Question

What is the most efficient way to sample a data frame under a certain constraint?

For example, say I have a directory of Names and Salaries, how do I select 3 such that their sum does not exceed some value. I’m just using a while loop but that seems pretty inefficient.

Answer 1

You could face a combinatorial explosion. This simulates the selection of 3 combinations of the EE’s from a set of 20 with salaries at a mean of 60 and sd 20. It shows that from the enumeration of the 1140 combinations you will find only 263 having sum of salaries less than 150.

> sum( apply( combn(1:20,3) , 2, function(x) sum(salry[x, "sals"]) < 150))
[1] 200
> set.seed(123)

> salry <- data.frame(EEnams = sapply(1:20 , 
                                       function(x){paste(sample(letters[1:20], 6) , 
                                             collapse="")}), sals = rnorm(20, 60, 20))
> head(salry)
  EEnams     sals
1 fohpqa 67.59279
2 kqjhpg 49.95353
3 nkbpda 53.33585
4 gsqlko 39.62849
5 ntjkec 38.56418
6 trmnah 66.07057

> sum( apply( combn(1:NROW(salry), 3) , 2, function(x) sum(salry[x, "sals"]) < 150))
[1] 263

If you had 1000 EE’s then you would have:

> choose(1000, 3)   # Combination possibilities
# [1] 166,167,000   Commas added to output