Hi:

A ddply solution:

library(plyr)
somedata = data.frame(week=rep(1:26,rep(5,26)),day=rep(1:5,26))
# sample two rows out of five per week
daysamp <- function(x) x[sample(1:5, 2), ]
# Ram it through ddply:
ddply(somedata, .(week), daysamp)

First part of output:
   week day
1     1   4
2     1   3
3     2   2
4     2   1
5     3   4
6     3   1
7     4   1
8     4   5

(52 rows in all, as expected)

HTH,
Dennis

On Fri, Mar 12, 2010 at 12:06 PM, Hosack, Michael <mhosack@state.pa.us>wrote:

> Fellow R users,
>
> I am stumped on what would seem to be something fairly simple.
> I have a dataframe that has a variable named 'WEEK' that takes
> the numbers 1:26 (26 week time-period) with each number repeated
> five times consecutively (once for each weekday, Monday through
> Friday). Ex. 111112222233333.....2626262626. I would like to
> randomly extract two weekdays per five day week for each of
> 26 weeks and store this data as a separate dataframe. I have
> been unable to get the sample function to work properly.
> I have also tried using the runif function to assign random
> numbers to each row of my dataframe, sort the dataframe first
> by week number then by random number value, and finally select
> the first two elements from each week subset (26 weeks total,
> giving 52 randomly selected values).  I can't figure out how
> to select the first two elements. My goal is to randomly
> select two weekdays per week (without replacement) for each of
> 26 consecutive weeks. Any advice would be greatly appreciated.
>
> Thank you,
>
> Mike
>
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