Serguei Kaniovski <[EMAIL PROTECTED]> writes:
> Hi all,
>
> starting from a vector "v[1:n]" I would like to compute the coefficients
> of the polynomial (1+x^v[1])*(1+x^v[2])*...*(1+x^v[n]). The following
> code works but is extremely slow for a large "n" due to, I believe, the
> polynomial being factorized. I wanted to try the package "polynom"
> command "unclass" but I could not figure out how to input a factorized
> polynomial. Can you please help?
>
> Serguei
>
> v<-c(1,1,2,3,4,6)
>
> e<-parse(text=paste("(1+x^", v, sep="", collapse="*", ")"))
>
> DD<-function(expr, name, order = 0) {
> if(order == 0)expr
> else DD(D(expr, name), name, order - 1)
> }
>
> sapply(0:18, function(i) eval(DD(e, "x", i), list(x=0)))/factorial(0:18)
You can do this much more efficiently by working directly with the
polynomial: Represent them as vectors of length sum(v)+1, then
c(1, 0, ..., 0)
is the polynomial 1, multiply by x^n is right-shift by n, and add is
add.
> v<-c(1,1,2,3,4,6)
> y <- rep(0,18)
> y[1] <-1
> for (i in v) y <- y+c(rep(0,i),y)[1:18]
> y
[1] 1 2 2 3 4 4 5 6 5 5 6 5 4 4 3 2 2 1
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907
______________________________________________
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
