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How to create frequency table of data.table in R?
If we have an data.table object or a data frame converted to a data.table and it has a factor column then we might want to create a frequency table that shows the number of values each factor has or the count of factor levels. This is a data summarization method which helps us to understand the variation in the occurrences of factor levels. This can be easily done with a single line of code if we have a data.table object, otherwise we first need to convert the object.
Example
Consider the below data frame −
Group<-sample(c("A","B","C","D"),20,replace=TRUE)
Frequency<-sample(1:50,20)
df1<-data.frame(Group,Frequency)
df1Output
Group Frequency 1 A 11 2 B 19 3 A 41 4 D 24 5 A 22 6 B 26 7 C 46 8 C 4 9 A 45 10 C 44 11 A 50 12 C 20 13 B 27 14 D 12 15 A 34 16 B 6 17 C 35 18 D 32 19 A 42 20 D 30
Loading data.table package and reading the data frame df1 as data.table −
Example
library(data.table) df1<-as.data.table(df1)
Finding the frequency table of df1 which is a data.table object now −
Example
df1[,.N,by=Group]
Output
Group N 1: A 7 2: B 4 3: D 4 4: C 5
Let’s have a look at another example −
Example
Class<-sample(c("1","2","3","4","5"),20,replace=TRUE)
Scores<-sample(1:100,20)
df2<-data.frame(Class,Scores)
df2Output
Class Scores 1 2 45 2 5 66 3 4 61 4 5 96 5 3 27 6 5 94 7 5 51 8 3 98 9 1 14 10 3 83 11 3 6 12 1 16 13 1 18 14 1 35 15 2 70 16 4 67 17 1 63 18 5 31 19 1 58 20 4 15
Example
df2<-as.data.table(df2) df2[,.N,by=Class]
Output
Class N 1: 2 2 2: 5 5 3: 4 3 4: 3 4 5: 1 6
Updated on: 08-Sep-2020
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