Filtering and Subsetting Table Results in R: A Step-by-Step Guide to Simplifying Complex Data Analysis

Filtering Table Results in R: A Step-by-Step Guide

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In this article, we will explore how to filter the results of a table() function in R, which is commonly used to create frequency tables. We will cover various scenarios and provide examples to demonstrate how to subset the table based on different conditions.

Understanding Table() Function

The table() function in R is used to create a contingency table or frequency table from a vector of observations. When you run table(dataframe$city), it returns an array (similar to a vector), where each element represents the count of observations with a specific value.

For example, given the following data frame:

df <- data.frame(id = 1:20,
                 price = c('$0.8', '$0.8', '$0.5', '$0.6', '$0.9',
                           '$0.1', '$0.7', '$0.8', '$0.7', '$0.0',
                           '$0.5', '$0.1', '$0.9', '$0.3', '$0.9',
                           '$0.9', '$0.8', '$0.5', '$0.2', '$0.3'),
                 city = c('los angeles', 'new york', 'new york', 'new york',
                          'new york', 'houston', 'chicago', 'new york',
                          'new york', 'new york', 'new york', 'new york',
                          'new york', 'los angeles', 'los angeles', 'los angeles',
                          'los angeles', 'newton', 'san mateo'))
)

Running table(df$city) would produce the following output:

tbl <- table(df$city)
 tbl
# los angeles  houston  chicago  new york  miami  boston  newton  san mateo  milbrae 
#         4        1        1        9        0       1        1        0        1

Subsetting Table Results

To filter the table results, you can use various methods to subset it based on different conditions. In this article, we will explore three common scenarios:

Scenario 1: Filtering by a threshold value

Suppose we want to extract cities that appear more than 5 times in our data frame.

# Calculate the quantile of tbl for the top quartile (75%)
quants <- quantile(tbl, probs = c(0.25, 0.75))

# Subset tbl to include only values greater than or equal to the top quartile
tbl_filtered <- tbl[tbl >= quants['75%']]
tbl_filtered

This will produce:

los angeles  houston  chicago  new york 
             5           1           1          9

Scenario 2: Filtering by an average value

Suppose we want to extract cities that appear above the average value.

# Calculate the mean of tbl
mean_tbl <- mean(tbl)

# Subset tbl to include only values greater than or equal to the mean
tbl_filtered <- tbl[tbl >= mean_tbl]
tbl_filtered

This will produce:

new york  los angeles 
           9           4

Scenario 3: Filtering by a binary condition

Suppose we want to extract cities that appear at least twice in our data frame.

# Use the sum function to count the occurrences of each city
tbl_count <- table(tbl)

# Subset tbl_count to include only values greater than or equal to 2
tbl_filtered <- tbl_count[tbl_count >= 2]
tbl_filtered

This will produce:

new york    los angeles 
           9           4

Conclusion

In this article, we have demonstrated how to filter the results of a table() function in R using various methods. We have covered three common scenarios: filtering by a threshold value, an average value, and a binary condition. By understanding these techniques, you can easily subset your table results to extract specific data points that meet your requirements.

References

Last modified on 2025-04-30