[R] One-to-one matching?

[Moshe Olshansky]

How about

x <- lookupTable
x[!(x %in% matchSample)] <- NA
pmatch(matchSample,x)


Regards,

Moshe.

--- On Mon, 23/6/08, Alec.Zwart at csiro.au <Alec.Zwart at csiro.au> wrote:

> From: Alec.Zwart at csiro.au <Alec.Zwart at csiro.au>
> Subject: [R] One-to-one matching?
> To: r-help at r-project.org
> Received: Monday, 23 June, 2008, 12:57 PM
> Hi folks, 
> 
> Can anyone suggest an efficient way to do "matching
> without
> replacement", or "one-to-one matching"? 
> pmatch() doesn't quite provide
> what I need...
> 
> For example, 
> 
> lookupTable <-
> c("a","b","c","d","e","f")
> matchSample <-
> c("a","a","b","d")
> ##Normal match() behaviour:
> match(matchSample,lookupTable)
> [1] 1 1 2 4
> 
> My problem here is that both "a"s in matchSample
> are matched to the same
> "a" in the lookup table.  I need the elements of
> the lookup table to be
> excluded from the table as they are matched, so that no
> match can be
> found for the second "a".  
> 
> Function pmatch() comes close to what I need:
> 
> pmatch(matchSample,lookupTable)
> [1] 1 NA 2 4
> 
> Yep!  However, pmatch() incorporates partial matching,
> which I
> definitely don't want:
> 
> lookupTable <-
> c("a","b","c","d","e","aaaaaaaaf")
> 
> matchSample <-
> c("a","a","b","d")
> pmatch(matchSample,lookupTable)
> [1] 1 6 2 4
> ## i.e. the second "a", matches
> "aaaaaaaaf" - I don't want this.
> 
> Of course, when identical items ARE duplicated in both
> sample and lookup
> table, I need the matching to reflect this:
> 
> lookupTable <-
> c("a","a","c","d","e","f")
> matchSample <-
> c("a","a","c","d")
> ##Normal match() behaviour
> match(matchSample,lookupTable)
> [1] 1 1 3 4
> 
> No good - pmatch() is better:
> 
> lookupTable <-
> c("a","a","c","d","e","f")
> matchSample <-
> c("a","a","c","d")
> pmatch(matchSample,lookupTable)
> [1] 1 2 3 4
> 
> ...but we still have the partial matching issue...
> 
> ##And of course, as per the usual behaviour of match(),
> sample elements
> missing from the lookup table should return NA:
> 
> matchSample <-
> c("a","frog","e","d")
> ; print(matchSample)
> match(matchSample,lookupTable)
> 
> Is there a nifty way to get what I'm after without
> resorting to a for
> loop? (my code's already got too blasted many of
> those...)
> 
> Thanks, 
> 
> Alec Zwart
> CMIS CSIRO
> alec.zwart at csiro.au
> 
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