[R] Avoiding for-loops

[Deepayan Sarkar]

On Thursday 25 November 2004 11:15, John wrote:
> Hello R-users,
>
> I have a symmetric matrix of numerical values and I
> want to obtain those values in the upper or lower
> triangle of the matrix in a vector. I tried to do the
> job by using two for-loops but it doens't seem to be a
> clever way, and I'd like to know a more efficient code
> for a large matrix of thousands of rows and columns.
> Below is my code for your reference.

Try 

mtx[lower.tri(mtx)]

which should give you the same order as your code (untested).

HTH,

Deepayan


>
> Thanks a lot.
>
> John
>
> ####################
>
> # mtx.sym is a symmetric matrix
>
> > my.ftn <- function(size_mtx, mtx) {
>
> + my.vector <- c()
> + for ( i in 1:size_mtx ) {
> + cat(".")
> + for ( j in 1:size_mtx ) {
> + if ( upper.tri(mtx)[i,j] ) {
> + my.vector <- c(my.vector, mtx[i,j])
> + }}}
> + cat("\n")
> + }
>
> # if I have a matrix, mtx.sym, of 100x100
>
> > my.ftn(100, mtx.sym)