[R] how to efficiently compute set unique?

[David Winsemius]

On Jun 21, 2010, at 9:18 PM, Duncan Murdoch wrote:

> On 21/06/2010 9:06 PM, G FANG wrote:
>> Hi,
>>
>> I want to get the unique set from a large numeric k by 1 vector, k is
>> in tens of millions
>>
>> when I used the matlab function unique, it takes less than 10 secs
>>
>> but when I tried to use the unique in R with similar CPU and memory,
>> it is not done in minutes
>>
>> I am wondering, am I using the function in the right way?
>>
>> dim(cntxtn)
>> [1] 13584763        1
>> uniqueCntxt = unique(cntxtn);    # this is taking really long
>
> What type is cntxtn?  If I do that sort of thing on a numeric  
> vector, it's quite fast:
>
> > x <- sample(100000, size=13584763, replace=T)
> > system.time(unique(x))
>  user  system elapsed
>  3.61    0.14    3.75

If it's a factor, it could be as simple as:

levels(cntxtn)  # since the work of "unique-ification" has already  
been done.

 > x <- factor(sample(100000, size=13584763, replace=T))
 > system.time(levels(x))
    user  system elapsed
       0       0       0
 > system.time(y <- levels(x))
    user  system elapsed
       0       0       0


-- 

David Winsemius, MD
West Hartford, CT