Function definition in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function in R?

A function in R is a block of code designed to perform a specific task. It can take inputs, called arguments, and return an output.

beginner

How do you define a simple function in R that adds two numbers?

You use the syntax:

add <- function(x, y) {
  return(x + y)
}

This creates a function named 'add' that returns the sum of x and y.

beginner

What does the 'return()' statement do inside an R function?

The 'return()' statement sends the result back to where the function was called. It ends the function and outputs the value inside return().

beginner

Can an R function have no arguments? Give an example.

Yes, an R function can have no arguments. Example:

hello <- function() {
  print("Hello, world!")
}

This function prints a greeting when called.

beginner

How do you call a function named 'multiply' with arguments 3 and 4 in R?

You call it by writing:

multiply(3, 4)

This runs the function with 3 and 4 as inputs.

What keyword is used to define a function in R?

A function

B def

C func

D fun

In R, the keyword 'function' is used to define a function.

What does the following R code do?

f <- function(x) { x * 2 }

A Prints x times 2

B Calls a function named f

C Creates a variable x with value 2

D Defines a function that doubles x

The code defines a function 'f' that takes input x and returns x multiplied by 2.

How do you return a value from a function in R?

A Using return()

B Using output()

C Using print()

D Using send()

The 'return()' function sends a value back from the function.

Which of these is a valid function call in R?

A myfunc 5, 10

B myfunc(5, 10)

C call myfunc(5, 10)

D myfunc->(5,10)

Functions in R are called by writing the function name followed by parentheses with arguments inside.

Can an R function have no arguments?

A Only if it returns NULL

B No

C Yes

D Only if it is anonymous

Functions in R can be defined without any arguments.

Explain how to define and call a function in R that takes two numbers and returns their sum.

Think about the syntax: name <- function(args) { ... } and how to use return()

Describe what happens when you call a function in R and how the return value is used.

Imagine the function as a machine that takes input and gives output.

Practice

1. What is the correct way to start defining a function named greet in R?

easy

A. greet <- function() { }

B. function greet() { }

C. def greet() { }

D. function <- greet() { }

Solution

1. Step 1: Recognize R function syntax

   In R, functions are defined by assigning a function() to a name using <-.

2. Step 2: Match the correct syntax

   greet <- function() { } uses the correct syntax: name <- function() { }.

3. Final Answer:

   greet <- function() { } -> Option A

4. Quick Check:

   Function definition = name <- function() [OK]

Hint: Use name <- function() to define functions in R [OK]

Common Mistakes:

• Using def instead of function
• Writing function name() without <-
• Placing function keyword after name

2. Which of the following is the correct syntax to define a function add that takes two arguments x and y in R?

easy

A. function add(x, y) { x + y }

B. add(x, y) <- function { return x + y }

C. add <- function(x, y) { return(x + y) }

D. add <- function x, y { return(x + y) }

Solution

1. Step 1: Check function definition format

   In R, arguments go inside parentheses after function keyword, and the body is inside braces.

2. Step 2: Verify return statement and assignment

   add <- function(x, y) { return(x + y) } correctly assigns function with arguments and uses return() properly.

3. Final Answer:

   add <- function(x, y) { return(x + y) } -> Option C

4. Quick Check:

   Arguments inside function() and return() used [OK]

Hint: Put arguments inside function() parentheses [OK]

Common Mistakes:

• Placing arguments outside function()
• Missing braces {} around function body
• Incorrect assignment syntax

3. What will be the output of the following R code?

multiply <- function(a, b) {
  result <- a * b
  return(result)
}
multiply(3, 4)

medium

A. 12

B. 7

C. 34

D. Error

Solution

1. Step 1: Understand function behavior

   The function multiply takes two numbers and returns their product.

2. Step 2: Calculate the product of 3 and 4

   3 * 4 equals 12, so the function returns 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs and return result to get output [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Forgetting to return the result

4. Identify the error in this R function definition:

sum_values <- function(x, y) 
  total <- x + y
  return(total)
}

medium

A. Function name is invalid

B. Missing opening brace { after function()

C. Incorrect use of return()

D. Arguments should be inside square brackets []

Solution

1. Step 1: Check function body braces

   The function body must be enclosed in braces { } immediately after function() declaration.

2. Step 2: Locate missing brace

   The code lacks an opening brace { after function(x, y), causing syntax error.

3. Final Answer:

   Missing opening brace { after function() -> Option B

4. Quick Check:

   Function body needs { } [OK]

Hint: Always put { after function() to start body [OK]

Common Mistakes:

• Omitting braces around function body
• Placing return outside function
• Using wrong brackets for arguments

5. You want to write a function filter_positive that takes a numeric vector and returns only the positive numbers. Which function definition correctly does this in R?

hard

A. filter_positive <- function(vec) { return(vec[vec == 0]) }

B. filter_positive <- function(vec) { return(vec[vec < 0]) }

C. filter_positive <- function(vec) { vec > 0 }

D. filter_positive <- function(vec) { return(vec[vec > 0]) }

Solution

1. Step 1: Understand filtering in R

   To get positive numbers, select elements where vec > 0.

2. Step 2: Check return statement

   filter_positive <- function(vec) { return(vec[vec > 0]) } returns vec elements where condition vec > 0 is TRUE, correctly filtering positives.

3. Final Answer:

   filter_positive <- function(vec) { return(vec[vec > 0]) } -> Option D

4. Quick Check:

   Filter with vec[vec > 0] returns positives [OK]

Hint: Use vec[vec > 0] to filter positive numbers [OK]

Common Mistakes:

• Using wrong comparison operator
• Not returning filtered vector
• Returning logical vector instead of values