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 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

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