Polyfit with centering and scaling values

Hi All,

I want to output centering and scaling values from the command prompt.

Here is the sample data I took from the matlab help:

    x = (0: 0.1: 2.5)';
    y = erf(x);

When I tried polyfit command here is the result:

     ppp = polyfit(x,y,3)

ppp =

    9.284046212199847e-02    -6.231661786330390e-01     1.384345932284717e+00    -1.993673645226568e-02

Second trial with centering and scaling values as output requests gives

>> [ppp1,SSS,muu]= polyfit(x,y, 3)

ppp1 =

     4.154044003296898e-02    -1.608834507201939e-01     2.001009389523068e-01     9.181275523715360e-01

SSS =

        R: [4x4 double]
       df: 22
    normr: 3.578557910406126e-02

muu =

     1.250000000000000e+00
     7.648529270389177e-01

I just don't understand why the ppp is differing from ppp1.

Please let me know the mistake in my understanding... Thanks for your time.

Answer 1

[Mohammad Abouali]

because ppp fits y to x

but ppp1 fits y to x^ where x^ = (x-mu1)./mu2;

check this code:

x = (0: 0.1: 2.5)';
y = erf(x);
ppp = polyfit(x,y,3)

ppp =

    0.0928   -0.6232    1.3843   -0.0199

[ppp1,SSS,muu]= polyfit(x,y, 3)

ppp1 =

    0.0415   -0.1609    0.2001    0.9181

plot(x,y,'r.')
hold on
plot(x,polyval(ppp,x))
plot(x,polyval(ppp1,x),'k')
plot(x,polyval(ppp1,(x-muu(1))./muu(2)),'g')
legend('Data','ppp','polyval(ppp1,x)','polyval(ppp1,(x-muu(1))./muu(2))');

[Raj Raj]

Got it. Thanks Mr. Mohammad..