Understanding Quoted Variables in Dplyr’s Group_by() %>% mutate() Function Call

In the world of data manipulation and analysis, functions like dplyr’s group_by() and mutate() are incredibly powerful tools. However, they can also be a bit finicky when it comes to quoting variables. In this post, we’ll delve into the intricacies of quoted variables in these function calls and explore how to use them effectively.

Reproducible Example

Let’s start with a simple example using dplyr and RStudio’s enquo() function. We’re going to create a dataset called “cats” that includes information about feline friends.

# Load necessary libraries
library(dplyr)
library(rlang)

# Create the cats dataset
cats <- data.frame(
  name = c(letters[1:10]),
  weight = c(rnorm(5, 10, 1), rnorm(5, 20, 3)),
  type = c(rep("not_fat", 5), rep("fat", 5))
)

get_means <- function(df, metric, group) {
  
  # Use enquo() to quote the variable names
  metric <- enquo(metric)
  group <- enquo(group)
  
  # Group by the specified column and calculate the mean
  df %>%
    group_by(!!group) %>% 
    summarise(mean_stat = mean(!!metric)) %>% 
    pull(mean_stat)
}

# Test the function
get_means(cats, weight, type)

In this example, we’re using enquo() to quote the variable names. This allows us to use tidy evaluation operators like !! in our pipeline.

What Went Wrong?

The code didn’t work as expected because of how R handles quotes and variable names. When you pass a string to a function in R, it gets interpreted as that string. So when we used metric = c(letters[1:10]), R treated c(letters[1:10]) as the value for metric, not as the name of a column.

The Solution

To fix this issue, we need to use enquo() to quote the variable names. This allows us to treat the variable name like a symbol in R. We can then use tidy evaluation operators like !! to access that variable within our pipeline.

Here’s how you can do it:

get_means <- function(df, metric, group) {
  
  # Use enquo() to quote the variable names
  metric <- enquo(metric)
  group <- enquo(group)
  
  # Group by the specified column and calculate the mean
  df %>%
    group_by(!!group) %>% 
    summarise(mean_stat = mean(!!metric)) %>% 
    pull(mean_stat)
}

This way, when we use !! in our pipeline, it knows that metric and group are variables, not strings.

Using Quoted Variables with ggplot()

Quoted variables can also be used with ggplot(). Let’s see how:

get_means <- function(df, metric, group) {
  
  # Use enquo() to quote the variable names
  metric <- enquo(metric)
  group <- enquo(group)
  
  # Group by the specified column and calculate the mean
  df %>%
    group_by(!!group) %>% 
    summarise(mean_stat = mean(!!metric)) %>% 
    ggplot(aes(!!group, !!metric)) + 
    geom_point()
}

In this example, we’re using enquo() to quote the variable names in our aes function. This allows us to use tidy evaluation operators like !! to access that variable within our plot.

Allowing for Any Number of Grouping Variables

If you want to allow your function to take any number of grouping variables, including none, you can use the ... argument and enquos() instead of enquo(). Here’s how:

get_means <- function(df, ...) {
  
  # Use enquos() to quote the variable names
  groups <- enquos(...)

  # Group by the specified columns and calculate the mean
  df %>%
    group_by(!!!groups) %>% 
    summarise(mean_stat = mean(value)) %>% 
    pull(mean_stat)
}

get_means(mtcars, mpg, cyl, vs)

In this example, we’re using enquos() to quote the variable names. This allows us to use tidy evaluation operators like !!! to access that variable within our pipeline.

Conclusion

Quoted variables can be a bit tricky when working with dplyr’s group_by() and mutate() functions, but they can also be incredibly powerful tools. By using enquo() or enquos(), you can use tidy evaluation operators like !! or !!! to access your variable names within your pipeline.

With these techniques under your belt, you’ll be able to create more complex and efficient data manipulation pipelines that get the job done. Happy coding!

Last modified on 2024-02-26