Anonymous functions in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an anonymous function in R?

An anonymous function in R is a function without a name. It is often used for short tasks and passed directly as an argument to other functions.

beginner

How do you define an anonymous function in R?

You define it using the syntax: function(args) expression. For example, function(x) x + 1 is an anonymous function that adds 1 to its input.

intermediate

Why use anonymous functions instead of named functions?

Anonymous functions are useful for quick, simple tasks where defining a full named function is unnecessary. They keep code concise and are often used inside functions like lapply() or sapply().

beginner

Example: What does this code do?
lapply(1:3, function(x) x * 2)

It applies the anonymous function function(x) x * 2 to each element of the list 1, 2, 3. The result is a list with elements 2, 4, and 6.

intermediate

Can anonymous functions have multiple arguments in R?

Yes, anonymous functions can have multiple arguments. For example, function(x, y) x + y adds two inputs without needing a name.

What is the correct way to write an anonymous function that squares a number in R?

A function(x) x^2

B square <- function(x) x^2

C def square(x): x^2

D func(x) {x * x}

Anonymous functions in R use the syntax function(args) expression. Option A correctly defines an anonymous function that squares x.

Which function commonly uses anonymous functions as arguments in R?

A read.csv()

B print()

C lapply()

D setwd()

lapply() applies a function to each element of a list and often uses anonymous functions for quick operations.

What is the main benefit of using anonymous functions?

A They allow quick, inline function definitions without naming.

B They run faster than named functions.

C They can only be used once in a program.

D They automatically save results to a file.

Anonymous functions let you write small functions quickly without creating a separate named function.

Can anonymous functions in R have no arguments?

A No, they must have at least one argument.

B Yes, you can write function() expression.

C Only if you assign them to a variable.

D Only inside loops.

Anonymous functions can have zero or more arguments. For example, function() 42 returns 42.

What does this code return?
sapply(1:3, function(x) x + 5)

A A single number 8

B A list: 6, 7, 8

C An error

D A vector: 6, 7, 8

sapply() returns a simplified vector applying the anonymous function to each element, adding 5 to 1, 2, and 3.

Explain what an anonymous function is in R and give a simple example.

Think about functions without names used directly inside other functions.

Describe a situation where using an anonymous function is better than a named function in R.

Consider when you want to do something simple just once.

Practice

1. What is an anonymous function in R?

easy

A. A function that must be saved to a variable

B. A function that always returns NULL

C. A function without a name, created on the spot

D. A function that can only be used once

Solution

1. Step 1: Understand the definition of anonymous functions

   Anonymous functions are functions created without giving them a name, used immediately.

2. Step 2: Compare options with the definition

   Only 'A function without a name, created on the spot' correctly describes an anonymous function.

3. Final Answer:

   A function without a name, created on the spot -> Option C

4. Quick Check:

   Anonymous function = unnamed function [OK]

Hint: Anonymous means no name, created instantly [OK]

Common Mistakes:

• Thinking anonymous functions must be saved to variables
• Believing anonymous functions always return NULL
• Confusing anonymous functions with one-time use functions

2. Which of the following is the correct syntax for an anonymous function in R?

easy

A. lambda x: x + 1

B. fun(x) { return x + 1 }

C. def(x) x + 1

D. function(x) x + 1

Solution

1. Step 1: Recall R syntax for anonymous functions

   In R, anonymous functions are written as function(arguments) expression.

2. Step 2: Check each option

   function(x) x + 1 matches R syntax. Options B, C, and D use syntax from other languages or incorrect forms.

3. Final Answer:

   function(x) x + 1 -> Option D

4. Quick Check:

   R anonymous function syntax = function(args) expr [OK]

Hint: R uses 'function' keyword for anonymous functions [OK]

Common Mistakes:

• Using 'def' or 'lambda' which are Python keywords
• Writing function name instead of 'function'
• Adding unnecessary return statements inside anonymous functions

3. What is the output of this R code?

lapply(1:3, function(x) x * 2)

medium

A. [[1]] 2 [[2]] 4 [[3]] 6

B. [1] 2 4 6

C. [1] 1 2 3

D. Error: object 'x' not found

Solution

1. Step 1: Understand lapply with anonymous function

   lapply applies the function to each element of 1:3, returning a list of results.

2. Step 2: Calculate each element

   For x=1,2,3, function(x) x*2 returns 2,4,6 respectively, stored as list elements.

3. Final Answer:

   [[1]] 2 [[2]] 4 [[3]] 6 -> Option A

4. Quick Check:

   lapply returns list of doubled values [OK]

Hint: lapply returns a list, not a vector [OK]

Common Mistakes:

• Expecting a vector output instead of a list
• Confusing lapply with sapply output
• Thinking anonymous function causes error

4. Identify the error in this R code using an anonymous function:

result <- lapply(1:3, function(x) { x + })

medium

A. Anonymous functions cannot be used inside lapply

B. Missing value after '+' operator

C. Incorrect use of curly braces

D. Variable 'result' is not defined

Solution

1. Step 1: Analyze the anonymous function body

   The function body has 'x +', which is incomplete because '+' needs a value after it.

2. Step 2: Identify the syntax error

   This causes a syntax error due to missing operand after '+'. Other options are incorrect because anonymous functions are valid in lapply, curly braces are correctly used, and 'result' is defined.

3. Final Answer:

   Missing value after '+' operator -> Option B

4. Quick Check:

   Incomplete operator usage = syntax error [OK]

Hint: Check operators have both operands [OK]

Common Mistakes:

• Assuming anonymous functions can't be used in lapply
• Ignoring incomplete expressions inside function
• Confusing variable definition errors with syntax errors

5. How can you use an anonymous function to filter even numbers from a vector v <- 1:6 using Filter() in R?

hard

A. Filter(function(x) x %% 2 == 0, v)

B. Filter(function(x) x / 2 == 0, v)

C. Filter(function(x) x %% 2 = 0, v)

D. Filter(function(x) x == 0, v)

Solution

1. Step 1: Understand filtering even numbers

   Even numbers have zero remainder when divided by 2, so condition is x %% 2 == 0.

2. Step 2: Check each option's condition

   Filter(function(x) x %% 2 == 0, v) correctly uses 'x %% 2 == 0'. Filter(function(x) x / 2 == 0, v) uses division instead of modulo, Filter(function(x) x %% 2 = 0, v) uses single '=', which is assignment not comparison, and Filter(function(x) x == 0, v) checks if x equals zero only.

3. Final Answer:

   Filter(function(x) x %% 2 == 0, v) -> Option A

4. Quick Check:

   Modulo equals zero filters even numbers [OK]

Hint: Use '%% 2 == 0' to check even numbers [OK]

Common Mistakes:

• Using single '=' instead of '==' for comparison
• Using division '/' instead of modulo '%%'
• Filtering for x == 0 instead of even numbers