[R] elegant matrix creation
Uwe Ligges
ligges at statistik.uni-dortmund.de
Tue Jul 12 14:35:42 CEST 2005
Robin Hankin wrote:
What about
foo <- function(a){
n <- length(a)
X <- diag(n+1)
X[lower.tri(X)] <- unlist(lapply(seq(n),
function(x) cumprod(c(1, a)[-seq(x)])))
X
}
foo(c(10,7,2))
Uwe Ligges
> Hi
>
> I want to write a little function that takes a vector of arbitrary
> length "n" and returns a matrix of size n+1 by n+1.
>
> I can't easily describe it, but the following function that works for
> n=3 should convey what I'm trying to do:
>
>
> f <- function(x){
> matrix(c(
> 1 , 0 , 0 , 0,
> x[1] , 1 , 0 , 0,
> x[1]*x[2] , x[2] , 1 , 0,
> x[1]*x[2]*x[3], x[2]*x[3], x[3], 1
> ),
> 4,4, byrow=T)
> }
>
> f(c(10,7,2))
> [,1] [,2] [,3] [,4]
> [1,] 1 0 0 0
> [2,] 10 1 0 0
> [3,] 70 7 1 0
> [4,] 140 14 2 1
> >
>
>
> As one goes down column "i", the entries get multiplied by successive
> elements of x, starting with x[i], after the first "1"
>
> As one goes along a row, one takes a product of the tail end of x,
> until the zeroes kick in.
>
>
> Am I missing some clever solution?
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
> tel 023-8059-7743
>
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