[R] mean of subset of rows

[joris.dewolf at cropdesign.com]

data <- data.frame(ID = rep(letters[1:4],5),size=rnorm(20,0,1))


aggregate(data$size, by = list(data$ID),mean)










                                                                           
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             01/10/2007 17:57          [R] mean of subset of rows          
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




Dear list,
this must be an easy one:

I have a data.frame of two columns, "ID" with four different levels (A
to D) and numerical "size", and each of the 4 different IDs is
repeated a
different number of times. I would like to get the mean size for each
ID as another data.frame. I have tried the following:

>ID= as.character(unique(data[,1])) # I use unique() because "data"
will be larger in future
>nIDs = length(ID)
>for(i in 1:nIDs){
+  subdata = subset(data,V1==ID[i])
+  average = as.data.frame(cbind(1:i,ID[i],mean(subdata[,2]))
+ }

Unfortunately, my output only gets the last level of ID four times:
>average
     V1 V2               V3
1  1  D 179.777777777778
2  2  D 179.777777777778
3  3  D 179.777777777778
4  4  D 179.777777777778

How can I get what I need? there might be an easier way to do it, but
I guess my skills aren´t that good. Any suggestions are welcome

Regards,

David

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