How to count how many values per level in a given factor?
Or using the dplyr
library:
library(dplyr)
set.seed(1)
dat <- data.frame(ID = sample(letters,100,rep=TRUE))
dat %>%
group_by(ID) %>%
summarise(no_rows = length(ID))
Note the use of %>%
, which is similar to the use of pipes in bash. Effectively, the code above pipes dat
into group_by
, and the result of that operation is piped into summarise
.
The result is:
Source: local data frame [26 x 2]
ID no_rows
1 a 2
2 b 3
3 c 3
4 d 3
5 e 2
6 f 4
7 g 6
8 h 1
9 i 6
10 j 5
11 k 6
12 l 4
13 m 7
14 n 2
15 o 2
16 p 2
17 q 5
18 r 4
19 s 5
20 t 3
21 u 8
22 v 4
23 w 5
24 x 4
25 y 3
26 z 1
See the dplyr
introduction for some more context, and the documentation for details regarding the individual functions.
Here 2 ways to do it:
set.seed(1)
tt <- sample(letters,100,rep=TRUE)
## using table
table(tt)
tt
a b c d e f g h i j k l m n o p q r s t u v w x y z
2 3 3 3 2 4 6 1 6 5 6 4 7 2 2 2 5 4 5 3 8 4 5 4 3 1
## using tapply
tapply(tt,tt,length)
a b c d e f g h i j k l m n o p q r s t u v w x y z
2 3 3 3 2 4 6 1 6 5 6 4 7 2 2 2 5 4 5 3 8 4 5 4 3 1
Using plyr
package:
library(plyr)
count(mydf$V1)
It will return you a frequency of each value.
Using data.table
library(data.table)
setDT(dat)[, .N, keyby=ID] #(Using @Paul Hiemstra's `dat`)
Or using dplyr 0.3
res <- count(dat, ID)
head(res)
#Source: local data frame [6 x 2]
# ID n
#1 a 2
#2 b 3
#3 c 3
#4 d 3
#5 e 2
#6 f 4
Or
dat %>%
group_by(ID) %>%
tally()
Or
dat %>%
group_by(ID) %>%
summarise(n=n())
We can use summary
on factor column:
summary(myDF$factorColumn)