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How do I apply additional constraints to polyfit?
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exmachina zhao
May 20, 2009, 3:33:01 PM5/20/09
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I have some linear data set and I want to fit it to a second order polynomial equation a+bx+cx^2, but b=0, so really I want to fit it to a+cx^2
Tips?
Steven Lord
May 20, 2009, 5:14:17 PM5/20/09
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"exmachina zhao" <yto...@yahoo.com> wrote in message
news:gv1ltd$k27$1...@fred.mathworks.com...
If you don't want to fit the full polynomial, you can't use POLYFIT.
Instead, create the data matrix yourself with the terms you want and use
backslash.
x = (1:10).';
a = 5;
c = 2;
y = a+c*x.^2;
M = [x.^2, ones(size(x))]; % M*[c; a] = y
coeffs = M\y
differenceInA = a - coeffs(2)
differenceInC = c - coeffs(1)
The two difference* variables should be small.
--
Steve Lord
sl...@mathworks.com
John D'Errico
May 20, 2009, 11:49:01 PM5/20/09
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"exmachina zhao" <yto...@yahoo.com> wrote in message <gv1ltd$k27$1...@fred.mathworks.com>...
> I have some linear data set and I want to fit it to a second order polynomial equation a+bx+cx^2, but b=0, so really I want to fit it to a+cx^2
>
> Tips?
> I have some linear data set and I want to fit it to a second order polynomial equation a+bx+cx^2, but b=0, so really I want to fit it to a+cx^2
>
> Tips?
You can also use my polyfitn, which allows yo to
explicitly specify the model.
http://www.mathworks.com/matlabcentral/fileexchange/10065
John
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