Default argument values in R Programming - Mini Project: Build & Apply

Default argument values

📖 Scenario: Imagine you are creating a simple calculator function in R that adds two numbers. Sometimes, you want to add a number to zero by default if the second number is not given.

🎯 Goal: Build a function called add_numbers that adds two numbers, where the second number has a default value of 0.

📋 What You'll Learn

Create a function named add_numbers with two parameters: a and b

Set the default value of parameter b to 0

Return the sum of a and b

Call the function with one argument and print the result

Call the function with two arguments and print the result

💡 Why This Matters

🌍 Real World

Default argument values are useful in many programs where some inputs are optional or have common defaults, like setting a default tax rate or default color.

💼 Career

Understanding default arguments helps you write flexible and user-friendly functions, a key skill for software development and data analysis.

1

Create the function skeleton

Write a function named add_numbers with two parameters: a and b. Do not set any default values yet.

R Programming

add_numbers <- function(a, b) {
  # Your code here
}

Hint

Use the function keyword to create a function with parameters a and b.

2

Set default value for second parameter

Modify the function add_numbers so that the parameter b has a default value of 0.

R Programming

add_numbers <- function(a, b = 0) {
  # Your code here
}

Hint

Set the default value by using b = 0 inside the function parameters.

3

Add the core logic to sum the numbers

Inside the function add_numbers, return the sum of a and b.

R Programming

add_numbers <- function(a, b = 0) {
  return(a + b)  # Your code here
}

Hint

Use return(a + b) to send back the sum.

4

Test the function with one and two arguments

Call the function add_numbers with one argument 5 and print the result. Then call it with two arguments 5 and 3 and print the result.

R Programming

print(add_numbers(5))  # Your code here
print(add_numbers(5, 3))  # Your code here

Hint

Use print(add_numbers(5)) and print(add_numbers(5, 3)) to show the results.

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