Default argument values in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a default argument value in R functions?

A default argument value is a value assigned to a function parameter that is used if no value is provided when the function is called.

beginner

How do you define a default argument value in an R function?

You assign a value to the parameter in the function definition, like function(x = 5) where 5 is the default value.

beginner

What happens if you call a function without providing a value for a parameter that has a default value?

The function uses the default value assigned to that parameter.

intermediate

Can default argument values depend on other arguments in the same function in R?

Yes, but only if the other arguments appear before the one with the default value, because R evaluates defaults in order.

beginner

Why are default argument values useful in R programming?

They make functions easier to use by providing common values automatically, so you don't have to specify every argument every time.

How do you specify a default value for an argument in an R function?

A By calling the function with a value

B By assigning a value inside the function body

C By assigning a value in the function definition, like <code>function(x = 10)</code>

D By using the <code>default()</code> function

Default values are set in the function definition by assigning a value to the parameter.

What value does a function use if you do not provide an argument that has a default value?

A Zero

B The default value specified in the function definition

C An error is thrown

D NULL

If no argument is provided, the function uses the default value defined for that parameter.

Can default argument values in R depend on other arguments in the same function?

A Yes, if the other arguments come before in the parameter list

B No, default values must be constants

C Yes, but only if they are global variables

D No, default values cannot depend on anything

Default values can depend on earlier arguments because R evaluates defaults in order.

What happens if you provide a value for an argument that has a default value?

A The function ignores the argument

B The default value is always used

C An error occurs

D The provided value overrides the default

When you provide a value, it replaces the default value for that call.

Why might you want to use default argument values in your functions?

A To make functions easier to call with common values

B To make functions run faster

C To prevent users from passing arguments

D To make functions only work with one value

Default values simplify function calls by providing common values automatically.

Explain how default argument values work in R functions and why they are useful.

Think about what happens when you call a function without some arguments.

Describe how default argument values can depend on other arguments in R and what rules apply.

Consider the order of parameters in the function definition.

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