r - Named arrays, dataframes and matrices -

if split data matrix rows according class labels in vector y this, result 'names' this:

> x <- matrix(c(1,2,3,4,5,6,7,8),nrow=4,ncol=2) > y <- c(1,3,1,3) > x_split <- split(as.data.frame(x),y) $`1`   v1 v2 1  1  5 3  3  7  $`3`   v1 v2 2  2  6 4  4  8 

i want loop through results , operations on each matrix, example sum elements or sum columns. how access each matrix in loop can that?

labels = names(x_split) (k in labels) {     # how x_split[k] matrix?     sum_class = sum(x_split[k]) # doesn't work } 

in fact, don't want deal dataframes , named arrays @ all. there way can call split without as.data.frame , list of matrices or similar?

to split without converting data frame

x_split <- list(x[c(1, 3), ], x[c(2, 4), ])  

more generally, write in terms of vector y of length nrow(x), indicating group each row belongs, can write

x_split <- lapply(unique(y), function(i) x[y == i, ]) 

to sum results

x_sum <- lapply(x_split, sum)  # [[1]] # [1] 16  # [[2]] # [1] 20 

(or use sapply if want result vector)