlabcentral/answers/34401-loop-intial-guesses-in-fsolve#answer_43134

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Works for me. Some possible reasons for your error: p is already defined in your workspace, in a way that doesn't allow the assignment you have in the loop; or your "real" code is actually for a function of multiple variables, (so x is actually a vector).

In the first case, that's easily solved by preallocating p before the loop, which you should do anyway:

a=1:1:9;
x0=200:-10:110;
p = zeros(9,1);
for i=1:9
    y=@(x) x*exp(a(i))-x/2;
    f=@(x) a(i)*x^2+1/2*y(x)-300;
    p(i)=fsolve(f, x0(i));
end
plot(a,p,'o-')

In the second case, you'll just need to changing your indexing. x0 would have to be a matrix, as would p, and you'd have to index into a whole row or column (depending on your choice of orientation) on each iteration.