Matrix creation in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this matrix creation code?

Consider the following R code that creates a matrix. What will be the output when printed?

R Programming

m <- matrix(1:6, nrow=2, ncol=3)
print(m)

A

[,1] [,2]
[1,]    1    2
[2,]    3    4
[3,]    5    6

B

[,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6

C

[,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

D

[,1] [,2] [,3]
[1,]    6    5    4
[2,]    3    2    1

💡 Hint

Remember that R fills matrices by columns by default.

The matrix function fills the matrix column-wise by default. So the numbers 1 to 6 fill down the first column, then the second, then the third.

❓ Predict Output

intermediate

What is the value of element m[2,3] after this code?

Given the matrix creation below, what is the value stored at row 2, column 3?

R Programming

m <- matrix(seq(10, 60, by=10), nrow=3, ncol=2)
m[2,3] <- 99
print(m)

A Error: subscript out of bounds

B 99

C 30

D 60

💡 Hint

Check the dimensions of the matrix before assigning.

The matrix has 3 rows and 2 columns, so column 3 does not exist. Trying to assign m[2,3] causes an error.

🧠 Conceptual

advanced

How does the byrow argument affect matrix creation?

What is the difference in the matrix created by these two commands? m1 <- matrix(1:6, nrow=2, ncol=3) m2 <- matrix(1:6, nrow=2, ncol=3, byrow=TRUE) Choose the correct description.

A m1 fills the matrix by columns, m2 fills by rows

B Both fill by columns, but m2 transposes the matrix

C m1 fills the matrix by rows, m2 fills by columns

D Both fill by rows, but m2 reverses the order

💡 Hint

Check the default behavior of matrix filling and what byrow=TRUE changes.

By default, matrix fills by columns. Setting byrow=TRUE makes it fill by rows instead.

🔧 Debug

advanced

What error does this matrix creation code produce?

Examine the code below and identify the error it produces when run in R: m <- matrix(1:8, nrow=3, ncol=3) print(m)

A Warning: data length is not a sub-multiple or multiple of the number of rows or columns

B Error: invalid matrix dimensions

C Error: number of items to replace is not a multiple of replacement length

D No error, matrix is created with recycling of values

💡 Hint

Think about what happens when the data length does not match the matrix size exactly.

R recycles the vector 1:8 to fill the 3x3 matrix (9 elements), so no error occurs.

🚀 Application

expert

How many elements are in the matrix created by this code?

What is the total number of elements in the matrix created by this R code? m <- matrix(data=rep(5, times=12), nrow=4, ncol=3)

A 7

B 12

C 4

D 3

💡 Hint

Multiply the number of rows by the number of columns.

A matrix with 4 rows and 3 columns has 4*3=12 elements.