I have matrix of dimensions 2784x252 which represents monthly returns. I am trying to put them into annual returns which would return a matrix of 2784x21 (21 x 12 = 252). To do that I need to sum for each row, 12 column at a time. I've tried to use the function cumsum but it won't work because it returns an array with only one column which isn't what I need to do.
Since I need to repeat the sum of 12 column 21 times which is the number of years I have, I thought using a loop could help but I can't find a way that it woulld work.
Thanks for the help,