[R] hairy indexing problem

[vito muggeo]

See ?tapply of course.
E.g.
tapply(foo, subject, mean, na.rm=T) #mean of foo per subj

best,
vito

----- Original Message -----
From: "Russell Senior" <seniorr at aracnet.com>
To: <R-help at stat.math.ethz.ch>
Sent: Wednesday, June 05, 2002 3:00 AM
Subject: [R] hairy indexing problem


>
> I've got a data frame that looks like this:
>
>    subject   foo   bar
>       2      1.7   3.2
>       2      2.3   4.1
>       3      7.6   2.3
>       3      7.1   3.3
>       3      7.3   2.3
>       3      7.4   1.3
>       5      6.2   6.1
>       5      3.4   6.9
>      ...
>
> That is, I've got multiple rows per subject.  I need to compute
> summaries within categories where the subject has the same number of
> rows.  For example, subject 2 and 5 both have two rows.  I need to
> compute mean for those four values of foo.  This looks like a good
> candidate for index vectors, but I need some help.  I've tried
> something like:
>
>   table(data) -> tmp
>
> and:
>
>   tmp[tmp == 2]
>
> and even:
>
>   as.numeric(attr(tmp[tmp == 2],"names"))
>
> to get a vector of subject numbers that have two rows in the original
> data frame.  But I am getting stuck there.  I want some kind of
> "is.member" function to use in a subsequent index vector expression,
> like:
>
>   i <- as.numeric(attr(tmp[tmp == 2],"names"))
>   data[is.member($subject,i)]$foo
>
> but there isn't an is.member() function.  Can someone please give me a
> pointer on the canonical way to do this?
>
> Thanks!
>
> --
> Russell Senior         ``The two chiefs turned to each other.
> seniorr at aracnet.com      Bellison uncorked a flood of horrible
>                          profanity, which, translated meant, `This is
>                          extremely unusual.' ''
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