[R] how to convert a vector to an upper matrix

[Berend Hasselman]

Zhishi Wang wrote:
> 
> Hi all,
> 
> If I have a vector A<-1:6, I would like to convert A to an upper
> matrix M=[0 1 2 3 ; 0 0 4 5 ; 0 0 0 6; 0 0 0 0].
> That is, if the length of the known vector is n, then the order m of
> the matrix I'd like to have should satisfy m(m-1)/2=n
> How could I do this in R?
> I really don't want to do it using a loop. Is there a function in R to do
> this ?
> 

?upper.tri
?uniroot

A <- 1:6

n <- length(A)
m <- as.integer(uniroot(function(x) {x*(x-1)- 2*n}, c(1, n))$root)

U <- matrix(0,nrow=m,ncol=m)
U[upper.tri(U)] <- A
U


Berend


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