Iterative solution to approach desired value

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M Teaxdriv 2022-5-19

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https://ww2.mathworks.cn/matlabcentral/answers/1722825-iterative-solution-to-approach-desired-value

评论： M Teaxdriv 2022-5-20

采纳的回答： Walter Roberson

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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 = [
99471e+5	20.014;
99871e+5	4.89;
887596 7.735;
941823 7.733;
964903 7.732;
976825 7.731;
01 7.73
    ]

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采纳的回答

Walter Roberson 2022-5-19

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https://ww2.mathworks.cn/matlabcentral/answers/1722825-iterative-solution-to-approach-desired-value#answer_967215

编辑：Walter Roberson 2022-5-19

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.

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Torsten 2022-5-19

编辑：Torsten 2022-5-19

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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