polyfit with multiple dependent variables

I think

https://www.mathworks.com/help/matlab/ref/polyfit.html

can handle multiple dependent variables. But when I tried

Fit = polyfit([x1 x2], x3, 1);

I got an error message

Error using polyfit (line 44)
The first two inputs must have the same number of elements.

Although I checked x1, x2, x3 has the same length. What can one do?

Answer 1

[Bjorn Gustavsson]

The polyfit function only works for 1-D polynomials, as far as I understand things. However there is an n-dimensional polynomial-fitting function package on the file exchange: polyfitn. That should give you the tools necessary to solve this problem. I've used it without much problems and to sensible success.

HTH

[alpedhuez]

Thank you.

Answer 2

[Remy Lassalle-Balier]

I think polyfit does not suport multiple dependent variables. The documentation is quite clear about the fact the first parameter should be a vector and not a matrix.

You can still run your fit manually:

X = [ ones( numel( x1 ) , 1 ) , x1(:) , x2(:) ];
fitParam = X\x3(:);
Fittedx3 = X * fitParam;

This is better explained here.

[alpedhuez]

True.