Mean of elements in a list of data.frames

Question

Suppose I had a list of data.frames (of equal rows and columns)

dat1 <- as.data.frame(matrix(rnorm(25), ncol=5)) dat2 <- as.data.frame(matrix(rnorm(25), ncol=5)) dat3 <- as.data.frame(matrix(rnorm(25), ncol=5))  all.dat <- list(dat1=dat1, dat2=dat2, dat3=dat3) 

How can I return a single data.frame that is the mean (or sum, etc.) for each element in the data.frames across the list (e.g., mean of first row and first column from lists 1, 2, 3 and so on)? I have tried lapply and ldply in plyr but these return the statistic for each data.frame within the list.

Edit: For some reason, this was retagged as homework. Not that it matters either way, but this is not a homework question. I just don't know why I can't get this to work. Thanks for any insight!

Edit2: For further clarification: I can get the results using loops, but I was hoping that there were a way (a simpler and faster way because the data I am using has data.frames that are 12 rows by 100 columns and there is a list of 1000+ of these data frames).

z <- matrix(0, nrow(all.dat$dat1), ncol(all.dat$dat1))  for(l in 1:nrow(all.dat$dat1)){    for(m in 1:ncol(all.dat$dat1)){       z[l, m] <- mean(unlist(lapply(all.dat, `[`, i =l, j = m)))    } } 

With a result of the means:

> z         [,1]        [,2]        [,3]        [,4]       [,5] [1,] -0.64185488  0.06220447 -0.02153806  0.83567173  0.3978507 [2,] -0.27953054 -0.19567085  0.45718399 -0.02823715  0.4932950 [3,]  0.40506666  0.95157856  1.00017954  0.57434125 -0.5969884 [4,]  0.71972821 -0.29190645  0.16257478 -0.08897047  0.9703909 [5,] -0.05570302  0.62045662  0.93427522 -0.55295824  0.7064439 

I was wondering if there was a less clunky and faster way to do this. Thanks!

Answer 1

Here is a one liner with plyr. You can replace mean with any other function that you want.

ans1 = aaply(laply(all.dat, as.matrix), c(2, 3), mean)