return first index for each unique value in a vector
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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