[R] Summing up matrices in a list

[John Fox]

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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