[R] tapply and names

[Liaw, Andy]

> From: Göran Broström
> 
> 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?

Would this work?

  dat <- dat[ave(dat$year, dat$m.id, min) >= 1816, ]

Andy

> -- 
>  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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