[R] Avoiding for-loops

[Uwe Ligges]

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. 


See ?upper.tri


Uwe Ligges



> 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)
> 
> 
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