Mean of each element of a list of matrices

Question

I have a list with three matrixes:

a<-matrix(runif(100))
b<-matrix(runif(100))
c<-matrix(runif(100))

mylist<-list(a,b,c)

I wou

Answer 1

Maybe what you want is:

> set.seed(1)
> a<-matrix(runif(4)) 
> b<-matrix(runif(4))
> c<-matrix(runif(4))
> mylist<-list(a,b,c)  # a list of 3 matrices 
> 
> apply(simplify2array(mylist), c(1,2), mean)
          [,1]
[1,] 0.3654349
[2,] 0.4441000
[3,] 0.5745011
[4,] 0.5818541

The vector c(1,2) for MARGIN in the apply call indicates that the function mean should be applied to rows and columns (both at once), see ?apply for further details.

Another alternative is using Reduce function

> Reduce("+", mylist)/ length(mylist)
          [,1]
[1,] 0.3654349
[2,] 0.4441000
[3,] 0.5745011
[4,] 0.5818541

Answer 2

Your question is not clear.

For the mean of all elements of each matrix:

sapply(mylist, mean)

For the mean of every row of each matrix:

sapply(mylist, rowMeans)

For the mean of every column of each matrix:

sapply(mylist, colMeans)

Note that sapply will automatically simplify the results to a vector or matrix, if possible. In the first case, the result will be a vector, but in the second and third, it may be a list or matrix.

Example:

a <- matrix(1:6,2,3)
b <- matrix(7:10,2,2)
c <- matrix(11:16,3,2)
mylist <- list(a,b,c)

> mylist
[[1]]
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

[[2]]
     [,1] [,2]
[1,]    7    9
[2,]    8   10

[[3]]
     [,1] [,2]
[1,]   11   14
[2,]   12   15
[3,]   13   16

Results:

> sapply(mylist, mean)
[1]  3.5  8.5 13.5

> sapply(mylist, rowMeans)
[[1]]
[1] 3 4

[[2]]
[1] 8 9

[[3]]
[1] 12.5 13.5 14.5

> sapply(mylist, colMeans)
[[1]]
[1] 1.5 3.5 5.5

[[2]]
[1] 7.5 9.5

[[3]]
[1] 12 15