loop

Rather than using a loop, how can I remove all consequentially
repeated values as in this example?
I am guessing using diff would help but not quite sure how.

get
s

date          f  
1 1999-01-01 1
2 1999-01-02 1
3 1999-01-03 1
4 1999-01-04 2
5 1999-01-05 2

v <- s[1,'f']; for (i in 2:nrow(s)) { if (s[i,'f'] == v) s[i,'f'] <- NA else v <- s[i,'f'] }
s <- s[!is.na(s$f),]
s

date          f
1 1999-01-01 1
4 1999-01-04 2