[R] Randomly selecting one row for each factor level

[Kelly Hildner]

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