[R] elegant matrix creation

[Uwe Ligges]

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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