Help with indexing

# create a factor and then convert back to numeric
x$nb <- as.integer(factor(x$name, levels=unique(x$name))) + 99
x

Dear R Helpers,

I am missing something very elementary here, and I don't seem to get it from the help pages of the ave, seq and seq_along functions, so I wonder if you could offer a quick help.

To use an example from an earlier post on this list, I have a dataframe of this kind:

dat = data.frame(name = rep(c("Mary", "Sam", "John"), c(3,2,4)))
dat$freq = ave(seq_along(dat$name), dat$name, FUN = seq_along)

dat
?name freq
1 Mary ? ?1
2 Mary ? ?2
3 Mary ? ?3
4 ?Sam ? ?1
5 ?Sam ? ?2
6 John ? ?1
7 John ? ?2
8 John ? ?3
9 John ? ?4

What I need is another column assigning a number to each name starting from index 100, that is:

?name freq ?nb
1 Mary ? ?1 100
2 Mary ? ?2 100
3 Mary ? ?3 100
4 ?Sam ? ?1 101
5 ?Sam ? ?2 101
6 John ? ?1 102
7 John ? ?2 102
8 John ? ?3 102
9 John ? ?4 102

What is the easiest way to do this?

Thanks a lot for your kind help.

Dana