How can I go on with my code even if fsolve does not find a solution?

Question

daniele frollani 2012-10-16

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Hello everyone, this is something i've been looking for ages and for whihc I still haven't found a solution. The problem is easy to explain: consider I have a code like this:

a=1;
for i=1:5
.....
sol = fsolve(@myfun,x0,options,....)
if 'fsolve can't find the solution or there is any error'
sol(1) = 1;
sol(2) = 1;
a = a+sol(1)-sol(2);
else
a = a+sol(1)-sol(2);
end
end

Let's pretend that if there is not a solution for fsolve this means I somehow know the values. So if fsolve doesn't find a solution or it gives any error (such as Error using trustnleqn (line 28) Objective function is returning undefined values at initial point. FSOLVE cannot continue.Error in fsolve (line 399) [x,FVAL,JACOB,EXITFLAG,OUTPUT,msgData]=...) I just want to go on with the code like nothing has happened. Instead fsolve always kicks me out of the program.

I would be grateful if anyone could help me. Regards, Daniele

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Answer 1

Walter Roberson 2012-10-16

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Use the three output form of fsolve() and look for a negative exitflag.

If fsolve() is generating an error() then use try/catch to contain the error.

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daniele frollani 2012-10-17

sorry maybe i should have written in here...

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Answer 2

daniele frollani 2012-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/51013-how-can-i-go-on-with-my-code-even-if-fsolve-does-not-find-a-solution#answer_62240

在 MATLAB Online 中打开

Hello Walter, first of all thanks for your answer, I really appreciated that. I solved my problem in this way:

      a=1;
      for i=1:5
        .....
       [sol,err,flag] = fsolve(@myfun,x0,options,....)
         if flag<0 || flag==0
           sol(1) = 1;
           sol(2) = 1;
           a = a+sol(1)-sol(2);
         else
           a = a+sol(1)-sol(2);
         end
      end

I looked for the try/catch construction but I didn't understand how to apply it to the problem. Even though my problem has been solved (and thanks again) it would be helpful if you can give me a short example of the try/catch condtruction.

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Walter Roberson 2012-10-17

在 MATLAB Online 中打开

try
  [sol,err,flag] = fsolve(@myfun,x0,options,....)
       if flag<0 || flag==0
         sol(1) = 1;
         sol(2) = 1;
         a = a+sol(1)-sol(2);
       else
         a = a+sol(1)-sol(2);
       end
  catch ME
         sol(1) = 1;
         sol(2) = 1;
         a = a+sol(1)-sol(2);
  end