Avoiding for-loops

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. 

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)