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 is a simple solution with the function aggregate:

aggregate(order_no ~ name, myvec, function(x) length(unique(x)))