Subset observations that differ by at least 30 minutes time

Question

I have a data.table (~30 million rows) consisting of a datetime column in POSIXct format, an id column and a few other co

Answer 1

Using Rcpp:

library(Rcpp)
library(inline)
cppFunction(
  'LogicalVector selecttimes(const NumericVector x) {
   const int n = x.length();
   LogicalVector res(n);
   res(0) = true;
   double testval = x(0);
   for (int i=1; i 30 * 60) {
      testval = x(i);
      res(i) = true;
    }
   }
   return res;
  }')

DT[, keep1 := selecttimes(datetime), by = id]

DT[, all(keep == keep1)]
#[1] TRUE

Some additional testing should be done, it needs input validation, and the time difference could be made a parameter.