Select the n most frequent values in a variable

Question

I would like to find the most common values in a column in a data frame. I assume using table would be the best way to do this? I then want to filter/subset my data frame to only include these top-n values.

An example of my data frame is as follows. Here I want to find e.g. the top 2 IDs.

ID    col
A     blue
A     purple
A     green
B     green
B     red
C     red
C     blue
C     yellow
C     orange

I therefore want to output the following:

Top 2 values of ID are:
A and C

I will then select the rows corresponding to ID A and C:

ID    col
A     blue
A     purple
A     green
C     red
C     blue
C     yellow
C     orange

Answer 1

We can count the number of values using table, sort them in decreasing order and select first 2 (or 10) values, get the corresponding ID's and subset those ID's from the data frame.

df[df$ID %in% names(sort(table(df$ID), decreasing = TRUE)[1:2]), ]

#  ID    col
#1  A   blue
#2  A purple
#3  A  green
#6  C    red
#7  C   blue
#8  C yellow
#9  C orange