[R-SIG-Mac] polynomial issue
David Winsemius
dwinsemius at comcast.net
Sat Nov 6 02:30:18 CET 2010
On Nov 5, 2010, at 7:05 PM, Ivan Cabrera wrote:
> Hi all, I'm new to any programming and I've been doing some stuff in
> R the last couple of month, I hope you can help me here…
>
> I'm using lm() to do a 12 degree polynomial fit to a series of data
> x,y. Then I built a polynomial using the coefficients given by
> coef() function (replacing any NA by a 0), and evaluate it in x.
This does really look more like an Rhelp question.
>
> > x<-data[,1]; y <- data[,2]
> > pol <- paste(paste("I(x",1:12,sep="^"),")",sep="")
> > form <- as.formula(paste("y~", paste(pol,collapse="+")))
> > p12 <- lm(form)
> > coef(p12)
> (Intercept) I(x^1) I(x^2) I(x^3)
> I(x^4) I(x^5) I(x^6) I(x^7)
> I(x^8) I(x^9) I(x^10)
> 1.549683e+05 -2.801410e+02 2.238321e-01 -1.037342e-04 3.071376e-08
> -6.013217e-12 7.735856e-16 -6.168457e-20 2.493099e-24
> NA -3.184072e-33
> I(x^11) I(x^12)
> NA 3.973316e-42
> > pl12 <- coef(p12)[1] + coef(p12)[2]*(x^1) + coef(p12)[3]*(x^2) +
> coef(p12)[4]*(x^3) + coef(p12)[5]*(x^4) + coef(p12)[6]*(x^5) +
> coef(p12)[7]*(x^6) + coef(p12)[8]*(x^7) + coef(p12)[9]*(x^8) +
> coef(p12)[11]*(x^10) + coef(p12)[13]*(x^12)
>
> My problem comes when I replace the coef(p12)[ ] with their actual
> values. I get completely different values of the polynomial…
I am wondering what would happen if you tried:
fit12 <- lm(y ~ poly(x), degree=12)
p12 <- predict(fit12)
plot(p12)
(Untested in absence of reproducible data.)
--
David.
>
> > pl12.1 <- (1.549683e+05) + (-2.801410e+02)*(x^1) +
> (2.238321e-01)*(x^2) + (-1.037342e-04)*(x^3) + (3.071376e-08)*(x^4)
> + (-6.013217e-12)*(x^5) + (7.735856e-16)*(x^6) +
> (-6.168457e-20)*(x^7) + (2.493099e-24)*(x^8) +
> (-3.184072e-33)*(x^10) + (3.973316e-42)*(x^12)
> > rle(pl12==pl12.1)
> Run Length Encoding
> lengths: int 3412
> values : logi FALSE
>
> and not even the shape of the functions are alike when i plot them.
>
> can anyone give me a clue of what's happening and how to fix it?
>
> Thanks,
> Ivan
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David Winsemius, MD
West Hartford, CT
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