How to Write a case_when Statement in R

The case_when() function in R is part of the dplyr package and is an elegant way to perform multiple if-else statements within a mutate() or summarise() function, or even stand-alone. In this article, we will dive into how to write and understand the case_when() function in R, including its syntax, examples, and best practices.

Table of Contents

1. Prerequisites
2. Introduction to case_when()
3. Basic Syntax
4. Practical Examples
   • 4.1 Basic Usage
   • 4.2 Using with mutate()
   • 4.3 Using with summarise()
   • 4.4 Stand-alone Use
   • 4.5 Nested case_when()
5. Advanced Tips
6. Common Pitfalls and How to Avoid Them
7. Conclusion

1. Prerequisites

Before diving into the case_when() function, you should have install the dplyr package, which can be done using the following code:

install.packages("dplyr")

Load the package using:

library(dplyr)

2. Introduction to case_when( )

The case_when() function allows you to vectorize multiple if_else() conditions. It operates in a way similar to SQL’s CASE WHEN statement, helping you avoid a lengthy series of nested ifelse() statements.

3. Basic Syntax

The basic syntax of case_when() is as follows:

case_when(
  condition1 ~ value1,
  condition2 ~ value2,
  ...
  TRUE ~ value_default
)

• condition1, condition2, …: These are logical conditions that will be checked.
• value1, value2, …: These are the values that will be returned if the condition is TRUE.
• TRUE ~ value_default: A default value if none of the conditions are met.

4. Practical Examples

4.1 Basic Usage

Here’s a basic example using a vector of numbers:

nums <- c(1, 2, 3, 4, 5)

result <- case_when(
  nums <= 2 ~ "small",
  nums <= 4 ~ "medium",
  TRUE ~ "large"
)

print(result)

output:

[1] "small"  "small"  "medium" "medium" "large" 

4.2 Using with mutate( )

You can use case_when() within the mutate() function to create a new column based on some conditions.

data <- tibble(
  age = c(22, 45, 67, 34, 29)
)

data <- data %>% mutate(
  age_group = case_when(
    age < 30 ~ "young",
    age < 50 ~ "middle-aged",
    TRUE ~ "old"
  )
)

4.3 Using with summarise( )

Similarly, you can use case_when() within summarise() to aggregate data conditionally.

data %>% 
  summarise(
    num_young = sum(case_when(
      age < 30 ~ 1,
      TRUE ~ 0
    ))
  )

4.4 Stand-alone Use

You can use case_when() as a stand-alone function to perform calculations based on conditions.

result <- case_when(
  nums %% 2 == 0 ~ "even",
  TRUE ~ "odd"
)

4.5 Nested case_when( )

You can also nest case_when() functions for more complex logic.

result <- case_when(
  nums %% 2 == 0 ~ case_when(
    nums > 3 ~ "even and large",
    TRUE ~ "even and small"
  ),
  TRUE ~ "odd"
)

5. Advanced Tips

• Use case_when() in combination with other dplyr functions for cleaner and more efficient code.
• You can use other R functions within case_when() for complex calculations.

6. Common Pitfalls and How to Avoid Them

• Order Matters: Conditions are checked in order. Once a TRUE condition is found, case_when() will stop checking subsequent conditions.
• Missing Default Case: Always include a default case (TRUE ~ value_default) to catch cases that don’t meet any condition.
• Type Consistency: Make sure all the return values are of the same type.

7. Conclusion

The case_when() function in R, a part of the dplyr package, provides a clean and efficient way to perform multiple conditional statements. Its usage can be diverse, ranging from stand-alone cases to being embedded in other dplyr functions like mutate() and summarise(). It is essential to remember the order of conditions, include a default case, and maintain type consistency while using case_when().

With this comprehensive guide, you should now be able to employ case_when() effectively in your R programming.