# [R] One-to-one matching?

Gabor Grothendieck ggrothendieck at gmail.com
Mon Jun 23 05:22:03 CEST 2008

```Try this.  apseq() sorts the input and appends a
sequence number: 0, 1, ... to successive
occurrences of each value.  Apply that to both
vectors transforms it into a problem that works
with ordinary match:

> lookupTable <- c("a", "a","b","c","d","e","f")
> matchSample <- c("a", "a","a","b","d")
>
> # sort and append sequence no
> apseq <- function(x) {
+ x <- sort(x)
+ s <- cumsum(!duplicated(x))
+ paste(x, seq(s) - match(s, s))
+ }
>
> match(apseq(matchSample), apseq(lookupTable))
[1]  1  2 NA  3  5

On Sun, Jun 22, 2008 at 10:57 PM,  <Alec.Zwart at csiro.au> wrote:
> 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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