[R] Quickest way to match two vectors besides %in%?

[Weiwei Shi]

?match

> x
  X1 X2
1  1  5
2  2  6
3  3  7
4  4  8

> y
  Y1 Y4
1  1  8
2  2  9
3  3 10
4  4 11
5  1 12
6  2 13
7  3 14
8  4 15

> y.orig<-y # backup

> y$Y4<-x$X2[match(y$Y1, x$X1)]
> y
  Y1 Y4
1  1  5
2  2  6
3  3  7
4  4  8
5  1  5
6  2  6
7  3  7
8  4  8


HTH,

Weiwei

On 11/8/05, Pete Cap <peteoutside at yahoo.com> wrote:
> Hello list,
>
> I have two data frames, X (48469,2) and Y (79771,5).
>
> X[,1] contains distinct values of Y[,2].
> I want to match values in X[,1] and Y[,2], then take
> the corresponding value in [X,2] and place it in
> Y[,4].
>
> So far I have been doing it like so:
> for(i in 1:48469) {
> y[which(x[i,1]==y[,3]),4]<-x[i,2]
> }
>
> But it chunks along so very slowly that I can't help
> but wonder if there's a faster way, mainly because on
> my box it takes R about 30 seconds to simply COUNT to
> 48,469 in the for loop.
>
> I have already tried using %in%.  It tells me if the
> values in X[,1] are IN Y[,2], which is useful in
> removing unnecessary values from X[,1].  But it does
> not tell me exactly where they match.  which(X[,1]
> %in% Y[,2]) does but it only matches on the first
> instance.
>
> This is the slowest part of the script I'm working
> on--if I could improve it I could shave off some
> serious operating time.  Any pointers?
>
> Regards,
>
> Pete
>
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--
Weiwei Shi, Ph.D

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