extracting column at regular intervals starting from different columns
On 18-09-2012, at 19:21, eliza botto wrote:
Dear useRs, i had a matrix with 31 rows and 444 columns and i wanted to extract every 37th column of that matrix starting from 1. more precisely i wanted to select columns 1, 38,75, 112 and so on. then doing the same by starting from column number 2(2,39,76,113.......). i was advised to use
x[c(TRUE, rep(FALSE, 36)),]
i works if it is start from first column but as i am very new to R i wanted to know what to do to make it work if i want to start column selection from column 2 and then from column 3 and so on. sorry for bothering you once again..
When you use my method you could do the following
f3 <- function(x,M,start=1) {
x[c(start,seq_len((length(x)-1)/M)*M+start)]
}
M <- 37
A <- 1:(3*M)
f3(A,M)
f3(A,M,start=1)
f3(A,M,start=2)
When start is too large you may get NA's in the result.
You may have to get rid of them.
Berend