On Oct 9, 2011, at 1:04 PM, Maas James Dr (MED) wrote:
If possible I'd like to produce a function that applies a formula to
a column in a matrix (essentially calculating the mse) and then
inserts it between values of a an array ...
confusing I know, here is an example of what I'm trying to accomplish:
## create a matrix
(a <- matrix(c(3,6,4,8,5,9,12,15),nrow=4))
## get the mean of all columns
(b <- apply(a,2,mean))
## calculate the mse of column 1
(c <- (sd(a[,1])/sqrt(length(a[,1]))))
## calculate the mse of column 2
(d <- (sd(a[,2])/sqrt(length(a[,2]))))
The sd function, like the var function, returns its values based on
columns, so just sd(a) fives you a vector of columns. Construting an
interleaving vector of column indices can be done matrix manipulations:
c(b, sd(a))[
c(
matrix(1:(2*length(b)), 2, byrow=TRUE) # create a
row-oriented matrix
) # the interleaving matrix is then extracted as a
vector by columns
]
#[1] 5.250000 2.217356 10.250000 4.272002
( You could also use the apply method of returning function(x)
{ c(mean(x), sd(x) } from each column.) and then wrap the c() function
around that:
c( apply(a, 2, function(x){ c(mean(x), sd(x) ) }) )
#[1] 5.250000 2.217356 10.250000 4.272002
## now create a new vector with each mean value and its corresponding
## mse right beside it
(e <- c(b[1],c,b[2],d))
Would anyone have any suggestions how to accomplish this using an
apply statement?
Thanks a bunch,
J
===============================
Dr. Jim Maas
University of East Anglia
David Winsemius, MD
West Hartford, CT
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