R help - Mar 2012 - randomly subsample rows from subsets

Hi,

I have a list of 1787 fish from 948 full-sib families and their lengths. My
table looks like this,

fish	fam	length
1	a	71.46
2	a	71.06
3	a	62.94
4	b	79.46
5	b	52.38
6	b	56.78
7	b	92.08
8	c	96.86
9	d	98.09
10	d	17.23
11	d	98.35
12	d	82.43
13	e	83.85
14	e	33.92
15	e	23.16
16	e	31.39
17	e	57.08
18	e	27.05
19	f	62.38
20	f	83.21
21	f	18.72
22	f	84.32
23	g	15.99
24	h	40.33
25	h	92.73
26	h	59.08
27	i	29.05

I want to randomly select 2 fish from each family that has 2 or more
individuals and exclude those families that have just one fish. How can I do
that? Thanks,



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Hello,

Try

text="
fish fam length
1 a 71.46
2 a 71.06
3 a 62.94
4 b 79.46
5 b 52.38
6 b 56.78
7 b 92.08
8 c 96.86
9 d 98.09
10 d 17.23
11 d 98.35
12 d 82.43
13 e 83.85
14 e 33.92
15 e 23.16
16 e 31.39
17 e 57.08
18 e 27.05
19 f 62.38
20 f 83.21
21 f 18.72
22 f 84.32
23 g 15.99
24 h 40.33
25 h 92.73
26 h 59.08
27 i 29.05
"
fish <- read.table(textConnection(text), header=TRUE)
head(fish)

set.seed(1)
select <- lapply(split(fish, fish$fam),
	function(x) if(NROW(x) > 1) x[sample(NROW(x), 2), ])
select <- select[!sapply(select, is.null)]

# result as a list
select
# result as a data.frame
do.call(rbind, select)

Hope this helps,

Rui Barradas


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