如何添加到R中的矩阵列表？

Question

Given a dataset 'train' that has 1607 rows and 256 columns, I want to make each row (with 256 elements each) a 16x16 matrix and have a list to hold 1607 of such matrices. All observations in the train data are values between -1 and 1 . For example, a row vector in the train could be

a <- seq(from = -1, to = 1, length = 256)

When I try to run a loop like this,

x <- lapply(1:nrow(train), matrix, data=0, nrow=16, ncol=16) #creates list of 1607 matrices of 0's
for (i in length(x)) {
   x[[i]] <- matrix(X_train[i,], nrow=16, ncol=16) #replace each element (matrix) of list x with a matrix of the same dimension from train
}

I keep getting the same unmodified list x with matrices of 0 for all elements of x . What am I doing wrong?

Answer 1

 for (i in length(x)) { 

Here, you are looping over one value, namely, length(x) . You probably want to loop over all values from 1 to the length of x . You do this with

for (i in 1:length(x)) {

or

for (i in seq_along(x)) {

The latter has the benefit that it won't produce a bad result if x is empty. But really, you don't need to make a loop at all, or produce a list of blank matrices which you immediately discard. Instead, do:

rows <- seq_len(nrow(X_train))
x <- lapply(rows, function(row) matrix(X_train[row, ], nrow=16, ncol=16))