[R] Randomly sampling subsets of dataframe variable

[Stephan Kolassa]

Hi Mike,

take an index vector that selects Monday and Tuesday out of each week, 
and then run a restricted random permutation on this vector which only 
permutes indices within each week. rperm() is in the sna package.

library(sna)
foo <- rep(c(TRUE,TRUE,FALSE,FALSE,FALSE),26)
your.data[foo[rperm(rep(seq(1,26),each=5))],]

HTH,
Stephan


Hosack, Michael schrieb:
> 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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