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_rows1   a       22   b       33   c       34   d       35   e       26   f       47   g       68   h       19   i       610  j       511  k       612  l       413  m       714  n       215  o       216  p       217  q       518  r       419  s       520  t       321  u       822  v       423  w       524  x       425  y       326  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 tabletable(tt)tta 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 tapplytapply(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.