Hi R-Help!
I am trying to find a nicer way of extracting all the "complete" diagonals
of a matrix. I am working with very large matrices that have many more rows
than columns. I want to be able to extract each of the diagonals that are
as long as the number of columns in the matrix. I have written a rather
ugly function that presently does the job. It illustrates what I am trying
to do, but I feel like there must be a cleaner (and faster) way. Does
anybody have any ideas? Here is what I've done so far:
diagonals <- function(mat){
output <- matrix(0,(dim(mat)[1]-dim(mat)[2]+1),NCOL(mat))
for(i in 1:NROW(output)){
G <- c()
for(j in 1:NCOL(mat)){
G <- c(G,mat[(i+j-1),j])
}
output[i,] <- G
}
return(output)
}
example <- rbind(rep(1,3),rep(2,3),rep(3,3),rep(4,3),rep(5,3))
example
[,1] [,2] [,3]
[1,] 1 1 1
[2,] 2 2 2
[3,] 3 3 3
[4,] 4 4 4
[5,] 5 5 5
diagonals(example)
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 2 3 4
[3,] 3 4 5
Many thanks,
Peter
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
