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

A data.table approach

library(data.table)
DT <- data.table(myvec)

DT[, .(number_of_distinct_orders = length(unique(order_no))), by = name]

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data.table v >= 1.9.5 has a built in uniqueN function now

DT[, .(number_of_distinct_orders = uniqueN(order_no)), by = name]