[r] Repeat each row of data.frame the number of times specified in a column

Use expandRows() from the splitstackshape package:

library(splitstackshape)
expandRows(df, "freq")

Simple syntax, very fast, works on data.frame or data.table.

Result:

    var1 var2
1      a    d
2      b    e
2.1    b    e
3      c    f
3.1    c    f
3.2    c    f