R data.table - sample by group with different sampling proportion

Question

I would like to efficiently make a random sample by group from a data.table, but it should be possible to sample a different proportion for each group.

If I

Answer 1

You can use .GRP but to ensure a correct group is matched.. you might want to define group_col as a factor variable.

group_sampler <- function(data, group_col, sample_fractions) {
  # this function samples sample_fraction <0,1> from each group in the data.table
  # inputs:
  #   data - data.table
  #   group_col - column(s) used to group by
  #   sample_fraction - a value between 0 and 1 indicating what % of each group should be sampled
  stopifnot(length(sample_fractions) == uniqueN(data[[group_col]]))
  data[, .SD[sample(.N, ceiling(.N*sample_fractions[.GRP]))], keyby = group_col]
}

Edit in response to chinsoon12's comment:

It would be safer (instead of relying on correct order) to have the last line of the function:

data[, .SD[sample(.N, ceiling(.N*sample_fractions[[unlist(.BY)]]))], keyby = group_col]

And then you pass sample_fractions as a named vector:

group_sampler(DT, 'a', sample_fractions= c(x = 0.1, y = 0.9))