Default argument values in R Programming - Step-by-Step Execution

Concept Flow - Default argument values

Function call

↓

Check arguments

↓

Argument provided?

Yes→Use provided value

↓

No

→Use default value

↓

Run function body with arguments

↓

Return result

When a function is called, R checks if each argument is given. If not, it uses the default value set in the function definition.

Execution Sample

R Programming

add <- function(x, y = 5) {
  return(x + y)
}
add(3)
add(3, 2)

Defines a function with a default value for y, then calls it with and without y.

Execution Table

Step	Function Call	Argument x	Argument y	Used y Value	Calculation	Output
1	add(3)	3	missing	5 (default)	3 + 5	8
2	add(3, 2)	3	2	2 (provided)	3 + 2	5
3	End	-	-	-	-	Function calls complete

💡 No more function calls to execute

Variable Tracker

Variable	Start	Call 1	Call 2	Final
x	undefined	3	3	3
y	undefined	5 (default)	2 (provided)	2 (last call)
result	undefined	8	5	5

Key Moments - 2 Insights

Why does y use 5 in the first call but 2 in the second?

Because in the first call (row 1), y is not provided, so the default 5 is used. In the second call (row 2), y is given as 2, so that value is used instead.

What happens if no default is set and argument is missing?

R will give an error because it expects that argument to be provided. Here, y has a default, so no error occurs.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of add(3)?

A 5

B 8

C 3

D Error

💡 Hint

Check row 1 in the execution_table where add(3) is called and output is shown.

At which step does the function use the default value for y?

A Step 3

B Step 2

C Step 1

D Never

💡 Hint

Look at the 'Used y Value' column in the execution_table for each step.

If we call add(4, 10), what would be the output?

A 14

B 9

C 5

D Error

💡 Hint

Add x and y values; y is provided, so default is ignored.

Concept Snapshot

Function arguments can have default values.
If argument is missing in call, default is used.
If argument is provided, that value is used.
Helps avoid errors and simplify calls.
Syntax: function(arg = default) { ... }

Full Transcript

This example shows how R functions use default argument values. When calling add(3), y is missing, so default 5 is used, resulting in 8. When calling add(3, 2), y is provided as 2, so output is 5. The flow checks if arguments are given; if not, defaults apply. This prevents errors and makes functions flexible.

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