one efficient way to do this, avoiding loops, is using the rowsum()
function, e.g.,
mat <- matrix(rnorm(1710*244), 1710, 244)
id <- rep(seq_len(570), each = 3)
means <- rowsum(mat, id, FALSE) / 3
Regarding the second part of your question, indeed a "gold" rule of
efficient R programming when it comes to loops is to pre-allocate the
output object. Check for instance,
system.time({
x <- NULL
for (i in 1:5e04) x <- c(x, rnorm(1))
})
versus
system.time({
x <- numeric(5e04)
for (i in 1:5e04) x[i] <- rnorm(1)
})
I hope it helps.
Best,
Dimitris
On 10/15/2010 9:34 AM, David A. wrote:
Hi list,
I have a 1710x244 matrix of numerical values and I would like to calculate the
mean of every group of three consecutive values per column to obtain a new
matrix of 570x244. I could get it done using a for loop but how can I do that
using apply functions?
In addition to this, do I have to initizalize a 570x244 matrix with 0's to
store the calculated values or can the output matrix be generated while
calculating the mean values?
Cheers,
Dave
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--
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center
Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
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