[R] Taking all "complete" diagonals of a matrix

[Dennis Murphy]

Hi:

Does this work for you?

mydiags <- function(mat) diag(mat[seq_len(ncol(mat)), ])

# Example:
set.seed(103)
u <- matrix(rpois(200, 10), ncol = 10)
# > dim(u)
# [1] 20 10
mydiags(u)
# [1]  7 12  6 13 12  6  5  6 14  6
u[1:10, ]     # as a double check

HTH,
Dennis

On Tue, Jul 19, 2011 at 2:15 PM, Peter Lomas <peter.lomas at ucalgary.ca> wrote:
> 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
>
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>
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