[R] Conditional sampling

[Hosack, Michael]

R experts,

I need to sample two rows without replacement from the following data frame such that 
neither row contains the same 'DOW'. For example, I cannot select both a Monday morning 
and a Monday afternoon. I am using STRATA_NUM as an index to randomly select rows from, 
since this variable indexes all unique permutations of DOW, SITE, and TOD. I know how to 
use the sample function to select rows, I just don't know how to sample with a constraint. 
This seems simple, but I can't seem to find a simple solution. Any help would be greatly 
appreciated. 

Thank you,

Mike 



DF <-
structure(list(DOW = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 5L, 5L, 5L,
5L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L), .Label = c("Fri", "Mon", "Thu", "Tue", "Wed"), class = "factor"),
    SITE = c(101L, 101L, 102L, 102L, 103L, 103L, 104L, 104L,
    101L, 101L, 102L, 102L, 103L, 103L, 104L, 104L, 101L, 101L,
    102L, 102L, 103L, 103L, 104L, 104L, 101L, 101L, 102L, 102L,
    103L, 103L, 104L, 104L, 101L, 101L, 102L, 102L, 103L, 103L,
    104L, 104L), TOD = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L,
    2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L,
    1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L,
    2L, 1L, 2L), .Label = c("Aftn", "Morn"), class = "factor"),
    STRATA_NUM = c(2L, 1L, 12L, 11L, 22L, 21L, 32L, 31L, 4L,
    3L, 14L, 13L, 24L, 23L, 34L, 33L, 6L, 5L, 16L, 15L, 26L,
    25L, 36L, 35L, 8L, 7L, 18L, 17L, 28L, 27L, 38L, 37L, 10L,
    9L, 20L, 19L, 30L, 29L, 40L, 39L), DATE = structure(c(1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 4L, 4L, 4L,
    4L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L), .Label = c("5/3/2010",
    "5/4/2010", "5/5/2010", "5/6/2010", "5/7/2010"), class = "factor"),
    DOW_NUM = c(3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L,
    4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 6L, 6L, 6L,
    6L, 6L, 6L, 6L, 6L, 7L, 7L, 7L, 7L, 7L, 7L, 7L, 7L), WEEK = c(1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L)), .Names = c("DOW", "SITE",
"TOD", "STRATA_NUM", "DATE", "DOW_NUM", "WEEK"), class = "data.frame", row.names = c(NA,
-40L))