Counting unique / distinct values by group in a data frame

Question

Let\'s say I have the following data frame:

> myvec
    name order_no
1    Amy       12
2   Jack       14
3   Jack       16
4   Dave       11
5    Amy             


        

Answer 1

This should do the trick:

ddply(myvec,~name,summarise,number_of_distinct_orders=length(unique(order_no)))

This requires package plyr.