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first_index
2012 Aug 28
5
return first index for each unique value in a vector
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.
What I am trying to do is easily