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How to create a frequency table for categorical data in R ?

  • Last Updated : 30 Jun, 2021

In this article, we will see how to create a frequency table for categorical data in R Programming Language.

Method 1 : Using table() method

Tables in R are used for better organizing and summarizing the categorical variables. The table() method takes the cross-classifying factors belonging in a vector to build a contingency table of the counts at each combination of factor levels. A contingency table is basically a tabulation of the counts and/or percentages for multiple variables. It excludes the counting of any missing values from the factor variable supplied to the method. The output returned is in the form of a table. This method can be used for cross-tabulation and statistical analysis.

table (fac-vec, .. )

It can be converted to a data frame using as.data.frame() method which returns the output in the form of a tabular structure organized into two columns, the first one containing the input factor variable and second, the counts of the corresponding variables, designated by “Freq”. 

as.data.frame(table)

In case, a single argument is specified the number of rows in the resultant data frame is equivalent to the number of unique levels of the factor. The rep() method is also used to repeat the first argument n number of times. 



Code:

R




# creating a factor vector
vec <- factor(c(letters[1 : 12], 
                letters[8 : 24],
                letters[4 : 6]))
  
# replicate the vector 2 times
fac_vec <- rep(vec , 2)
  
# generating frequency table of 
# the factor vector
freq <- table(fac_vec)
print ("Frequency DataFrame")
  
# converting the table into dataframe
freq_df <- as.data.frame(freq)
print (freq_df)

Output:

[1] "Frequency DataFrame"
   fac_vec Freq
1        a    2
2        b    2
3        c    2
4        d    4
5        e    4
6        f    4
7        g    2
8        h    4
9        i    4
10       j    4
11       k    4
12       l    4
13       m    2
14       n    2
15       o    2
16       p    2
17       q    2
18       r    2
19       s    2
20       t    2
21       u    2
22       v    2
23       w    2
24       x    2

The table() method can take multiple arguments as input, and as a result, a data frame of all the possible unique combinations is returned. Each value of the first argument is mapped with the second argument value, and the frequency of this combination is returned. 

R




set.seed(1)  
  
# creating a dataframe
data_frame = data.frame(  
    "col1" sample(1:2, 8, replace = TRUE) ,
    "col2" = sample(letters[1:3], 8, replace = TRUE)
)
  
print ("Data Frame")
print (data_frame)
  
# generating frequency table of the
# factor vector
freq <- table(data_frame$col1, data_frame$col2)
print ("Frequency DataFrame")
  
# converting the table into dataframe
freq_df <- as.data.frame(freq)
print (freq_df)

Output:

[1] "Data Frame"
  col1 col2
1    1    b
2    2    b
3    1    c
4    1    c
5    2    a
6    1    a
7    1    a
8    1    b
[1] "Frequency DataFrame"
  Var1 Var2 Freq
1    1    a    2
2    2    a    1
3    1    b    2
4    2    b    1
5    1    c    2
6    2    c    0

The following program illustrates the concept of computing the frequency distribution over a large dataset and visualizing it:

R






library(data.table)
library("ggplot2")
  
# importing the dataset
file <- fread('ch11b.dat')
print ("Dataset 5 rows")
head(file)
  
data <- as.data.frame(file)
freq <- table(data$V5)
print ("Frequency count of column V5")
print (freq)
  
# re-order levels
compute <- function(x) {
  freq <- table(x)
  factor(x, levels = names(sort(freq)))
}
  
# plotting the data
ggplot(data, aes(x = compute(`V5`))) +
  geom_bar() +
  xlab("Gender"

Output:

[1] "Dataset 5 rows" 
V1  V2   V3    V4 V5 
1:  1 307  930 36.58  0 
2:  2 307  940 36.73  0 
3:  3 307  950 36.93  0 
4:  4 307 1000 37.15  0 
5:  5 307 1010 37.23  0 
6:  6 307 1020 37.24  0 
[1] "Frequency count of column V5" 
0  1  
38 62 

Frequency distribution over column V5

Method 2: Using count() method

The “plyr” package is used for data manipulation and modification, which can be installed into the working space using :

install.packages("plyr")

count() method in the plyr package is used to keep a frequency count of the categorical variables encountered. The method call is equivalent to as.data.frame(table(x)). However, it does not include combinations with zero counts. This method is considered to be better than table() method because it is faster and returns the output in the form of data frame, with direct headings for rows and columns. 

count( fac-vec, .. )

Code:

R




# importing required libraries
library("plyr")
  
# creating a factor vector
vec <- factor(c(letters[1:12], 
                letters[8:24], 
                letters[4:6]))
  
# replicate the vector 2 times
fac_vec <- rep(vec , 2)
  
# generating frequency table of the 
# factor vector
freq <- count(fac_vec)
print ("Frequency DataFrame")
print (freq)

Output

[1] "Frequency DataFrame"
         x Freq
1        a    2
2        b    2
3        c    2
4        d    4
5        e    4
6        f    4
7        g    2
8        h    4
9        i    4
10       j    4
11       k    4
12       l    4
13       m    2
14       n    2
15       o    2
16       p    2
17       q    2
18       r    2
19       s    2
20       t    2
21       u    2
22       v    2
23       w    2
24       x    2

The following code snippet illustrates the usage of the table() method over very large datasets of the power of 105 elements divided into two categories Male and Female: 

R




# creating a factor vector
fac_vec <- factor(sample(c("Male", "Female"),
                         100000 , replace = TRUE))
  
# generating frequency table of the factor vector
freq <- count(fac_vec)
print ("Frequency DataFrame")
  
# converting the table into data frame
freq_df <- as.data.frame(freq)
print (freq_df)

Output:

[1] "Frequency DataFrame" 
        x  freq
 1 Female 49936
 
2   Male 50064




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