Counting unique / distinct values by group in a data frame

Question

I have a dataframe:

> myvec
    name order_no
1   Nehal       12
2   sejal       14
3   sejal       16
4   shyam       11
5   Nehal       12
6   Sejal       16
7   Ram       19

For each name how to count the distinct order no. Value. I am expecting this output:

name    number_of_distinct_orders
Neha      2
Sejal       3
Shyam   1
Ram       1

Answer 1

If you have plyr package then use this:

ddply(myvec,~name,summarise,number_of_distinct_orders=length(unique(order_no)))

else use data.table approach

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

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