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.