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 should do the trick:

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

This requires package plyr.

Answer 2

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)

Answer 3

This is a simple solution with the function aggregate:

aggregate(order_no ~ name, myvec, function(x) length(unique(x)))

Answer 4

In dplyr you may use n_distinct to "count the number of unique values":

library(dplyr)
myvec %>%
  group_by(name) %>%
  summarise(n_distinct(order_no))

Answer 5

Here is a solution with sqldf

library("sqldf")

myvec <- read.table(header=TRUE, text=
"   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")
sqldf("SELECT name,COUNT(distinct(order_no)) as number_of_distinct_orders FROM myvec GROUP BY name")
# > sqldf("SELECT name,COUNT(distinct(order_no)) as number_of_distinct_orders FROM myvec GROUP BY name")
#    name number_of_distinct_orders
# 1   Amy                         2
# 2  Dave                         1
# 3  Jack                         3
# 4 Larry                         1
# 5   Tom                         2

Answer 6

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

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