R help - Apr 2006 - Randomly selecting one row for each factor level

I don't use R much, and I have been unable to figure out how to get the 
subset of my data frame that I would like.

For example, if this were my data frame:

 > dfr <- data.frame(x=rep(letters[1:3], 4), y=(1:12), z=(LETTERS[1:12]))
 > dfr
    x  y z
1  a  1 A
2  b  2 B
3  c  3 C
4  a  4 D
5  b  5 E
6  c  6 F
7  a  7 G
8  b  8 H
9  c  9 I
10 a 10 J
11 b 11 K
12 c 12 L

I would like to randomly select one row for each level of the factor x 
and create a new data frame with the results.  For example, the result 
might be:

    x  y z
1  a  1 A
5  b  5 E
6  c  6 F

Any help would be greatly appreciated!

Thanks,
Kelly

-- 
K. Kelly Hildner, Ph.D.
NOAA Fisheries
Southwest Fisheries Science Center
110 Shaffer Rd.
Santa Cruz, CA 95060

Try this:

set.seed(1)
f <- function(x) { dd <- dfr[dfr$x == x,]; dd[sample(nrow(dd),1),] }
do.call("rbind", lapply(levels(dfr$x), f ))

On 4/20/06, Kelly Hildner <Kelly.Hildner at noaa.gov>
wrote:
> I don't use R much, and I have been unable to figure out how to get the
> subset of my data frame that I would like.
>
> For example, if this were my data frame:
>
>  > dfr <- data.frame(x=rep(letters[1:3], 4), y=(1:12),
z=(LETTERS[1:12]))
>  > dfr
>    x  y z
> 1  a  1 A
> 2  b  2 B
> 3  c  3 C
> 4  a  4 D
> 5  b  5 E
> 6  c  6 F
> 7  a  7 G
> 8  b  8 H
> 9  c  9 I
> 10 a 10 J
> 11 b 11 K
> 12 c 12 L
>
> I would like to randomly select one row for each level of the factor x
> and create a new data frame with the results.  For example, the result
> might be:
>
>    x  y z
> 1  a  1 A
> 5  b  5 E
> 6  c  6 F
>
> Any help would be greatly appreciated!
>
> Thanks,
> Kelly
>
> --
> K. Kelly Hildner, Ph.D.
> NOAA Fisheries
> Southwest Fisheries Science Center
> 110 Shaffer Rd.
> Santa Cruz, CA 95060
>
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