[R] return first index for each unique value in a vector

[Robert Baer]

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.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
>>>
>>>          [[alternative HTML version deleted]]
>>>
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>> ______________________________________________
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>
>