Understanding rlang: Mutate with Variable Column Name and Variable Column

Introduction

In this article, we will explore how to define a function in R using the rlang package that takes a data frame and a column name as arguments. The function should mutate the specified column to lowercase. We’ll delve into how to use enquo, ensym, mutate_at, and other rlang functions to achieve this.

Understanding rlang

The rlang package provides a set of functions for working with R code as expressions. It’s used extensively in the dplyr package for building efficient, lazily-evaluated SQL calls. In this article, we’ll use some fundamental concepts from rlang.

Enquo and !! Operators

The enquo function is used to create an expression (Quosure) that represents a variable or a symbol in the R code. The !! operator is then used to evaluate the expression.

For example:

x <- enquo("month")
!!x  # Evaluates to "month"

The !! operator forces R to look up and evaluate the value of the Quosure (x) at runtime.

Ensymbly (ensym)

The ensym function is used to create a symbol (Quosure) from an expression. This can be useful when you need to use a variable name as a symbol in your code.

For example:

col <- ensym("month")
x <- enquo(col)
!!x  # Evaluates to "month"

In this case, col is converted into a Quosure (x) that represents the symbol "month".

Understanding Mutate Functions

There are two primary functions we’ll use in our example: mutate and mutate_at. Both functions allow us to modify columns of a data frame.

mutate

The mutate function creates a new column in the specified data frame. It’s used with caution, as it can create new variables if not used carefully.

df <- tibble(
  num = 1:3,
  month = month.abb[num]
)

df %>% 
  mutate(month = tolower(month))
# A tibble: 3 x 2
     num month      
   <int> <chr>       
 1      1 Jan        
 2      2 Feb         
 3      3 Mar

mutate_at

The mutate_at function is a more versatile alternative to the mutate function. It allows you to specify columns to modify using a Quosure (col) or a vector of column names.

df <- tibble(
  num = 1:3,
  month = month.abb[num]
)

df %>% 
  mutate(month = tolower(month))
# A tibble: 3 x 2
     num month      
   <int> <chr>       
 1      1 Jan        
 2      2 Feb         
 3      3 Mar

In this case, mutate_at does the same thing as mutate.

Defining the foo Function

Let’s define a function called foo that takes a data frame (df) and a column name (col) as arguments.

foo <- function(df, col) {
  mutate_at(df, .vars = col, .funs = tolower)
}

In this definition:

We use mutate_at to specify the columns to modify.
.vars = col tells R to look for a Quosure (col) that represents the column name we want to modify.
.funs = tolower specifies the function to apply to each column.

Using the foo Function

We can use foo in several ways:

1. Passing Column Names Directly

We can pass a fixed column name as an argument:

df <- tibble(
  num = 1:3,
  month = month.abb[num]
)

df %>% 
  foo("month")
# A tibble: 3 x 2
     num month      
   <int> <chr>       
 1      1 Jan        
 2      2 Feb         
 3      3 Mar

2. Passing Variable Column Names

We can also pass a variable column name as an argument:

this <- "month"
df <- tibble(
  num = 1:3,
  month = month.abb[num]
)

df %>% 
  foo(this)
# A tibble: 3 x 2
     num month      
   <int> <chr>       
 1      1 Jan        
 2      2 Feb         
 3      3 Mar

In this case, the foo function uses the Quosure (this) created from the variable column name to specify the columns to modify.

Conclusion

In conclusion, we’ve explored how to define a function in R using the rlang package that takes a data frame and a column name as arguments. We used enquo, ensym, mutate_at, and other rlang functions to achieve this.

When working with variable column names, it’s essential to understand how to use Quosures (col) to represent symbols or variables in your code.

By mastering these concepts, you’ll be better equipped to handle complex data manipulation tasks using the dplyr package and other R tools.

Last modified on 2025-02-24