Hi:

Is something like this what you were after?

x <- data.frame(A = sample(LETTERS[1:5], 1000, replace = TRUE),
                 B = rpois(1000, 50),
                 C = rnorm(1000))
x[unique(x$A), ]
  A  B          C
4 E 49 1.18424176
5 B 51 0.51911271
1 D 71 0.06266016
2 E 61 0.59862609
3 A 45 0.55970798

x[unique(x$B), ]   is a bit longer so is not included :)

HTH,
Dennis

On Thu, Oct 28, 2010 at 11:29 AM, boshao zhang <zboshao@yahoo.com> wrote:

> Dear everyone in the Mailing list:
>
> It is easy to get the unique elements in a column. But I would like to get
> rid of those rows that the elements of this column are redundant. Or
> sometimes, to have a look at the rows that the elements of this column are
> redundant is also important. I guess it boils down to throw out the index of
> the redundant elements.
>
> With millions of rows, how can I efficiently perform the task?
>
> Thank you in advance.
>
> Boshao
>
>
>
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>
>
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