MATLAB: Polyfit for 0 order polynomial

polyfit

Dear all,

I am trying to plot a horizontal line (y=b and b is a constant) which is the least-square fitting to a set of data. Is that legit to set 'n' in polyfit as 0, say p = polyfit(x, y, 0) to have a horizontal least square fitting line?

Thanks, Ren

Best Answer

It's legitimate, but it's also far simpler to do

p=mean(y);

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MATLAB: Fit and Polyfit (LAR vs. Least squares)

You can find out by asking for the output argument:

[fitobject,gof,output] = fit(x,y,fitType)
output.algorithm

But the real difference between fit and polyfit are the options that you can choose as well as polyfit is strictly limited to polynomials and uses the least squares method.

MATLAB: Polyfit: Polynomial is badly conditioned

I get "Subscripted assignment dimension mismatch." polyfit() gives you back coefficients, and the number of those is different depending on what order you choose: 2 for linear, 3 for quadratic, 6 for 5th order. So how are you going to stick those all (a varying number) into a column of p? What is the size of P? Try it like this:

N = 5;
% Create some sample data.
time = rand(20,3);
values = rand(20,5);
p = zeros(N, 6); % Preallocate the largest you think you'll need
for k=1:N
  [p(k,1:k+1), S, mu] = polyfit(time(:,3),values(:,5),k);
end

If it still complains, you'll have to use S and mu when you go to use polyval().

Best Answer

Related Solutions

MATLAB: Fit and Polyfit (LAR vs. Least squares)

MATLAB: Polyfit: Polynomial is badly conditioned

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