Running count based on field in R

Question

I have a data set of this format

User       
1 
2
3
2
3
1  
1      

Now I want to add a column saying count which counts the occurrence of the

Answer 1

This is fairly easy with ave and seq.int:

> ave(User,User, FUN= seq.int)
[1] 1 1 1 2 2 2 3

This is a common strategy and is often used when the items are adjacent to each other. The second argument is the grouping variable and in this case the first argument is really kind of a dummy argument since the only thing that it contributes is a length, and it is not a requirement for ave to have adjacent rows for the values determined within groupings.