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

You can just use the built-in R functions tapply with length

tapply(myvec$order_no, myvec$name, FUN = function(x) length(unique(x)))