column selection for aggregate()

I didn't understand from the help what really does the function rowMeans 

but it looks like it doesn't take into account the categorical variables 

(I want to calculate the means when the values of all categorical 
variables are the same, second part of aggregate). Moreover, ssfa_num 
contains only numeric variables, meaning that the categories will not be 

associated with it.
I'm kind of confused with this approach.
You think it would work for me?

Thanks
Ivan

b k a ?crit :

On Mon, Jan 18, 2010 at 10:17 AM, Ivan Calandra 
<ivan.calandra at uni-hamburg.de <mailto:ivan.calandra at uni-hamburg.de>> 
wrote:

    Thanks for your answer, but it doesn't work...

    Here is what I get:

    > ssfamean <- aggregate(ssfa[[10:24]],ssfa[c("SPECSHOR", "BONE",

    "TO_POS", "FACETTE", "SHEARFAC", "ENA_BA")],mean)
    Error in .subset2(x, i, exact = exact) :
      recursive indexing failed at level 2


Wouldn't you be better off with rowMeans() ? Split your dataframe into 

numeric matrix:

ssfa_num  <- ssfa[10:24]

ssfameans <- rowMeans(ssfa_num)


    Also col_index <- match("Asfc", ssfa) doesn't really work since
    col_index is composed of 1227 NAs...





Yes, it should be:

col_index <- match("Asfc", names(ssfa))

Ben

   [[alternative HTML version deleted]]