[R] Randomly sampling subsets of dataframe variable

[Hosack, Michael]

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