How to count the number of unique values by group? [duplicate]
ID= c('A', 'A', 'A', 'B', 'B', 'B')
color=c('white', 'green', 'orange', 'white', 'green', 'green')
d = data.frame (ID, color)
My desired result is
unique_colors=c(3,3,3,2,2,2)
d = data.frame (ID, color, unique_colors)
or more clear in a new dataframe c
ID= c('A','B')
unique_colors=c(3,2)
c = data.frame (ID,unique_colors)
I've tried different combinations of aggregate
and ave
as well as by
and with
and I suppose it is a combination of those functions.
The solution would include:
length(unique(d$color))
to calculate the number of unique elements.
Solution 1:
I think you've got it all wrong here. There is no need neither in plyr
or <-
when using data.table
.
Recent versions of data.table, v >= 1.9.6, have a new function uniqueN()
just for that.
library(data.table) ## >= v1.9.6
setDT(d)[, .(count = uniqueN(color)), by = ID]
# ID count
# 1: A 3
# 2: B 2
If you want to create a new column with the counts, use the :=
operator
setDT(d)[, count := uniqueN(color), by = ID]
Or with dplyr
use the n_distinct
function
library(dplyr)
d %>%
group_by(ID) %>%
summarise(count = n_distinct(color))
# Source: local data table [2 x 2]
#
# ID count
# 1 A 3
# 2 B 2
Or (if you want a new column) use mutate
instead of summary
d %>%
group_by(ID) %>%
mutate(count = n_distinct(color))