Catch Me If You Can: Exception Handling in R

The Importance of Error Handling

In the multifaceted world of R programming, particularly when navigating the intricate paths of data analysis, robust error handling is not merely a precaution; it’s an essential pillar of reliable and trustworthy code development. Picture this: you’re deep into a complex data analysis task, your script meticulously weaving through rows and columns of data, and suddenly, it grinds to a halt — an unhandled error has thrown a wrench into the gears. Such abrupt interruptions not only break the flow of your work but can also lead to misleading results if errors go unnoticed or are mismanaged. This scenario highlights the critical nature of error handling in programming — a skill paramount to ensuring the smooth execution and integrity of your code.

In this fourth episode of our series on enhancing R functions, titled “Catch Me If You Can,” we embark on a journey through the nuances of error and exception management in R. Our quest is to fortify our data_quality_report() function against the unexpected. We will explore R's built-in mechanisms for error handling, learn to predict and manage potential disruptions, and, most importantly, understand how to maintain the continuity and accuracy of our analyses in the face of errors. Mastering these techniques will empower you to handle unexpected situations gracefully, transforming potential obstacles into controlled, manageable events, thus elevating the robustness of your R functions.

Basics of Error Handling in R

Error handling in R is a multifaceted tool, essential for signaling and managing issues within your code. The functions stop(), warning(), and message() form the foundation of this system. stop() is used to throw an error and halt execution, signaling that something has gone fundamentally wrong. warning(), in contrast, indicates a potential issue or anomaly in the code but doesn't stop the execution; it serves as a caution sign, allowing the script to proceed but alerting the user to potential irregularities. message() is less severe; it's used for conveying information, such as status updates or confirmations, without implying any error or warning.

But the true art of error handling in R extends beyond just signaling a problem. It’s about how your program responds to these issues — whether it’s a full stop, a cautious continuation, or a simple notification. This is where R’s try() and tryCatch() functions become pivotal. try() allows you to attempt an operation that might generate an error, with the assurance that even if it fails, your entire script won't come to a standstill. tryCatch(), on the other hand, offers a more nuanced approach. It allows you to define specific actions based on different types of outcomes — whether it's an error, a warning, or a normal completion. This approach not only enhances the robustness of your code but also provides a safety net, ensuring that your script can gracefully handle and respond to various situations.

To illustrate these concepts in action, let’s consider an example that employs tryCatch():

example_function <- function(data) {
  result <- tryCatch({
    if (!is.numeric(data)) {
      stop("Data must be numeric")
    }
    sqrt(data)
  }, error = function(e) {
    message("Error: ", e$message)
    NA  # Returning NA in case of an error
  })
  return(result)
}

example_function(1)
#> [1] 1

example_function(64)
#> [1] 8

example_function("a")
#> Error: Data must be numeric
#> [1] NA

In this example, example_function is designed to compute the square root of a numeric input. However, if the input is non-numeric, stop() triggers an error, which is then elegantly handled by tryCatch. The function, instead of crashing, displays an error message and returns NA. This is a simple demonstration of how tryCatch can make your functions more resilient and user-friendly.

Implementing tryCatch in data_quality_report()

To enhance the robustness of the data_quality_report() function, incorporating tryCatch is crucial. It ensures that the function can handle errors gracefully, without disrupting the entire execution. Let's focus on integrating tryCatch into the outlier detection component of the function. Outlier detection involves numerical operations that might lead to errors, particularly when dealing with data of unexpected formats or types.

Here’s how to robustly implement tryCatch in the outlier detection part:

data_quality_report <- function(data) {
  missing_values <- data %>%
    summarize(across(everything(), ~sum(is.na(.)))) %>%
    pivot_longer(cols = everything(), names_to = "column", values_to = "missing_values")

  outliers <- tryCatch({
    data %>%
      select(where(is.numeric)) %>%
      imap(~{
        # only for show we are going to change na.rm values to FALSE
        qnt <- quantile(.x, probs = c(0.25, 0.75), na.rm = FALSE
        iqr <- IQR(.x, na.rm = FALSE)
        lower_bound <- qnt[1] - 1.5 * iqr
        upper_bound <- qnt[2] + 1.5 * iqr
        outlier_count <- sum(.x < lower_bound | .x > upper_bound, na.rm = FALSE)
        
        tibble(column = .y, lower_bound, upper_bound, outlier_count)
      }) %>%
      bind_rows()
  }, error = function(e) {
    message("Error in outlier detection: ", e$message)
    NULL  # Returning NULL in case of an error in outlier detection
  })

  data_types <- data %>%
    summarize(across(everything(), ~paste(class(.), collapse = ", "))) %>%
    pivot_longer(cols = everything(), names_to = "column", values_to = "data_type")
  
  list(
    MissingValues = missing_values,
    Outliers = outliers,
    DataTypes = data_types
  )
}

dummy_data <- tibble(
  normal_numeric_column = c(1, 2, 3, 4, 5),  # A normal numeric column
  problematic_column = c(1, 2, 3, 4, NA)    # A numeric column with an Inf value
)

data_quality_report(dummy_data)
#> Error in outlier detection: Error in outlier detection: In index: 2.

