Operating on multiple matrices in a for loop using R - r

Question

I have 1000 matrices named A1, A2, A3,...A1000.
In a for loop I would like to simply take the colMeans() of each matrix:
for (i in 1:1000){
means[i,]<-colMeans(A1)
}
I would like to do this for each matrix Ax. Is there a way to put Ai instead of A1 in the for loop?

Answer 1

So, one way is:
for (i in 1:1000){
means[i,]<-colMeans(get(paste('A', i, sep = '')))
}
but I think that misses the point of some of the comments, i.e., you probably had to do something like this:
csvs = lapply(list.files('.', pattern = 'A*.csv'), function(fname) {
read.csv(fname)
})
Then the answer to your question is:
means = lapply(csvs, colMeans)

Answer 2

I don't completely understand, but maybe you have assigned each matrix to a different variable name? That is not the best structure, but you can recover from it:
# Simulate the awful data structure.
matrix.names<-paste0('A',1:1000)
for (name in matrix.names) assign(name,matrix(rnorm(9),ncol=3))
# Pull it into an appropriate list
list.of.matrices<-lapply(matrix.names,get)
# Calculate the column means
column.mean.by.matrix<-sapply(list.of.matrices,colMeans)

Answer 3

You initial question asks for a 'for loop' solution. However, there is an easy way to get the desired
result if we use an 'apply' function.
Perhaps putting the matrices into a list, and then applying a function would prove worthwhile.
### Create matrices
A1 <- matrix(1:4, nrow = 2, ncol = 2)
A2 <- matrix(5:9, nrow = 2, ncol = 2)
A3 <- matrix(11:14, nrow = 2, ncol = 2)
### Create a vector of names
names <- paste0('A', 1:3)
### Create a list of matrices, and assign names
list <- lapply(names, get)
names(list) <- names
### Apply the function 'colMeans' to every matrix in our list
sapply(list, colMeans)
I hope this was useful!

Answer 4

As others wrote already, using a list is perhaps your best option. First you'll need to place your 1000 matrices in a list, most easily accomplished using a for-loop (see several posts above). Your next step is more important: using another for-loop to calculate the summary statistics (colMeans).
To apply a for-loop through an R object, in general you can do one of the two options:
Loop over by indices: for example:
for(i in 1:10){head(mat[i])} #simplistic example
Loop "directly"
for(i in mat){print(i)} #simplistic example
In the case of looping through R lists, the FIRST option will be much easier to set up. Here is the idea adapted to your example:
column_means <- rep(NA,1000) #empty vector to store column means
for (i in 1:length(list_of_matrices)){
mat <- list_of_matrices[[i]] #temporarily store individual matrices
##be sure also to use double brackets!
column_means <- c(column_means, colMeans(mat))