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

Use rollapply function from zoo package. See ?rollapply for more details.

library(zoo)
rollapply(matrix[,1], width=3, mean, by=3)  

Example:

> set.seed(1)
> Data <- matrix(rnorm(30, 100, 50), ncol=2)  # some random data
> rollapply(Data[,1], width=3, mean, by=3)  
[1]  78.69268 118.40534 130.02559 126.60393  71.48317
> # you could check this out by doing some verification as in:
> mean(Data[1:3, 1])
[1] 78.69268
> mean(Data[4:6, 1])
[1] 118.4053
> mean(Data[7:9, 1]) # and so on ...
[1] 130.0256

If you want the mean for all columns in your matrix, then just add by.column=TRUE in the rollapply call:

> rollapply(Data, width=3, mean, by=3, by.colum=TRUE)
          [,1]      [,2]
[1,]  78.69268 114.71187
[2,] 118.40534 138.90166
[3,] 130.02559  81.12249
[4,] 126.60393 106.79836
[5,]  71.48317  74.48399