take the mean of rows in a matrix r

Question

I have the following matrix of depth and temperature data (855 rows, 2 col) and would like to take the mean of every 3 rows within each column. For example:

Answer 1

Try to use tapply and apply:

R > f <- rep(c(1:3), each = 3)
R > f
[1] 1 1 1 2 2 2 3 3 3
R > x <- matrix(1:27, 9, 3)
R > x
      [,1] [,2] [,3]
 [1,]    1   10   19
 [2,]    2   11   20
 [3,]    3   12   21
 [4,]    4   13   22
 [5,]    5   14   23
 [6,]    6   15   24
 [7,]    7   16   25
 [8,]    8   17   26
 [9,]    9   18   27
R > apply(x, 2, function(t) tapply(t, f, mean))
  [,1] [,2] [,3]
1    2   11   20
2    5   14   23
3    8   17   26