R tips July 2025

R-tips
A practical, hands-on introduction to debugging in R. We explore core tools (browser(), debugonce(), traceback(), recover()) and demonstrate how to apply them in both scripts and Shiny applications, with live examples and step-by-step guidance to streamline your workflow.
Author

Nikita M.

Published

July 10, 2025

Debugging in R: Practical Tools and Techniques

Introduction

Debugging is essential in programming for identifying and resolving errors effectively. Good debugging skills enhance productivity and ensure code reliability.

Topics Covered:

  • browser()
  • debugonce()
  • traceback()
  • recover()
  • Debugging Shiny applications

The browser() Function

Purpose: Pauses execution, allowing interactive inspection.

Example:

my_function <- function(x) {
    y <- x + 10
    browser()
    z <- y / 2
    return(z)
}

my_function(5)

How to use:

  • Inspect variables directly.
  • Step through code (n for next, c to continue).

The debugonce() Function

Purpose: Debugs a function exactly once.

Example:

add_numbers <- function(a, b) {
    sum <- a + b
    return(sum)
}

debugonce(add_numbers)

add_numbers(3, "two")

How to use:

  • Automatically enters debug mode at next call.
  • Useful for inspecting unexpected errors inside functions.

The traceback() Function

Purpose: Identifies the call sequence leading to an error.

Example:

func_a <- function() { func_b() }
func_b <- function() { stop("An error occurred!") }

func_a()
traceback()

How to use:

  • Quickly identifies problematic calls.
  • Ideal for debugging nested functions.

The recover() Function

Purpose: Interactive debugging at the moment an error occurs.

Set this first:

options(error = recover)

f1 <- function(x) { f2(x) }
f2 <- function(x) { f3(x) }
f3 <- function(x) { stop("Unexpected error!") }

f1(10)

How to use:

  • Provides menu to enter error environments.
  • Inspect variables interactively.

Debugging Shiny Applications

Debugging Shiny can be challenging. Key approaches include:

  • Use browser() in reactive contexts:
library(shiny)

ui <- fluidPage(
    numericInput("num", "Number:", 10),
    textOutput("result")
)

server <- function(input, output) {
    output$result <- renderText({
        browser()
        paste("Double:", input$num * 2)
    })
}

shinyApp(ui, server)
  • Monitor logs and console outputs.

Summary: Key Takeaways

  • browser() - Interactive inspection.
  • debugonce() - Single-use debugging.
  • traceback() - Error call sequence.
  • recover() - Interactive error handling.
  • Shiny apps - Combine browser(), logging, and output checks.

Effective debugging greatly enhances coding efficiency and reliability.

References