Polynoom coëfficiënt from excel data

Question

Armando Marchena 2015-10-17

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/249143-polynoom-coefficient-from-excel-data

评论： Star Strider 2015-10-17

In order to calculate a parameters in a 5th order Polynoom function, im using "fzero" in a function. The input data is from an excel list (code shown below). I would like to function to work with any data inputted from an excel sheet.

    function y = Cp_REV02(~)
%clear
%clc
cp_data = xlsread('Data.xlsx', 'testing', 'P7:P44')
f = @(x)- 0.0006*x.^5 + 0.0104*x.^4 - 0.0602*x.^3 + 0.146*x.^2 - 0.108*x + 0.043 -cp_data;
init_guess = 3;
lambda = fzero(f, init_guess)
end

The error is shown below:

Operands to the and && operators must be convertible to logical scalar values.

Error in fzero (line 307) elseif ~isfinite(fx) ~isreal(fx)

Error in Cp_REV02 (line 9) lambda = fzero(f, init_guess)

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Answer 1

Star Strider 2015-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/249143-polynoom-coefficient-from-excel-data#answer_196323

在 MATLAB Online 中打开

To get the roots (zeros) of a polynomial, the easiest way is to use the roots function.

For instance:

cp_data = 1:5;                                                          % Create Data
for k1 = 1:length(cp_data)
    p = [0.0006 +0.0104 -0.0602 +0.146 -0.108 +(0.043 - cp_data(k1))];  % Polynomial
    r = roots(p);                                                       % All Roots
    real_roots(:,k1) = r(imag(r)==0);                                   % Real Roots
end

The fzero function only returns real roots, so I selected to retain only those. (If you want all of them, save ‘r’ instead of ‘real_roots’.) I used a cell array for ‘real_roots’ since the number of those may vary, depending on what ‘dp_data’ is in a particular iteration.

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Star Strider 2015-10-17

The fzero function does not like complex values. Also, you are searching for one real root near 3 (because that is where you’ve told fzero to start looking) and for whatever reason (since I don’t know what ‘cp_data’ are), fzero is encountering a problem with your function in finding that zero-crossing. Your function appears to be correct, but fzero, as good as it is, is not as robust to your problem as you want it to be.

Also, you are not passing individual values of your ‘cp_data’ vector to your function in each loop, but instead the entire vector, and you are not saving the different results at each iteration, so lambda will have the results only of the last iteration. If you want to use ‘ii’ as an index vector, it has to begin with 1, not 0. All subscripts in MATLAB have to be positive integers. (Zero is not a positive integer.)

You really need to use my code as I posted it, because it actually will work in your application. If you want a specific root near 3 (or whatever value you want), post some values of ‘cp_data’ so I can run my code with it. (A bit of additional code should be able to isolate your single desired root, if you only want one. I just need some values of ‘cp_data’ to test my code, and to know the criteria to select the root you want.)