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Polyfit and polyval plot intercepting zero

185 次查看（过去 30 天）
Johan Lilliestråle
评论： TEDDY OKOLO 2021-8-20
Hi Folks
I have a question about the polynomial regression function polyfit. Lets say that we have two random vector, for example; a = [2.1484, 2.1118, 2.0759, 2.0550, 2.0106]; b = [20.530, 20.221, 19.972, 19.721, 19.693];
If we want to do a polynomial regression it is very easily done by the inbuilt function: p = polyfit(a,b,n) If I want to plot the points and the polynomial curve it is also very easily done by the polyval function;
p = polyfit(p,a,n); q = polyval(p,q); plot(a,b,'r',a,q,'b')
(I think this is the right code, this first part illustrates the points in read and the second part the polyval function in blue) But I FOUND IT VERY HARD to make an polynomial regression which is forced to intercept the zero (this is in contrary pretty easy in excel). How can I achieve the polynomial evaluation, polyval, for an curve that intercept zero and easily put the function in a plot command with the points from vector a and b above? By the way, if it is possible, would it be possible to also combine all this, dots from a and b vectors, the polynomial through zero in a plot for standard deviation (the errorbar(a,b) function) and plot standard deviations for all the points from a and b.
Would really appreciate answer, have a nice day, regards from Sweden

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回答（2 个）

Torsten
Torsten 2015-2-19
math man
math man 2018-12-18
Another, more vulgar way to solve this is to simply force where you want your Origin to be (Y at 0):
Where your varargin is a set of forced input pairs such as [0,0].
function [XY,Fit] = XY_ForceAndFitPoly(XY,PolyParamCount,varargin)
ForcedPairsRow = cell2mat(varargin);
ForcedPairsCount = size(ForcedPairsRow,2)/2;
for pair = 1:ForcedPairsCount
ForcedPair = ForcedPairsRow(1,1+(pair-1)*2:2+(pair-1)*2);
ForcedBlock = repmat(ForcedPair,10000,1);
XY = vertcat(XY,ForcedBlock);
end
Fit = polyfit(XY(:,1),XY(:,2),PolyParamCount);
XPlotVals = linspace(-.2,.2,31);
FitVals = polyval(Fit,XPlotVals);
hold on;
plot(XPlotVals,FitVals,'bo')
end

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