sample_and_replace_value: Replace given value in a random pct of rows (by grouping)
In m-dz/mdmisc:

Function modifies the input dt!

1 2	sample_and_replace_value(dt, pct, col_name, value, grouping = NULL, sort = FALSE)

grouping

set.seed(2016)
size <- 10
dt <- data.table(
  id = 1:size, A = sample(letters[1:3], size, replace = TRUE), B = 'N',
  C = sample(1:100, size, replace = TRUE) + sample(30:70, size, replace = TRUE))
set.seed(2016)
sample_and_replace_value(dt, pct = 0.5, col_name = 'B', value = 'Y', grouping = 'A')
dt[, table(A, B)]
sample_and_replace_value(dt, pct = 0.5, col_name = 'B', value = 'Y', grouping = 'A', sort = TRUE)
dt