return first index for each unique value in a vector
Sheesh! I would have thought that someone would have noticed that on the ?unique Help page there is a link to ?duplicated, which gives a _logical_ vector of the duplicates. From this, everything else can be quickly derived -- and packaged in a simple Matlab like function, if you insist on that. e.g. unik <- !duplicated(A) ## logical vector of unique values seq_along(A)[unik] ## indices A[unik] ## the values If you want the indices in increasing order, see ?order -- Bert On Tue, Aug 28, 2012 at 3:32 PM, R. Michael Weylandt
<michael.weylandt at gmail.com> wrote:
On Tue, Aug 28, 2012 at 2:58 PM, Bronwyn Rayfield <bronwynrayfield at gmail.com> wrote:
I would like to efficiently find the first index of each unique value in a very large vector. For example, if I have a vector A<-c(9,2,9,5) I would like to return not only the unique values (2,5,9) but also their first indices (2,4,1). I tried using a for loop with which(A==unique(A)[i])[1] to find the first index of each unique value but it is very slow.
You'll get marginally more speed from which.max() but I'm sure there's a better way. I'll write if I can think of it. Michael
What I am trying to do is easily and quickly done with the "unique" function in MATLAB (see http://www.mathworks.com/help/techdoc/ref/unique.html). Thank you for your help, Bronwyn [[alternative HTML version deleted]]
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______________________________________________ R-help at r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm