Q:

How do one select first in a lower.tri output?

I have an out from a lower.tri output and wnat to do the mean on the first column.
How to do that?

A:

An easy way to do it (and also get any column mean) would be to (instead of using lower.tri directly on the matrix) do the following:
#Assume a 5x5 matrix
a <- matrix(1:25, nrow=5)
#set upper.tri and the diag to NA
#so what you are left with is just the lower.tri values in a matrix
a[upper.tri(a)] <- diag(a) <- NA

Then it is easy to do:
colMeans(a, na.rm=TRUE)
[1]  3.5  9.0 14.5 20.0  NaN

And get all of them. colMeans(a, na.rm=TRUE)[1] will just give you the first.

