Apply functions on matrices in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of apply() on matrix rows

What is the output of this R code that uses apply() on matrix rows?

R Programming

mat <- matrix(1:9, nrow=3, byrow=TRUE)
result <- apply(mat, 1, sum)
print(result)

A [1] 12 15 18

B [1] 6 15 24

C [1] 3 6 9

D [1] 1 2 3

💡 Hint

Remember, apply() with margin 1 applies the function to rows.

The matrix has rows: (1,2,3), (4,5,6), (7,8,9). Summing each row gives 6, 15, and 24.

❓ Predict Output

intermediate

Using apply() on matrix columns

What does this R code print when applying mean on matrix columns?

R Programming

mat <- matrix(c(2,4,6,8,10,12), nrow=2)
result <- apply(mat, 2, mean)
print(result)

A [1] 2 4 6

B [1] 4 6 8

C [1] 3 5 7

D [1] 5 7 9

💡 Hint

Margin 2 means apply function to columns.

The matrix columns are (2,8), (4,10), (6,12). Their means are (5,7,9).

❓ Predict Output

advanced

Output of apply() with a custom function

What is the output of this R code applying a custom function to each row of a matrix?

R Programming

mat <- matrix(1:6, nrow=2)
result <- apply(mat, 1, function(x) paste(x, collapse='-'))
print(result)

A [1] "1-2-3" "4-5-6"

B [1] "1-4" "2-5"

C [1] "1-3-5" "2-4-6"

D [1] "1-2" "3-4"

💡 Hint

Margin 1 applies function to rows; matrix is filled column-wise by default.

Matrix rows are (1,3,5) and (2,4,6). The function joins elements with '-'.

🔧 Debug

advanced

Identify the error in apply() usage

What error does this R code produce?

R Programming

mat <- matrix(1:9, nrow=3)
result <- apply(mat, 3, sum)
print(result)

A Error in apply(mat, 3, sum) : MARGIN must be between 1 and 2

B Error in apply(mat, 3, sum) : subscript out of bounds

C Error in apply(mat, 3, sum) : incorrect number of dimensions

D No error, prints sums of third dimension

💡 Hint

Check the dimensions of the matrix and the margin argument.

The matrix has only 2 dimensions, margin 3 is invalid, causing the error "MARGIN must be between 1 and 2".

🧠 Conceptual

expert

Understanding apply() output type

Given this R code, what is the type and length of result?

R Programming

mat <- matrix(c(1,2,3,4), nrow=2)
result <- apply(mat, 1, function(x) x * 2)
str(result)

A "num [1:2, 1:2]" (a 2x2 numeric matrix)

B "num [1:2]" (a numeric vector of length 2)

C "list of 2" (a list with 2 elements)

D "num [1:4]" (a numeric vector of length 4)

💡 Hint

When the function returns a vector, apply() simplifies to a matrix.

The function doubles each row vector (length 2), so apply returns a 2x2 matrix.