Debugging tools in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the browser() function in R?

The browser() function pauses the execution of R code and allows you to inspect variables and step through the code interactively to find bugs.

beginner

How does traceback() help in debugging R code?

traceback() shows the sequence of function calls that led to an error, helping you understand where the problem happened.

intermediate

What is the use of debug() in R?

debug() marks a function so that when it is called, R enters the browser mode inside that function, allowing step-by-step execution.

intermediate

Explain the difference between browser() and debug().

browser() is placed manually inside code to pause execution at that point, while debug() is used to mark a function so that every time it runs, it pauses at the start.

advanced

What does the recover() function do in R debugging?

recover() allows you to choose which environment to enter after an error occurs, letting you inspect the state of the program at different call levels.

Which R function shows the call stack after an error?

A traceback()

B browser()

C debug()

D recover()

traceback() prints the sequence of calls that led to an error, helping locate the problem.

What happens when you use debug() on a function?

A The function runs normally without interruption

B Execution pauses at the start of the function for step-by-step debugging

C The function is skipped

D An error is thrown immediately

debug() causes R to pause inside the function so you can step through the code.

Which debugging tool lets you interactively inspect variables at any point in your code?

A traceback()

B debug()

C recover()

D browser()

browser() pauses execution and lets you check variables and run commands interactively.

How does recover() improve error handling?

A It automatically fixes errors

B It prevents errors from occurring

C It lets you choose which environment to inspect after an error

D It logs errors to a file

recover() allows you to select the call frame to debug after an error happens.

When would you use browser() instead of debug()?

A To pause execution at a specific line inside a function

B To debug all calls to a function

C To print the call stack

D To recover from an error

browser() is placed at a specific spot in code to pause there, unlike debug() which pauses at function start.

Describe how you would use debug() and browser() to find a bug in an R function.

Think about when you want to pause the whole function vs a specific spot.

Explain the role of traceback() and recover() in understanding errors in R.

One shows the path, the other lets you explore the problem.

Practice

1. Which R function allows you to step through a function line-by-line to find errors?

easy

A. traceback()

B. debug()

C. browser()

D. stop()

Solution

1. Step 1: Understand the purpose of debug()

   The debug() function lets you step through a function line-by-line to see what happens inside.

2. Step 2: Compare with other functions

   traceback() shows the call stack after an error, browser() pauses execution but debug() is designed for stepping through.

3. Final Answer:

   debug() -> Option B

4. Quick Check:

   Step through function = debug() [OK]

Hint: Use debug() to step through function code line-by-line [OK]

Common Mistakes:

• Confusing traceback() with stepping through code
• Thinking browser() automatically steps through all lines
• Using stop() which just halts execution

2. Which of the following is the correct way to start debugging a function named myFunc in R?

easy

A. debug myFunc()

B. debug:myFunc()

C. debug->myFunc()

D. debug(myFunc)

Solution

1. Step 1: Recall correct debug() syntax

   The debug() function is called with the function name as an argument: debug(myFunc).

2. Step 2: Identify invalid syntax

   Options A, B, and D use invalid R syntax for function calls or operators.

3. Final Answer:

   debug(myFunc) -> Option D

4. Quick Check:

   Correct debug syntax = debug(myFunc) [OK]

Hint: Use parentheses with debug() and function name [OK]

Common Mistakes:

• Omitting parentheses after debug
• Using colon or arrow instead of parentheses
• Adding spaces incorrectly in function call

3. What will be the output after running this R code?

f <- function(x) {
  debug()
  y <- x + 1
  return(y)
}
f(3)

medium

A. Starts debugging mode and waits for user input

B. Returns 4 immediately without debugging

C. Throws an error because debug() needs a function name

D. Prints 3 and exits

Solution

1. Step 1: Understand debug() usage

   debug() is used with a function name outside the function, not inside it.

2. Step 2: Analyze code behavior

   Calling debug() inside a function without arguments causes an error because debug expects a function name.

3. Final Answer:

   Throws an error because debug() needs a function name -> Option C

4. Quick Check:

   debug() inside function without name = error [OK]

Hint: debug() needs a function name, not called inside function body [OK]

Common Mistakes:

• Thinking debug() works inside function without arguments
• Expecting immediate return without debugging
• Confusing debug() with browser()

4. You run a function and get an error. Which R command helps you see the sequence of function calls that led to the error?

medium

A. traceback()

B. debug()

C. browser()

D. stop()

Solution

1. Step 1: Identify purpose of traceback()

   traceback() shows the call stack, listing functions called before the error.

2. Step 2: Differentiate from other tools

   debug() and browser() help step through code, stop() halts execution but does not show call history.

3. Final Answer:

   traceback() -> Option A

4. Quick Check:

   Call stack after error = traceback() [OK]

Hint: Use traceback() to see error call sequence [OK]

Common Mistakes:

• Using debug() to see call stack after error
• Confusing browser() with traceback()
• Expecting stop() to show call history

5. You want to debug a complex function calcStats that calls other functions internally. Which approach helps you pause execution at any line inside calcStats and inspect variables interactively?

hard

A. Use browser() inside calcStats at desired lines

B. Use debug(calcStats) only once before calling

C. Use traceback() after running calcStats

D. Use stop() to halt execution at errors

Solution

1. Step 1: Understand browser() usage

   Inserting browser() pauses execution at that line and lets you inspect variables interactively.

2. Step 2: Compare with debug() and traceback()

   debug() steps through entire function but cannot pause at arbitrary lines; traceback() only shows call stack after error.

3. Step 3: Recognize stop() role

   stop() halts execution with an error but does not allow interactive inspection.

4. Final Answer:

   Use browser() inside calcStats at desired lines -> Option A

5. Quick Check:

   Pause anywhere interactively = browser() [OK]

Hint: Insert browser() where you want to pause and inspect [OK]

Common Mistakes:

• Relying only on debug() to pause anywhere
• Using traceback() before error occurs
• Expecting stop() to allow variable inspection