polyfit step by step or analytics

Hello

x = array of max 10 items

y = array of max 10 items

x and y has the same length

i use polyfit(x,y,3)

i get the desired result

but there is a way to have all the steps done for reach the result, or there is a way to have the analytic result?

Thanks

[Steven Lord]

but there is a way to have all the steps done for reach the result

For what purpose are you hoping to see "all the steps"? Is it for pedagogical purposes (to learn what polyfit does) or is it for some other reason?

If the former, see the Algorithms section on the polyfit documentation page or the Least Squares chapter in Cleve Moler's Numerical Computing with MATLAB.

[John D'Errico]

I don't think a step by step explanation of what polyfit does will help you, at least not without an understanding of the linear algebra, but also of MATLAB itself. You are admittedly a MATLAB novice, so much of what happens inside polyfit will seem quite strange, since it worries about things like tall arrays, etc.

Anyway, that suggests to not worry about exactly what the code does, but to go instead to a good text on numerical linear algebra. Start with the Moler text as suggested by @Steven Lord.

Answer 1

[Bruno Luong]

polyfit is the mfile you can step through in debugging mode.

If you call with one outout argument (non normalized) it build vandermond matrix of size 10 x 4 then solve least square linear

system.

There is a formula for 4 x 4 system but it will have a moderately long formula.

[Luca]

sorry but how i can debug i'm totally new

Thanks

[Bruno Luong]

Start reading the debug doc

Then type this in MATLAB command window to see what polyfit does.

dbstop in polyfit
x = 1:10;
P = 1:4;
y = polyval(P, x);
polyfit(x, y, 3);