How to Select Top N Values By Groups in R

One of the common tasks in data manipulation and analysis is to select the top N values by groups. For instance, you might want to pick the top 3 salespersons in each region, or you may wish to select the top 5 products for each category based on their ratings. In this article, we will explore how to accomplish this in R, leveraging a variety of methods from Base R to dplyr and data.table.

Table of Contents

1. Introduction
2. Using Base R
   • Sorting and Subsetting
   • The by() Function
   • split() and lapply()
3. Using dplyr
   • The top_n() Function
   • arrange() and slice_head()
4. Using data.table
   • Basic Usage
   • Index-based Selection
5. Custom Functions
6. Practical Applications
7. Conclusion

1. Introduction

Selecting the top N values by groups is a task that you’ll likely encounter in many data analysis projects. This operation allows you to isolate the best-performing items within each category, facilitating more nuanced insights.

2. Using Base R

Sorting and Subsetting

One of the straightforward ways to achieve this in Base R is by sorting the data frame first and then subsetting.

# Sample data
df <- data.frame(Category = c('A', 'A', 'A', 'B', 'B', 'C', 'C'),
                 Value = c(4, 2, 1, 5, 3, 6, 2))

# Sort the data frame
sorted_df <- df[order(df$Category, -df$Value), ]

# Subset to get top 2 rows for each Category
top_2_by_category <- by(sorted_df, sorted_df$Category, head, n = 2)

The by( ) Function

The by() function can be very handy. It’s designed to apply a function to a data frame split by factors.

top_2_by_category <- by(df, df$Category, function(x) {
  x_sorted <- x[order(-x$Value), ]
  return(head(x_sorted, 2))
})

split( ) and lapply( )

You can also use split() to divide the data frame by groups and then use lapply() to apply a function to each group.

split_data <- split(df, df$Category)
top_2_by_category <- lapply(split_data, function(x) {
  x_sorted <- x[order(-x$Value), ]
  return(head(x_sorted, 2))
})

3. Using dplyr

dplyr from the tidyverse package collection makes data manipulation tasks more straightforward and readable.

The top_n( ) Function

The top_n() function makes it straightforward to select the top N rows for each group.

library(dplyr)
top_2_by_category <- df %>%
  group_by(Category) %>%
  top_n(2, Value)

arrange( ) and slice_head( )

You can use arrange() in conjunction with slice_head() to accomplish the same task.

top_2_by_category <- df %>%
  group_by(Category) %>%
  arrange(desc(Value)) %>%
  slice_head(n = 2)

4. Using data.table

data.table offers high-performance and memory-efficient options, especially useful for large datasets.

Basic Usage

library(data.table)
dt <- as.data.table(df)
top_2_by_category <- dt[, head(.SD[order(-Value)], 2), by = Category]

Index-based Selection

If you set keys for your data table, data.table can perform the operation even faster.

setkey(dt, Category, Value)
top_2_by_category <- dt[, head(.SD, 2), by = Category]

5. Custom Functions

You can define a custom function to encapsulate the logic for selecting the top N values by category.

get_top_n_by_category <- function(data, category_col, value_col, n = 2) {
  data %>% 
    group_by(across(all_of(category_col))) %>% 
    arrange(desc(across(all_of(value_col)))) %>% 
    slice_head(n = n)
}

# Usage
top_2_by_category <- get_top_n_by_category(df, "Category", "Value", 2)

6. Practical Applications

• Sales Data: Identifying the top-selling products in each region.
• Educational Data: Finding the top-performing students in each class or subject.
• E-commerce: Sorting the most viewed or best-rated items by category.
• Stock Market: Selecting the top-performing stocks in each sector for a given period.

7. Conclusion

Selecting the top N values by groups in R can be achieved in multiple ways, each with its own advantages and drawbacks. Base R methods like sorting and subsetting, or using by() and split() functions, are simple but can be slow for large data. The dplyr and data.table packages offer more efficient and readable options. The method you choose will often depend on your specific requirements, including the data size and structure, and your preferred syntax.