[Rd] Documentation detail [was: Merging factor levels.]

[François Pinard]

Hi to R developers.

In the "Details:" section of "?levels", it would be nice including the 
following sentence, taken from a reply from Brian Ripley:

    If you set two levels to be the same label, they get merged.

Granted, the "Examples:" section does have a terse comment and examples 
from which users may imply this behaviour.  Yet, the documentation would 
be friendlier if it was clearly stating the fact.

Brian's message stated in full below, for reference:

----- Forwarded message from Prof Brian D Ripley <ripley at stats.ox.ac.uk> -----

From: Prof Brian D Ripley <ripley at stats.ox.ac.uk>
Date: Thu Apr 10 10:43:28 2003
Subject: [R] Merging factor levels.

On Mon, 12 Nov 2001, [iso-8859-1] Göran Broström wrote:

> This must be a FAQ, but I can't find the answer:
> I have a factor 'okk':

> > table(okk)
> okk
>      Other   STORKAGE SKELLEFTEA    FALMARK       BOLE  NEAR_TOWN
>      14064        560        346        526        332         34

> Now I want to put the cases in 'NEAR_TOWN' into the 'Other' category',
> so I try:

> > okk <- ifelse(okk == "NEAR_TOWN", "Other", okk)
> > table(okk)
> okk
>     1     2     3     4     5 Other
> 14064   560   346   526   332    34
> > is.factor(okk)
> [1] FALSE

> This is not what I intended. I can do what I want the hard way (i.e., by
> converting  okk  to character an then back again) but there
> must be something simpler?

Something like

levels(okk) <- c(levels(okk)[1:5], "Other")

If you set two levels to be the same label, they get merged.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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----- End forwarded message -----

-- 
François Pinard   http://pinard.progiciels-bpi.ca