R help - Jun 2003 - Multiple match function?

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

?which

HTH

Edith Hodgen
Statistician and Data Manager
New Zealand Council for Educational Research
Phone: +64-4-384 7939 x 812
Fax:      +64-4-384 7933
edith.hodgen at nzcer.org.nz

           Web site http://www.nzcer.org.nz
>>> Jonck van der Kogel <jonck at vanderkogel.net> 11/06/2003
13:28:38 >>>
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

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://www.stat.math.ethz.ch/mailman/listinfo/r-help

On 06/11/03 03:28, 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".
one way is
which(x==2)

I'm sure there are others.

-- 
Jonathan Baron, Professor of Psychology, University of Pennsylvania
R page:               http://finzi.psych.upenn.edu/

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
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help