Re: need help with polyfit and polyval

[Dmitri A. Sergatskov]

On Mon, May 6, 2019 at 9:31 AM Doug Stewart <address@hidden> wrote:

I am trying to convert some data from wikipedia to octave and I ran into this problem.

here is the code

**********************************************

data = ""   4.0000    3.1000

    5.0000    4.7400

    6.0000    6.1300

    7.0000    7.2600

    8.0000    8.1000

    9.0000    8.1400

   10.0000    8.7400

   11.0000    8.7700

   12.0000    9.1300

   13.0000    9.1400

   14.0000    9.2600  ];

  x = data(:,1);

  y = data(:,2);

  [P, S, MU] = polyfit (x, y, 1)

y3=S.yf

y2=polyval(P,x)

plot(x,y,'x',x,y,x,y2,x,y3)

**********************************************

poly fit  should return the polynomial in P

But when i use it in polyval it is obviously wrong.

but polyfit also returns the new calculated y values in S.yf.

S.yf look correct ut P seems to be wrong.

I get 

P =   1.8410   7.5009

which is      y=x*1.841 + 7.5009

and this is obviously wrong.

Yes. It looks like

[P, S, MU] = polyfit (x, y, 1) form has a problem.

if you just do p = polyfit (x, y, 1)

you get a correct p:

P=polyfit (x,y,1)
P =

   0.55509   2.50509

octave:12> y2=polyval(P,x)
y2 =

    4.7255
    5.2805
    5.8356
    6.3907
    6.9458
    7.5009
    8.0560
    8.6111
    9.1662
    9.7213
   10.2764

Dmitri.

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