$Outliers
NULL

In this enhanced data_quality_report() function, the tryCatch block ensures that if an error occurs during the outlier detection process , it doesn't cause the entire function to fail. Instead, it gracefully handles the error, outputs an informative message, and continues execution. This addition significantly improves the function's resilience and user-friendliness.

Utilizing safely() from purrr

Another elegant approach to managing potential errors in R is using the safely() function from the purrr package. safely() wraps any function and returns a new version of that function that never throws an error. Instead, it returns a list containing two elements: result (the original function's output) and error (an error object if an error occurred, otherwise NULL).

Let’s apply safely() to a hypothetical example within our data_quality_report() function. Imagine we have a custom calculation that could fail under certain conditions, such as when dealing with extreme values:

custom_division <- function(x, y) {
  if (y == 0) {
    stop("Division by zero error")
  }
  x / y
}

# Wrap the custom function with safely
safe_division <- safely(custom_division)


data_quality_report <- function(data) {
  missing_values <- data %>%
    summarize(across(everything(), ~sum(is.na(.)))) %>%
    pivot_longer(cols = everything(), names_to = "column", values_to = "missing_values")
  
  outliers <- tryCatch({
    data %>%
      select(where(is.numeric)) %>%
      imap(~{
        qnt <- quantile(.x, probs = c(0.25, 0.75), na.rm = TRUE)
        iqr <- IQR(.x, na.rm = TRUE)
        lower_bound <- qnt[1] - 1.5 * iqr
        upper_bound <- qnt[2] + 1.5 * iqr
        outlier_count <- sum(.x < lower_bound | .x > upper_bound, na.rm = TRUE)
        
        tibble(column = .y, lower_bound, upper_bound, outlier_count)
      }) %>%
      bind_rows()
  }, error = function(e) {
    message("Error in outlier detection: ", e$message)
    NULL  # Returning NULL in case of an error in outlier detection
  })
  
  data_types <- data %>%
    summarize(across(everything(), ~paste(class(.), collapse = ", "))) %>%
    pivot_longer(cols = everything(), names_to = "column", values_to = "data_type")
  
  # Applying the safe_division to a column
  # Assuming 'data' has columns 'numerator' and 'denominator'
  division_results <- map2(data$numerator, data$denominator, ~ safe_division(.x, .y))

  # Extract results and handle errors
  division_values <- map(division_results, "result")
  division_errors <- map(division_results, "error")

  # Check and handle if any errors occurred
  if (any(!map_lgl(division_errors, is.null))) {
    message("Errors occurred in division calculations.")
    # Additional error handling logic
  }
  
  list(
    MissingValues = missing_values,
    Outliers = outliers,
    DataTypes = data_types,
    # We ar adding those two to check what was wrong
    Division_values = division_values,
    Division_errors = division_errors
  )
}

dummy_data <- tibble(
  numerator = c(10, 20, 30, 40),
  denominator = c(2, 4, 0, 5)  # The third element will cause division by zero
)


result <- data_quality_report(dummy_data)
#> Errors occurred in division calculations.

result[["Division errors"]]
[[1]]
NULL

[[2]]
NULL

[[3]]
<simpleError in .f(...): Division by zero error>
  
[[4]]
NULL

In this implementation, safe_division ensures that even if custom_division fails , the data_quality_report() function doesn't halt execution. Instead, it captures the error, allowing for a more controlled and informative response.

Best Practices for Error Handling

Effective error handling in R encompasses a set of best practices that collectively enhance the resilience and user-friendliness of your code:

• Use Meaningful Error Messages: Error messages should be clear and informative, helping users understand what went wrong and how to potentially fix it. Avoid vague or overly technical jargon.
• Fail Early and Clearly: If a function encounters a situation where it cannot proceed correctly, it’s often better to halt its execution early with a clear and informative message. This prevents the propagation of errors and ambiguities in the later stages of the script.
• Consider the User’s Perspective: Design your error handling with the end-user in mind. Provide clear instructions or alternatives when an error occurs, enabling users to understand and address the issue.
• Log Errors for Future Reference: In more complex applications, consider logging errors to a file or a logging service. This can be invaluable for debugging and improving your application over time.
• Test Your Error Handling: Just as you test your functions for correct results, also test them for correct error handling. Ensure that your function responds as expected in various error scenarios.

Through robust error handling, our R functions become not only more reliable but also more user-friendly. Anticipating and managing potential errors ensure that our scripts are resilient and dependable. As we continue our series on enhancing R functions, remember that error handling is an integral part of writing excellent code. Embrace these techniques to make your R code robust and professional, capable of gracefully handling whatever challenges it encounters.

Catch Me If You Can: Exception Handling in R was originally published in Numbers around us on Medium, where people are continuing the conversation by highlighting and responding to this story.