Counting unique / distinct values by group in a data frame
dataframe, distinct-values, r, r-faq
Solution
A `data.table` approach
library(data.table)
DT <- data.table(myvec)
DT[, .(number_of_distinct_orders = length(unique(order_no))), by = name]
`data.table` v >= 1.9.5 has a built in `uniqueN` function now
DT[, .(number_of_distinct_orders = uniqueN(order_no)), by = name]
Problem
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 12 6 Jack 16 7 Tom 19 8 Larry 22 9 Tom 19 10 Dave 11 11 Jack 17 12 Tom 20 13 Amy 23 14 Jack 16 ``` I want to count the number of distinct `order_no` values for each `name`. It should produce the following result: ``` name number_of_distinct_orders Amy 2 Jack 3 Dave 1 Tom 2 Larry 1 ``` How can I do that?