[R] Multiple match function?

[Tony Plate]

At Wednesday 03:28 AM 6/11/2003 +0200, Jonck van der Kogel wrote:
>Hi all,
>I have (yet another) question about a function in R. What I would like to 
>do is test for the presence of a certain value in a vector, and have the 
>positions that this value is at returned to me.
>For example, let's say I have a vector:
>x <- c(1,1,2,2,3,3,4,4)
>
>Now I would like a function that would return positions 3 and 4 should I 
>test for the value "2". Or 5 and 6 should I test for "3".
>
>Could someone please tell me how I should do this? The "match" function 
>only returns the first position that a value is found at.

Actually, there are different ways of using match().  I think the following 
does what you want:

 > x <- c(1,1,2,2,3,3,4,4)
 > # how you were using it
 > match(2, x)
[1] 3
 > # switch the arguments around to get a non-NA value for each position of 
x that matches at least one element of a set
 > match(x, 2)
[1] NA NA  1  1 NA NA NA NA
 > # see which elements of x are in the set {2}
 > which(!is.na(match(x, 2)))
[1] 3 4
 > # see which elements of x are in the set {3}
 > which(!is.na(match(x, 3)))
[1] 5 6
 > # see which elements of x are in the set {2,3}
 > which(!is.na(match(x, c(2,3))))
[1] 3 4 5 6
 >

The operator %in% provides a more intuitive interface for this usage of 
match().

>Of course I could write my own function that loops through the vector and 
>tests for the presence of each value manually but it seems likely that a 
>function that does this is already present in R. No need to re-invent the 
>wheel :-)

As others have already pointed out, which() together with "==" may be all 
you need.

>Thanks very much, Jonck
>
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