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 would also work but is less eloquent than the plyr solution:

x <- sapply(split(myvec, myvec$name),  function(x) length(unique(x[, 2]))) 
data.frame(names=names(x), number_of_distinct_orders=x, row.names = NULL)