lapply and sapply in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of lapply with a function

What is the output of this R code?

R Programming

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

A list(a = 6, b = 15)

B list(6, 15)

C c(6, 15)

D c(a = 6, b = 15)

💡 Hint

lapply returns a list with the same names as input.

lapply applies the function to each element and returns a list preserving names.

❓ Predict Output

intermediate

Output of sapply with a function

What is the output of this R code?

R Programming

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

A list(a = 6, b = 15)

B c(6, 15)

C c(a = 6, b = 15)

D matrix(c(6, 15), nrow=1)

💡 Hint

sapply tries to simplify the result to a vector or matrix.

sapply returns a named vector when possible, here sums become a named numeric vector.

❓ Predict Output

advanced

Difference in output types between lapply and sapply

What is the class of the output for these two calls? x <- list(a = 1:3, b = 4:6) 1) lapply(x, mean) 2) sapply(x, mean)

A 1) numeric, 2) numeric

B 1) numeric, 2) list

C 1) list, 2) list

D 1) list, 2) numeric

💡 Hint

lapply always returns a list, sapply tries to simplify to vector.

lapply returns a list of means, sapply returns a numeric vector of means.

🧠 Conceptual

advanced

When does sapply return a matrix instead of a vector?

Given a list where each element is a numeric vector of length 2, what will sapply return?

A A numeric matrix with 2 rows and columns equal to the number of list elements

B A list of numeric vectors

C A numeric vector with length equal to the number of list elements

D An error because sapply cannot handle vectors

💡 Hint

sapply simplifies to matrix when each element returns a vector of same length.

When each element returns a vector of length > 1, sapply returns a matrix with rows = vector length.

🔧 Debug

expert

Why does this sapply call produce unexpected output?

Consider this code: x <- list(a = 1:3, b = 4:6) sapply(x, function(v) v > 2) What is the output and why might it be confusing?

A A list of logical vectors, one per list element

B A logical matrix with rows for each element in vectors and columns for list elements

C A logical vector with length equal to list length

D An error because the function returns a vector

💡 Hint

Check how sapply simplifies when function returns vectors longer than 1.

The function returns logical vectors of length 3, sapply simplifies to a matrix with 3 rows and 2 columns.

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