[R-SIG-Mac] polynomial issue

[David Winsemius]

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