From ripley at stats.ox.ac.uk  Wed Nov 28 10:50:15 2007
From: ripley at stats.ox.ac.uk (ripley at stats.ox.ac.uk)
Date: Wed, 28 Nov 2007 10:50:15 +0100 (CET)
Subject: [Rd] hash access to data.frame uses prefix? (PR#10474)
Message-ID: <20071128095015.720E2283416B@mail.pubhealth.ku.dk>

If you do want only exact matching, use match().  E.g. in your example

x[match("V41", row.names(x)), ]

will give NA as you expected for a construction documented to do something 
else.

On Wed, 28 Nov 2007, Prof Brian Ripley wrote:

> Exact matching has preference over partial matching: see ?pmatch.
>
> Your version of R is three versions obsolete: the latest version explains 
> this in detail under ?`[.data.frame` and ?`[` (and maybe 2.5.0 does too).
>
> Please do your homework before sending non-bugs to R-bugs.
>
>
> On Wed, 28 Nov 2007, pkensche at cmbi.ru.nl wrote:
>
>> Full_Name: Philip Kensche
>> Version: R version 2.5.0 (2007-04-23)
>> OS: Linux
>> Submission from: (NULL) (131.174.146.31)
>> 
>> 
>> I want to access a row of a data.frame() by using the row names as hash 
>> keys.
>
> Hmm, you mean you use character vector indices.  No hashing is involved.
>
>> This works fine for most keys.
>> 
>> Now consider the following data.frame().
>> 
>>> x <- data.frame(v=c(V40="a", V411="b"))
>> 
>>> x
>>     v
>> V40  a
>> V411 b
>> 
>> If I query for "V41", which does not exist in the data.frame() the call 
>> does not return NA as I would expect but the row "V411".
>> 
>>> x[ "V41", ]
>> [1] b
>> Levels: a b
>> 
>> If there the prefix is not unique the query does not return a results, i.e.
>> 
>>> x <- data.frame(v=c(V412="a", V411="b"))
>> 
>>> x
>>     v
>> V412 a
>> V411 b
>> 
>>> x[ "V41", ]
>> [1] <NA>
>> Levels: a b
>> 
>> 
>> sessionInfo() output:
>>> sessionInfo()
>> R version 2.5.0 (2007-04-23)
>> i686-pc-linux-gnu
>> 
>> locale:
>> LC_CTYPE=en_US;LC_NUMERIC=C;LC_TIME=en_US;LC_COLLATE=en_US;LC_MONETARY=en_US;LC_MESSAGES=en_US;LC_PAPER=en_US;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US;LC_IDENTIFICATION=C
>> 
>> attached base packages:
>> [1] "stats"     "graphics"  "grDevices" "utils"     "datasets"  "methods"
>> [7] "base"
>> 
>> other attached packages:
>> lattice
>> "0.15-4"
>> 
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>> 
>
>

-- 
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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595



[Rd] hash access to data.frame uses prefix? (PR#10474)
ripley at stats.ox.ac.uk