Say I have a data.frame
object:
df <- data.frame(name=c('black','black','black','red','red'),
type=c('chair','chair','sofa','sofa','plate'),
num=c(4,5,12,4,3))
Now I want to count the number of rows (observations) of for each combination of name
and type
. This can be done like so:
table(df[ , c("name","type")])
or possibly also with plyr
, (though I am not sure how).
However, how do I get the results incorporated into the original data frame? So that the results will look like this:
df
# name type num count
# 1 black chair 4 2
# 2 black chair 5 2
# 3 black sofa 12 1
# 4 red sofa 4 1
# 5 red plate 3 1
where count
now stores the results from the aggregation.
A solution with plyr
could be interesting to learn as well, though I would like to see how this is done with base R.
Using data.table
:
library(data.table)
dt = as.data.table(df)
# or coerce to data.table by reference:
# setDT(df)
dt[ , count := .N, by = .(name, type)]
For pre-data.table 1.8.2
alternative, see edit history.
Using dplyr
:
library(dplyr)
df %>%
group_by(name, type) %>%
mutate(count = n())
Or simply:
add_count(df, name, type)
Using plyr
:
plyr::ddply(df, .(name, type), transform, count = length(num))