# [R] polynomial

Thomas Lumley tlumley at u.washington.edu
Thu Oct 10 00:42:55 CEST 2002

```On Wed, 9 Oct 2002, Ben Bolker wrote:

>
>   Any better (more efficient, built-in) ideas for computing
>
>  coef[1]+coef[2]*x+coef[3]*x^2+ ...
>
>  than
>
> polynom <- function(coef,x) {
>   n <- length(coef)
>
> sum(coef*apply(matrix(c(rep(x,n),seq(0,n-1)),ncol=2),1,function(z)z[1]^z[2]))
> }
>

Well if x is a scalar as in your example
n<-length(coef)-1
sum(coef*x^(0:n))
seems simpler, or if x is a vector then
n<-length(coef)-1
rowSums(coef*outer(0:n,x,"^"))
or if it's a long enough vector that you don't want n copies of it
rval<-coef[1]
xn<-x
n<-length(coef)
for(i in 2:n){
rval<-rval+coef[i]*xn
xn<-xn*x
}

Unlike lapply, which is actually faster than a for() loop, apply() is
basically a clarity optimisation rather than a speed optimisation, and in
this case I don't think it's clearer.

-thomas

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