lapply and sapply in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to apply the function to each element of the list using lapply.

R Programming

result <- lapply(my_list, [1])

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

Alist

Bsum

Cc

Dmean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that does not accept a vector input.

Using a function that returns a list instead of a numeric value.

The lapply function applies the given function to each element of the list and returns a list of results. Here, sum is the function applied.

2fill in blank

medium

Complete the code to apply the function to each element of the list and simplify the result using sapply.

R Programming

result <- sapply(my_list, [1])

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

Alength

Blist

Csum

Dc

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that returns a list, which prevents simplification.

Using a function that returns the length, which is correct but not the intended answer here.

The sapply function applies the function to each element and tries to simplify the result. Using sum will return a numeric vector of sums.

3fill in blank

hard

Fix the error in the code to correctly apply the mean function to each element using lapply.

R Programming

result <- lapply(my_list, [1])

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

Amean

Bsum

Clength

Dmedian

Attempts:

3 left

💡 Hint

Common Mistakes

Including parentheses after the function name inside lapply.

Using a function that does not calculate the mean.

The lapply function expects a function name without parentheses. Using mean without parentheses correctly passes the function.

4fill in blank

hard

Fill both blanks to create a named vector of the lengths of each element in the list using sapply.

R Programming

lengths <- sapply([1], [2])

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

Amy_list

Blength

Csum

Dmean

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum instead of length for counting elements.

Using a variable name that does not exist.

The sapply function applies length to each element of my_list, returning a named vector of lengths.

5fill in blank

hard

Fill both blanks to create a list of squared values for each element in the list using lapply.

R Programming

squares <- lapply(my_list, function(x) x[1][2])

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

A*

B2

C^

D**

Attempts:

3 left

💡 Hint

Common Mistakes

Using the multiplication operator '*' instead of the power operator '^'.

Using the wrong exponent.

The anonymous function squares each element by using the power operator ^ with exponent 2: function(x) x^2. lapply applies it to each element and returns a list.

Practice

1. What is the main difference between lapply and sapply in R?

easy

A. lapply modifies the original list, sapply does not.

B. lapply works only on vectors, sapply works only on lists.

C. lapply always returns a list, while sapply tries to simplify the result.

D. lapply is faster than sapply in all cases.

Solution

1. Step 1: Understand return types of lapply and sapply

   lapply always returns a list regardless of the function applied.

2. Step 2: Understand simplification behavior of sapply

   sapply tries to simplify the output to a vector or matrix if possible, otherwise returns a list.

3. Final Answer:

   lapply always returns a list, while sapply tries to simplify the result. -> Option C

4. Quick Check:

   Return type difference = lapply always returns a list, while sapply tries to simplify the result. [OK]

Hint: Remember: lapply = list output, sapply = simpler output if possible [OK]

Common Mistakes:

• Thinking lapply works only on vectors
• Believing sapply always returns a list
• Assuming lapply modifies original data

2. Which of the following is the correct syntax to apply the function sqrt to each element of a list my_list using lapply?

easy

A. lapply(my_list, sqrt)

B. lapply(my_list, sqrt())

C. lapply(sqrt, my_list)

D. lapply(my_list sqrt)

Solution

1. Step 1: Identify correct function call syntax

   The function name should be passed without parentheses to lapply, so sqrt not sqrt().

2. Step 2: Confirm argument order

   The first argument is the list, second is the function to apply: lapply(my_list, sqrt).

3. Final Answer:

   lapply(my_list, sqrt) -> Option A

4. Quick Check:

   Function without () and correct order = lapply(my_list, sqrt) [OK]

Hint: Pass function name without () as second argument in lapply [OK]

Common Mistakes:

• Using parentheses after function name inside lapply
• Swapping argument order
• Missing comma between arguments

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

nums <- list(a = 1:3, b = 4:6)
lapply(nums, sum)
sapply(nums, sum)

medium

A. Both return a list with sums: list(a=6, b=15)

B. lapply returns list(a=6, b=15), sapply returns numeric vector c(a=6, b=15)

C. lapply returns numeric vector, sapply returns list

D. Both return numeric vector c(6, 15)

Solution

1. Step 1: Calculate sums for each list element

   Sum of 1:3 is 6, sum of 4:6 is 15.

2. Step 2: Understand output types of lapply and sapply

   lapply returns a list with sums named by list elements; sapply simplifies to a numeric vector with names.

3. Final Answer:

   lapply returns list(a=6, b=15), sapply returns numeric vector c(a=6, b=15) -> Option B

4. Quick Check:

   lapply = list, sapply = vector if possible [OK]

Hint: lapply always list; sapply tries vector or matrix [OK]

Common Mistakes:

• Confusing output types of lapply and sapply
• Assuming both return same structure
• Ignoring named elements in output

4. Consider this code snippet:

my_list <- list(1, 2, 3)
lapply(my_list, function(x) x^2)

What is wrong with this code if the goal is to get a numeric vector of squared values?

medium

A. Nothing is wrong; it returns a numeric vector.

B. The anonymous function is incorrect; it should use x*x instead of x^2.

C. The list elements should be named for lapply to work.

D. Using lapply returns a list, not a numeric vector.

Solution

1. Step 1: Understand output type of lapply

   lapply always returns a list, even if function returns single values.

2. Step 2: Identify how to get numeric vector output

   To get a numeric vector, sapply should be used instead of lapply.

3. Final Answer:

   Using lapply returns a list, not a numeric vector. -> Option D

4. Quick Check:

   lapply output is list, not vector [OK]

Hint: Use sapply for vector output, lapply always returns list [OK]

Common Mistakes:

• Thinking lapply returns vector
• Believing function syntax is wrong
• Assuming names are required for lapply

5. You have a list of numeric vectors: data_list <- list(c(1,2), c(3,4,5), c(6)). You want to create a numeric vector of the means of each vector, but if a vector is empty, return NA for that element. Which code correctly does this using lapply or sapply?

hard

A. sapply(data_list, function(x) if(length(x) == 0) NA else mean(x))

B. lapply(data_list, function(x) if(length(x) == 0) NA else mean(x))

C. sapply(data_list, mean, na.rm=TRUE)

D. lapply(data_list, mean, na.rm=TRUE)

Solution

1. Step 1: Handle empty vectors with conditional inside function

   The function checks if length is zero, returns NA; else returns mean.

2. Step 2: Use sapply to get numeric vector output

   Using sapply simplifies the result to a numeric vector, which is desired.

3. Final Answer:

   sapply(data_list, function(x) if(length(x) == 0) NA else mean(x)) -> Option A

4. Quick Check:

   Conditional function + sapply for vector output [OK]

Hint: Use sapply with if-else inside function for conditional output [OK]

Common Mistakes:

• Using lapply and expecting vector output
• Not handling empty vectors explicitly
• Passing na.rm=TRUE without checking empty vectors