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Multiple match function?

3 messages · Jonck van der Kogel, Jonathan Baron, Tony Plate

Jonck van der Kogel
# 
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. 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 
:-)

Thanks very much, Jonck
Jonathan Baron
#
On 06/11/03 03:28, Jonck van der Kogel wrote:
one way is
which(x==2)

I'm sure there are others.
Tony Plate
#
At Wednesday 03:28 AM 6/11/2003 +0200, Jonck van der Kogel wrote:
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().
As others have already pointed out, which() together with "==" may be all 
you need.