[R] apply mean to a three-dimension data

[David Winsemius]

Are you taking the same online course as Barth Riley, who posed a very  
similar question only this morning?

On Mar 24, 2011, at 11:07 AM, Hui Du wrote:

> Hi All,
>
>                Suppose I have data like
>
> b[[1]] = matrix(1:4, 2, 2)
> b[[2]] = matrix(10:13, 2, 2)
> b[[3]] = matrix(20:23, 2, 2)
>
> [[1]]
>     [,1] [,2]
> [1,]    1    3
> [2,]    2    4
>
> [[2]]
>     [,1] [,2]
> [1,]   10   12
> [2,]   11   13
>
> [[3]]
>     [,1] [,2]
> [1,]   20   22
> [2,]   21   23
>
>                Now I want to calculate the mean of each cell across  
> the list. For example mean of (b[[1]][1,1], b[[2]][1,1], b[[3]] 
> [1,1]), mean of (b[[1]][1,2], b[[2]][1,2], b[[3]][1,2]) etc. e.g.  
> mean of (1, 10, 20), mean of(3, 12, 22). Could somebody tell me how  
> to do it? Thank you in advance.

 > vec <- do.call("c", b)  # turns values into a long vector
 > dim(vec) <- c(length(b[[1]], length(b)) # "folds" it, so it is  
three columns "wide"
 > rowMeans(vec)
[1] 10.33333 11.33333 12.33333 13.33333


This is done in row-major order as are all of R's matrix operations,  
so it's not exactly what you asked for, but you will need to get used  
to row-first order if you're going to use R.
-- 

David Winsemius, MD
West Hartford, CT