# [R] elegant matrix creation

Arne Henningsen ahenningsen at email.uni-kiel.de
Tue Jul 12 13:53:06 CEST 2005

```Maybe not elegant, but I guess this function does what you want:

f <- function( x ) {
n <- length( x ) + 1
m <- matrix( 0, nrow = n, ncol = n )
diag(m) <- 1
for( j in 1:( n-1 ) ) {
for( i in ( j+1 ):n ) {
m[ i, j ] <- prod( x[ j:(i-1) ] )
}
}
return( m )
}

Best wishes,
Arne

On Tuesday 12 July 2005 13:11, Robin Hankin wrote:
> 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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--
Arne Henningsen
Department of Agricultural Economics
University of Kiel
Olshausenstr. 40
D-24098 Kiel (Germany)
Tel: +49-431-880 4445
Fax: +49-431-880 1397
ahenningsen at agric-econ.uni-kiel.de
http://www.uni-kiel.de/agrarpol/ahenningsen/

```