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
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>>> 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