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

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