[R] compute variance of every column in a matrix without a loop
Thomas Lumley
tlumley at u.washington.edu
Sun Mar 17 23:50:00 CET 2002
On Sat, 16 Mar 2002, Francisco J Molina wrote:
> Is it possible to compute the variance of every column in a matrix
> without a loop?
No, but you can make it look as if there's no loop.
You can hide the for() loop inside a function with
apply(m, 2, var)
if the reason you want to avoid a loop is to make your code look simpler.
In 1.5.0 you will be able to do
colMeans(m*m)-colMeans(m)^2
if the reason is to have the loop take place in compiled code and be
faster.
-thomas
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