On 8/28/2012 5:52 PM, Bert Gunter wrote:
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
Another way that works is:
as.numeric(rownames(unique(data.frame(A)[1])))
However, this may not be too efficient.
On Tue, Aug 28, 2012 at 3:32 PM, R. Michael Weylandt
<michael.weyla...@gmail.com> wrote:
On Tue, Aug 28, 2012 at 2:58 PM, Bronwyn Rayfield
<bronwynrayfi...@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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