Hi Phil,

thanks, that already helps: Reduce() gets me the means I need!

Unfortunately, Reduce() apparently won't help me with trimming or winsorizing the means, judging from its "successively" philosophy... Any other ideas out there?

Best,
Stephan


Phil Spector schrieb:
Stephan -
If you try to apply mean directly to a matrix, it will just return a scalar. It's easy to get row means and column
means, but to preserve each element of the matrix, I think
it's better to do the computations directly:

Reduce('+',foo) / length(foo)

                                       - Phil Spector
                     Statistical Computing Facility
                     Department of Statistics
                     UC Berkeley
                     spec...@stat.berkeley.edu


On Tue, 30 Dec 2008, Stephan Kolassa wrote:

Dear useRs,

I have a list, each entry of which is a matrix of constant dimensions. Is there a good way (i.e., not using a for loop) to apply a mean to each matrix entry *across list entries*?

Example:

foo <- list(rbind(c(1,2,3),c(4,5,6)),rbind(c(7,8,9),c(10,11,12)))
some.sort.of.apply(foo,FUN=mean)

I'm looking for a componentwise mean across the two entries of foo, i.e., the following output:

    [,1] [,2] [,3]
[1,]    4    5    6
[2,]    7    8    9

[NB. My "real" application involves trimming and psych::winsor(), so anything that generalizes to this would be extra good.]

I've been looking at apply and {s,l,m,t}apply, by, with and aggregate and searched the list archives... any ideas?

Thanks a lot,
Stephan

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