[R] Summing up matrices in a list
John Fox
jfox at mcmaster.ca
Wed Mar 16 17:30:07 CET 2005
Dear Vicky,
Actually, this question was asked before (about a year ago, I think).
Looping turns out to be not so bad a solution; check out the following
example (from a short-course that I taught):
------- snip --------
# to loop or not to loop?
# Example: summing a list of matrices
matrices <- as.list(1:1000) # reasonable
system.time(for (i in 1:1000) matrices[[i]] <- matrix(rnorm(100), 10, 10))
matrices <- list() # problematic
system.time(for (i in 1:1000) matrices <- c(matrices,
list(matrix(rnorm(100), 10,
10))))
S <- matrix(0, 10, 10) # simple
system.time(for (i in 1:length(matrices)) S <- S + matrices[[i]])
S
# "clever"
system.time(S <- apply(array(unlist(matrices), dim = c(10, 10, 1000)), 1:2,
sum))
S
------- snip --------
I hope this helps,
John
--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox
--------------------------------
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Vicky Landsman
> Sent: Wednesday, March 16, 2005 11:22 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Summing up matrices in a list
>
> Dear all,
> I think that my question is very simple but I failed to solve it.
> I have a list which elements are matrices like this:
>
> >mylist
> [[1]]
> [,1] [,2] [,3]
> [1,] 1 3 5
> [2,] 2 4 6
>
> [[2]]
> [,1] [,2] [,3]
> [1,] 7 9 11
> [2,] 8 10 12
>
> I'd like to create a matrix M<-mylist[[1]]+mylist[[2]]
> [,1] [,2] [,3]
> [1,] 8 12 16
> [2,] 10 14 18
>
> Is there a way to create M without looping?
> Thanks a lot,
> Vicky Landsman.
> [[alternative HTML version deleted]]
>
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