Environment and closures in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a function that returns another function adding a fixed number.

R Programming

makeAdder <- function(x) {
  function(y) {
    return(y [1] x)
  }
}

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Forgetting to use the operator between y and x.

The inner function adds y and x. The + operator is needed.

2fill in blank

medium

Complete the code to assign a value to a variable in the parent environment inside a closure.

R Programming

counter <- function() {
  count <- 0
  function() {
    count [1] count + 1
    return(count)
  }
}

Drag options to blanks, or click blank then click option'

A->

B=

C<-

D<<-

Attempts:

3 left

💡 Hint

Common Mistakes

Using <- which creates a new local variable instead of updating the parent.

Using = which does not assign in the parent environment.

The <<− operator assigns the value to count in the parent environment, allowing the counter to update.

3fill in blank

hard

Fix the error in the closure that tries to access a variable from the wrong environment.

R Programming

makeMultiplier <- function(factor) {
  function(x) {
    return(x * [1])
  }
}

mult2 <- makeMultiplier(2)
mult2(5)

Drag options to blanks, or click blank then click option'

Afactor

Bx

CmakeMultiplier

Dmult2

Attempts:

3 left

💡 Hint

Common Mistakes

Using x instead of factor inside the return statement.

Using the function names instead of variables.

The inner function must use factor from the parent environment to multiply x.

4fill in blank

hard

Fill both blanks to create a closure that stores and retrieves a value.

R Programming

makeStorage <- function() {
  value <- NULL
  list(
    set = function(x) {
      value [1] x
    },
    get = function() {
      return(value [2])
    }
  )
}

Drag options to blanks, or click blank then click option'

A<<-

Bvalue

Creturn

D<-

Attempts:

3 left

💡 Hint

Common Mistakes

Using <- in set which creates a local variable.

Not returning the stored value in get.

The set function uses <<− to update value in the parent environment. The get function returns value.

5fill in blank

hard

Fill all three blanks to create a closure that counts how many times it has been called and returns the count.

R Programming

makeCounter <- function() {
  count <- 0
  function() {
    count [1] count + 1
    return([2])
  }
}

counter <- makeCounter()
counter()
counter()  # returns [3]

Drag options to blanks, or click blank then click option'

A<-

Bcount

C<<-

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using <- which does not update the parent variable.

Returning a literal number instead of the count variable.

Incorrect expected return value after two calls.

The counter updates count in the parent environment using <<−. It returns the updated count. After two calls, the count is 2.

Practice

1. What is an environment in R programming?

easy

A. A type of loop for repeating code

B. A place where variables and their values are stored

C. A function that takes another function as input

D. A package that extends R's capabilities

Solution

1. Step 1: Understand the definition of environment

   In R, an environment is a collection where variables and their values are stored together.

2. Step 2: Differentiate from other options

   Options B, C, and D describe loops, higher-order functions, and packages, which are not environments.

3. Final Answer:

   A place where variables and their values are stored -> Option B

4. Quick Check:

   Environment = variable storage [OK]

Hint: Remember: environment stores variables like a box [OK]

Common Mistakes:

• Confusing environment with a function
• Thinking environment is a loop
• Mixing environment with packages

2. Which of the following is the correct way to create a closure in R?

easy

A. f <- function(x) { function() x + 1 }

B. f <- function(x) { x + 1 }

C. f <- function(x) { x <- x + 1 }

D. f <- function(x) { print(x) }

Solution

1. Step 1: Identify closure structure

   A closure is a function that returns another function which remembers variables from its environment.

2. Step 2: Check each option

   f <- function(x) { function() x + 1 } returns a function that uses 'x' from the outer function, creating a closure. Others do not return a function.

3. Final Answer:

   f <- function(x) { function() x + 1 } -> Option A

4. Quick Check:

   Closure returns function using outer variable [OK]

Hint: Closure = function returning function using outer variables [OK]

Common Mistakes:

• Not returning a function inside another function
• Modifying variable without returning a function
• Confusing print with returning a function

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

make_counter <- function() {
  count <- 0
  function() {
    count <<- count + 1
    count
  }
}
counter <- make_counter()
counter()
counter()

medium

A. Error due to undefined variable

B. [1] 0 followed by [1] 1

C. [1] 1 followed by [1] 1

D. [1] 1 followed by [1] 2

Solution

1. Step 1: Understand the closure and <<- operator

   The inner function modifies 'count' in the parent environment using <<- and increments it by 1 each call.

2. Step 2: Trace the calls to counter()

   First call returns 1 (0+1), second call returns 2 (1+1).

3. Final Answer:

   [1] 1 followed by [1] 2 -> Option D

4. Quick Check:

   Closure increments count with <<- [OK]

Hint: <<- changes outer variable, so count increases each call [OK]

Common Mistakes:

• Thinking count resets each call
• Confusing <- with <<- operator
• Expecting error due to variable scope

4. Identify the error in this R code that tries to create a closure:

make_adder <- function(x) {
  function(y) {
    x + y
  }
}
add5 <- make_adder(5)
add5(3)
rm(x)

medium

A. The function make_adder is missing a return statement

B. Removing 'x' breaks the closure, causing an error on add5(3)

C. No error; code runs fine

D. The inner function should use <<- to modify x

Solution

1. Step 1: Understand closure variable dependency

   The inner function depends on 'x' stored in its own dedicated environment created by make_adder.

2. Step 2: Effect of removing 'x'

   rm(x) removes 'x' from the global environment if it exists, but the closure's environment retains its own 'x' = 5, so add5(3) works fine.

3. Final Answer:

   No error; code runs fine -> Option C

4. Quick Check:

   Closure environments are independent of global [OK]

Hint: rm(x) doesn't affect closure's private x [OK]

Common Mistakes:

• Thinking functions need explicit return
• Believing <<- is needed to read outer variables
• Mistakenly thinking rm(x) removes closure's x

5. You want to create a function in R that remembers a secret number and returns a function to check guesses. Which approach correctly uses closures to keep the secret safe?

hard

A. secret_keeper <- function(secret) { function(guess) { guess == secret } }

B. secret_keeper <- function(secret) { secret <- NULL; function(guess) { guess == secret } }

C. secret_keeper <- function(secret) { function(guess) { secret <- guess; TRUE } }

D. secret_keeper <- function(secret) { function(guess) { print(secret); guess } }

Solution

1. Step 1: Identify closure usage to keep secret

   secret_keeper <- function(secret) { function(guess) { guess == secret } } returns a function that compares guess to secret stored in the closure environment, keeping secret safe.

2. Step 2: Analyze other options for flaws

   secret_keeper <- function(secret) { secret <- NULL; function(guess) { guess == secret } } sets secret to NULL, losing the secret. secret_keeper <- function(secret) { function(guess) { secret <- guess; TRUE } } changes secret to guess, breaking secrecy. secret_keeper <- function(secret) { function(guess) { print(secret); guess } } prints secret, exposing it.

3. Final Answer:

   secret_keeper <- function(secret) { function(guess) { guess == secret } } -> Option A

4. Quick Check:

   Closure keeps secret hidden and accessible only inside [OK]

Hint: Closure hides secret; don't overwrite or expose it [OK]

Common Mistakes:

• Overwriting secret variable inside closure
• Exposing secret with print statements
• Setting secret to NULL before use