Debugging tools in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of debugged function with browser()

Consider the following R function that uses browser() for debugging. What will be the output when calling sum_squares(3)?

R Programming

sum_squares <- function(n) {
  browser()
  total <- 0
  for (i in 1:n) {
    total <- total + i^2
  }
  return(total)
}
sum_squares(3)

A [1] 14

B [1] 6

C Error in sum_squares(3) : object 'total' not found

D [1] 9

💡 Hint

Remember that the function sums squares of numbers from 1 to n.

The function sums 1^2 + 2^2 + 3^2 = 1 + 4 + 9 = 14. The browser() pauses execution but does not change the output.

❓ Predict Output

intermediate

Effect of traceback() after error

What will be the output of the following R code snippet?

R Programming

f <- function(x) {
  g(x)
}
g <- function(y) {
  stop("Error occurred")
}
tryCatch(f(5), error = function(e) {traceback(); print("Handled error")})

A No output, program stops

B Error in g(x) : Error occurred

C

1: stop("Error occurred")
[1] "Handled error"

D

3: g(x)
2: f(5)
[1] "Handled error"

💡 Hint

traceback() shows the call stack at the error point.

traceback() prints the call stack showing g called by f, then the error handler prints "Handled error".

❓ Predict Output

advanced

Output of debug() with step-through

Given the code below, what will be printed when debugged_sum(2) is called and the user steps through the function until it returns?

R Programming

debugged_sum <- function(n) {
  debug()
  s <- 0
  for (i in 1:n) {
    s <- s + i
  }
  return(s)
}
debugged_sum(2)

A [1] 3

B [1] 2

C Error: debug() requires a function argument

D [1] 0

💡 Hint

Check how debug() is used in R.

debug() must be called with a function name as argument, calling debug() inside a function without argument causes an error.

❓ Predict Output

advanced

Behavior of options(error = recover)

What happens when you run the following code and an error occurs?

R Programming

options(error = recover)
f <- function(x) {
  g(x)
}
g <- function(y) {
  stop("Oops")
}
f(1)

A No output, silently fails

B Interactive menu to choose a frame to debug

C Program stops with error message 'Oops'

D Prints traceback and continues execution

💡 Hint

options(error = recover) triggers a debugging menu on error.

When an error occurs, recover() lets you select which function frame to inspect interactively.

🧠 Conceptual

expert

Identifying the cause of silent failure in R script

You run an R script that uses try() to catch errors but notice that some errors do not appear and the script continues silently. Which of the following best explains this behavior?

A <code>try()</code> suppresses errors and returns an object of class 'try-error' without stopping execution

B <code>try()</code> always stops execution on error and prints the error message

C <code>try()</code> converts errors into warnings that are printed immediately

D <code>try()</code> logs errors to a file but does not affect console output

💡 Hint

Think about how try() handles errors differently than stop().

try() catches errors and returns an object indicating failure but does not stop the script, so errors may be silent unless checked.

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