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Iterative solution to approach desired value

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M Teaxdriv
M Teaxdriv am 19 Mai 2022
Kommentiert: M Teaxdriv am 20 Mai 2022
Hello,
I would like to find desired value Q = 7.76 using starting external solver in my Matlab script. Input for the external solver is value p. From several runs of the external solver I found the following values of p = pQ(:,1) and corresponding Q = pQ(:,2). How to find desired value 7.76? I found several examples of algorithms like Newton-Raphson but they use analytical equation, what I don't have here. Could you help to write such script?
Best regards
Michal
pQ = [
0.99471e+5 20.014;
0.99871e+5 4.89;
99795.887596 7.735;
99795.941823 7.733;
99795.964903 7.732;
99795.976825 7.731;
99796.01 7.73
]

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Walter Roberson
Walter Roberson am 19 Mai 2022
Bearbeitet: Walter Roberson am 19 Mai 2022
fzero() or fsolve().
If you need to be able to look at the complete source code then https://www.mathworks.com/matlabcentral/fileexchange/72478-bisection-method
Note that when you do not reply to my suggestions such as https://www.mathworks.com/matlabcentral/answers/1721605-iterative-solution-to-achieve-convergence#comment_2165925 then it is difficult for the volunteers to guess what you need different than what has already been suggested.
  8 Kommentare
Torsten
Torsten am 19 Mai 2022
Bearbeitet: Torsten am 19 Mai 2022
I restricted the search interval to (90000:100000):
p0 = 99.925e+3 ; % some start value for p in [90000:100000] such that f(p) = 7.76
p0_trans = tan( (p0-95000)/5000 * pi/2);
p = fzero(@fun,p0_trans);
p_backtrans = 2/pi*atan(p)*5000 + 95000
function res = fun(p)
p_backtrans = 2/pi*atan(p)*5000 + 95000;
% Calculate Q as a function of p
save('p_value.mat',num2str(p_backtrans))
run external_solver
results = readtable('Results.txt')
Q = results.Q
res = Q - 7.76;
end
M Teaxdriv
M Teaxdriv am 20 Mai 2022
Thanks a lot for your help!

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