Default argument values in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with default argument

What is the output of this R code when calling greet()?

R Programming

greet <- function(name = "friend") {
  paste("Hello", name)
}
greet()

A "Hello friend"

B "Hello name"

C Error: object 'name' not found

D "Hello"

💡 Hint

Check what value the function uses when no argument is given.

The function greet has a default argument 'name' set to "friend". Calling greet() uses this default and returns "Hello friend".

❓ Predict Output

intermediate

Effect of missing argument with default

What will be the output of this R code?

R Programming

multiply <- function(x, y = 2) {
  x * y
}
multiply(5)

A Error: argument 'y' missing

B 10

C 5

D NULL

💡 Hint

Look at the default value of y and how multiply is called.

The function multiply uses y=2 by default. Calling multiply(5) means x=5 and y=2, so 5*2=10.

🔧 Debug

advanced

Identify the error in default argument usage

What error does this R code produce when run?

R Programming

f <- function(x = y, y = 3) {
  x + y
}
f()

A Error: argument 'x' missing

B 3

C 6

D Error: object 'y' not found

💡 Hint

Default arguments are evaluated when the function is called, in order.

The default value for x is y, but y is not defined before x's default is evaluated, causing an error.

❓ Predict Output

advanced

Output with default argument as a function call

What is the output of this R code?

R Programming

get_default <- function() {
  7
}
add <- function(x, y = get_default()) {
  x + y
}
add(3)

A 10

B 3

C Error: object 'get_default' not found

D NULL

💡 Hint

Default arguments can be set by calling other functions.

The default value for y is the result of get_default(), which returns 7. So add(3) returns 3+7=10.

🧠 Conceptual

expert

Understanding evaluation timing of default arguments

Consider this R code. What is the value of result after running it?

R Programming

x <- 5
f <- function(y = x) {
  x <- 10
  y
}
result <- f()

A Error: object 'x' not found

B 10

C 5

D NULL

💡 Hint

Default arguments are evaluated when the function is called, before the function body runs.

The default argument y = x uses the global x = 5 at call time. The local x = 10 inside the function does not affect y's default.

Practice

1. What happens if you call an R function with a default argument but do not provide that argument?

easy

A. The function asks the user to input the missing argument.

B. The function returns an error because the argument is missing.

C. The function uses the default value specified in the function definition.

D. The function ignores the argument and runs without it.

Solution

1. Step 1: Understand default arguments in R

   Default arguments provide a preset value if no value is given when calling the function.

2. Step 2: Behavior when argument is missing

   If the argument is not provided, R uses the default value automatically without error.

3. Final Answer:

   The function uses the default value specified in the function definition. -> Option C

4. Quick Check:

   Missing argument uses default value [OK]

Hint: If argument missing, default value is used automatically [OK]

Common Mistakes:

• Thinking missing argument causes error
• Assuming function asks for input
• Believing function ignores missing argument silently

2. Which of the following is the correct way to define a function in R with a default argument x set to 10?

easy

A. myfunc <- function(x <- 10) { return(x) }

B. myfunc <- function(x = 10) { return(x) }

C. myfunc <- function(x: 10) { return(x) }

D. myfunc <- function(x == 10) { return(x) }

Solution

1. Step 1: Recall R function default argument syntax

   In R, default values are assigned with = inside the function definition.

2. Step 2: Check each option

   Only myfunc <- function(x = 10) { return(x) } uses x = 10 correctly. Others use invalid syntax like :, <-, or ==.

3. Final Answer:

   myfunc <- function(x = 10) { return(x) } -> Option B

4. Quick Check:

   Default argument uses = in function definition [OK]

Hint: Use = to assign default values in function arguments [OK]

Common Mistakes:

• Using <- or : instead of = for default values
• Using == which is a comparison, not assignment
• Placing default outside function parentheses

3. What is the output of this R code?

add <- function(a, b = 5) {
  return(a + b)
}
add(3)

medium

A. 8

B. 3

C. Error: argument 'b' missing

D. 5

Solution

1. Step 1: Identify default argument usage

   The function add has b defaulting to 5 if not provided.

2. Step 2: Call function with only a=3

   Since b is missing, it uses default 5, so result is 3 + 5 = 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   3 + default 5 = 8 [OK]

Hint: If argument missing, default value is used [OK]

Common Mistakes:

• Expecting error for missing argument
• Ignoring default and using only provided argument
• Confusing output with just the first argument

4. Consider this R function:

multiply <- function(x, y = 2) {
  x * y
}
multiply(4, )

What is the problem with this code?

medium

A. Function returns 8 because y defaults to 2.

B. Function throws error because x is missing.

C. Function ignores y and multiplies by 1.

D. Syntax error due to trailing comma without value.

Solution

1. Step 1: Analyze function call syntax

   The call multiply(4, ) has a comma but no second argument value, which is invalid syntax in R.

2. Step 2: Understand R argument rules

   Trailing commas without values cause syntax errors; you must omit the comma if skipping optional arguments.

3. Final Answer:

   Syntax error due to trailing comma without value. -> Option D

4. Quick Check:

   Trailing comma without value causes syntax error [OK]

Hint: No trailing commas without values in function calls [OK]

Common Mistakes:

• Assuming default used despite syntax error
• Thinking function returns result ignoring syntax
• Confusing missing argument with missing value

5. You want to write an R function greet that prints a greeting message. It should take a name argument with default "Guest" and a punctuation argument with default "!". Which function definition correctly implements this?

hard

A. greet <- function(name = "Guest", punctuation = "!") { paste0("Hello, ", name, punctuation) }

B. greet <- function(name, punctuation) { paste0("Hello, ", name = "Guest", punctuation = "!") }

C. greet <- function(name = Guest, punctuation = !) { paste0("Hello, ", name, punctuation) }

D. greet <- function(name = "Guest", punctuation) { paste0("Hello, ", name, punctuation = "!") }

Solution

1. Step 1: Check default argument syntax

   Defaults must be assigned inside the function definition using = and quoted strings for text.

2. Step 2: Validate each option

   greet <- function(name = "Guest", punctuation = "!") { paste0("Hello, ", name, punctuation) } correctly sets defaults with quotes and uses them in paste0. Other options misuse default assignment by placing it inside the function body, omitting defaults for optional arguments, or lacking quotes around strings (e.g., name = Guest, punctuation = !).

3. Final Answer:

   greet <- function(name = "Guest", punctuation = "!") { paste0("Hello, ", name, punctuation) } -> Option A

4. Quick Check:

   Default arguments set with = and quoted strings [OK]

Hint: Set defaults inside function parentheses with = and quotes for text [OK]

Common Mistakes:

• Assigning defaults inside function body
• Forgetting quotes around string defaults
• Omitting default for all optional arguments