How to subset a data.frame?

Question

I have a data set like this

a <- data.frame(var1 = c("patientA", "patientA", "patientA", "patientB", "patientB", "patientB", "patientB"),
                var2 = as.Date(c("2015-01-02","2015-01-04","2015-02-02","2015-02-06","2015-01-02","2015-01-07","2015-04-02")),
                var3 = c(F, T, F, F, F, T, F)               
                )
sequ <- rle(as.character(a$var1))
a$sequ <- sequence(sequ$lengths)

producing

> a
      var1       var2  var3 sequ
1 patientA 2015-01-02 FALSE    1
2 patientA 2015-01-04  TRUE    2
3 patientA 2015-02-02 FALSE    3
4 patientB 2015-02-06 FALSE    1
5 patientB 2015-01-02 FALSE    2
6 patientB 2015-01-07  TRUE    3
7 patientB 2015-04-02 FALSE    4

How can I subset/filter this data set so that I get all the rows which var3 == TRUE and var2 date value is greater than in the row where var3 == TRUE (by patient, var1? I tried

subset(a, (var3 == TRUE) & (var2 > var3))

but this does not produce a correct result set. The correct one is

#       var1       var2  var3 sequ
# 1 patientA 2015-01-04  TRUE    2
# 2 patientA 2015-02-02 FALSE    3
# 3 patientB 2015-02-06 FALSE    1
# 4 patientB 2015-01-07  TRUE    3
# 5 patientB 2015-04-02 FALSE    4

Answer 1

You may try with data.table. Here, we convert the 'data.frame' to 'data.table' (setDT(a)), grouped by 'var1', we get a logical index for 'var2' elements that are greater than or equal to corresponding 'var2' elements for which 'var3' is TRUE and subset the dataset .SD.

library(data.table)
setDT(a)[,.SD[var2 >= var2[var3]], var1]
#       var1       var2  var3 sequ
#1: patientA 2015-01-04  TRUE    2
#2: patientA 2015-02-02 FALSE    3
#3: patientB 2015-02-06 FALSE    1
#4: patientB 2015-01-07  TRUE    3
#5: patientB 2015-04-02 FALSE    4

An option using base R (assuming that the data is ordered by 'var1')

a[with(a, var2>=rep(var2[var3], table(var1))),]
#      var1       var2  var3 sequ
#2 patientA 2015-01-04  TRUE    2
#3 patientA 2015-02-02 FALSE    3
#4 patientB 2015-02-06 FALSE    1
#6 patientB 2015-01-07  TRUE    3
#7 patientB 2015-04-02 FALSE    4