On 08/16/2011 07:56 AM, Eik Vettorazzi wrote:
> Hi Lao,
> you tried to reinvent the wheel. Have a look at ?tapply
>
> tapply(sleep$extra,sleep$group,mean)
or take a look at the plyr package:
library(plyr)
data(sleep)
ddply(sleep, .(group), summarise, m = mean(extra))
cheers,
Paul
> Cheers
>
> Am 16.08.2011 09:41, schrieb Lao Meng:
>> Hi all:
>> My data:data(sleep)
>>
>> If I wanna calculate each group's extra,what I can do is:
>> #method1
>> attach(sleep)
>> mean(extra[group==1])
>> mean(extra[group==1])
>>
>>
>> #method2
>> result<-matrix(,0,2)
>> g<-split(sleep,sleep$group)
>> for(i in 1:length(g))
>> {
>> result<-rbind(result,data.frame(unique(g[[i]]$group),mean(g[[i]]$extra)))
>> }
>> colnames(result)<-c("name","mean")
>>
>> But the above 2 method is a little bit tedious.Is there a "short cut" manner
>> to get the same result?
>>
>> Thanks a lot!
>>
>> My best
>>
>> [[alternative HTML version deleted]]
>>
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