[R] mean of subset of rows

joris.dewolf at cropdesign.com joris.dewolf at cropdesign.com
Mon Oct 1 18:16:21 CEST 2007

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

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

             <darteta001 at ikasl                                             
             Sent by:                                                   To 
             r-help-bounces at r-         r-help at r-project.org                
             project.org                                                cc 
             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:
     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



R-help at r-project.org mailing list
PLEASE do read the posting guide
and provide commented, minimal, self-contained, reproducible code.

More information about the R-help mailing list