Summing up matrices in a list

lst <- list(matrix(1:4, 2, 2), matrix(5:8, 2, 2), matrix(9:12, 2, 2))
m <- matrix(0, 2, 2)
m[] <- rowSums(do.call("cbind", lapply(lst, c)))
m

From: Dimitris Rizopoulos

try this:

matSums <- function(lis, na.rm=FALSE){
    if(!is.list(lis) || !all(sapply(lis, is.matrix))) 
stop("'lis' must 
be a list containing 2-dimensional arrays")
    dims <- sapply(lis, dim)
    n <- dims[1, 1]
    p <- dims[2, 1]
    if(!all(n == dims[1, ]) || !all(p == dims[2, ])) stop("the 
matrices must have the same dimensions")
    mat <- matrix(unlist(lis), n * p, length(lis))
    matrix(rowSums(mat, na.rm=na.rm), n, p)
}
#########
mylist <- list(matrix(1:6, 2), matrix(7:12, 2))
matSums(mylist)


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Vicky Landsman" <msvika at mscc.huji.ac.il>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, March 16, 2005 5:21 PM
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.
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