Complex values computed by fit.m function (Curve fitting Tool)

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Matti Rademacher
Matti Rademacher 2019 年 6 月 26 日
コメント済み: Matti Rademacher 2019 年 6 月 26 日
Hi,
I am trying to use the fit.m function, but matlab throws the "complexValueComputed" exception, even though my x variable could not created complex values.
My x variable is: x = 1 - T./Tcrit and T is an array with max(Tsat) < Tcrit.
My y variable is an irrational function: y = a1 + a2 .*x.^(1/3) + a3 .* x.^(2/3)+ a4 .* x.^(3/3)+...
So there is just a possibilty to create complex values for Tsat > Tcrit, but this is not possible.
I even try to solve this problem by using the abs() function for my x variable, but nothing changed.
The error message is: "Complex value computed by model function, fitting cannot continue.Try using or tightening upper and lower bounds on coefficients."
and appears in fit.m in line 116 or for the iFit.m function in line 348.
Any help for resolving this problem would be greatly appreciated!
  6 件のコメント
Stephen23
Stephen23 2019 年 6 月 26 日
編集済み: Stephen23 2019 年 6 月 26 日
Matti Rademacher
Matti Rademacher 2019 年 6 月 26 日
I know that the eval-function is not the best solution, but the preworker of the scripts used it everywhere and it is not so easy to replace them all.

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回答 (1 件)

Torsten
Torsten 2019 年 6 月 26 日
編集済み: Torsten 2019 年 6 月 26 日
Since your fitting problem is linear in the coefficients a1,a2,a3,..., this is all you need to get the best fit parameters:
A = [ones(1368,1), Tsat.^(1/3), Tsat.^(2/3), Tsat.^(3/3), Tsat.^(4/3)];
b = psat;
sol = A\b;
a1 = sol(1)
a2 = sol(2)
a3 = sol(3)
a4 = sol(4)
a5 = sol(5)
Best wishes
Torsten.
  7 件のコメント
Torsten
Torsten 2019 年 6 月 26 日
So mag(1,1) is equal to 4 in your code ?
Matti Rademacher
Matti Rademacher 2019 年 6 月 26 日
Yes, but anyway I think I found the problem's source: the fitoption normalize. This could be an explanation for complex values.

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