count frequency of rows based on a column value in R

Question

I understand that this is quite a simple question, but I haven't been able to find an answer to this.

I have a data frame which gives you the id of a person and his hobby. Since a person may have many hobbies, the id field may be repeated in multiple rows, each with a different hobby. I have been trying to print out only those rows which have more than one hobbies. I was able to get the frequencies using table.

1. But how do I apply the condition to print only when the frequency is greater than one.

2. Secondly, is there a better way to find frequencies without using table.

This is my attempt with table without the filter for frequency greater than one

> id=c(1,2,2,3,2,4,3,1)
> hobby = c('play','swim','play','movies','golf','basketball','playstation','gameboy')
> df = data.frame(id, hobby)
> table(df$id)

1 2 3 4 
2 3 2 1 

Answer 1

This example assumes you are trying to filter df

id=c(1,2,2,3,2,4,3,1)
hobby = c('play','swim','play','movies','golf','basketball',
'playstation','gameboy')
df = data.frame(id, hobby)
table(df$id)

Get all those ids that have more than one hobby

tmp <- as.data.frame(table(df$id))
tmp <- tmp[tmp$Freq > 1,]

Using that information - select their IDs in df

df1 <- df[df$id  %in% tmp$Var1,]
df1

Answer 2

Try using data table, I find it more readable than using table() functions:

library(data.table)
id=c(1,2,2,3,2,4,3,1)
hobby = c('play','swim','play','movies',
          'golf','basketball','playstation','gameboy')
df = data.frame(id=id, hobby=hobby)
dt = as.data.table(df)
dt[,hobbies:=.N, by=id]

You will get, for your condition:

> dt[hobbies >1,]
    id       hobby hobbies
1:  1        play       2
2:  2        swim       3
3:  2        play       3
4:  3      movies       2
5:  2        golf       3
6:  3 playstation       2
7:  1     gameboy       2