[R] tapply and names

[Dimitris Rizopoulos]

your approach, after omitting the "as.numeric()" in the second line, 
seems to work even for `m.id' being factor, i.e.,

dat <- data.frame(m.id=rep(letters[1:10], 10), year=sample(1805:1950, 
100, TRUE))
###########
mid <- tapply(dat$year, dat$m.id, min)
mid <- names(mid)[mid >= 1816]
dat. <- dat[dat$m.id %in% mid, ]
dat; dat.

but maybe there is something better.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Göran Broström" <gb at tal.stat.umu.se>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, January 25, 2005 3:55 PM
Subject: [R] tapply and names


>I have a data frame containing children, with variables 'year' = 
>birth
> year, and 'm.id' = mother's id number. Let's assume that all the 
> births of
> each mother is represented in the data frame.
>
> Now I want to create a subset of this data frame containing all 
> children,
> whose mother's first birth was in the year 1816 or later. This seems 
> to
> work:
>
>    mid <- tapply(dat$year, dat$m.id, min)
>    mid <- as.numeric(names(mid)[mid >= 1816])
>    dat <- dat[dat$m.id %in% mid, ]
>
> but I'm worried about the second line, because the output from 
> 'tapply'
> isn't documented to have a 'dimnames' attribute (although it has 
> one, at
> least in R-2.1.0, 2005-01-19). Another aspect is that this code 
> relies on
> m.id being numeric; I would have to change it if the type of m.id 
> changes
> to, eg, character.
>
> So, question: Is there a better way of doing this?
> -- 
> Göran Broström                    tel: +46 90 786 5223
> Department of Statistics          fax: +46 90 786 6614
> Umeå University                   http://www.stat.umu.se/egna/gb/
> SE-90187 Umeå, Sweden             e-mail: gb at stat.umu.se
>
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