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.

Practice

1. What does the matrix() function in R primarily do?

easy

A. Creates a table of values arranged in rows and columns

B. Generates a list of random numbers

C. Sorts a vector in ascending order

D. Calculates the sum of all elements in a vector

Solution

1. Step 1: Understand the purpose of matrix()

   The matrix() function is designed to create a matrix, which is a table of values arranged in rows and columns.

2. Step 2: Compare with other options

   Options A, B, and C describe different functions or operations unrelated to matrix creation.

3. Final Answer:

   Creates a table of values arranged in rows and columns -> Option A

4. Quick Check:

   matrix() creates tables = D [OK]

Hint: matrix() makes tables with rows and columns [OK]

Common Mistakes:

• Confusing matrix() with functions that generate random numbers
• Thinking matrix() sorts data
• Assuming matrix() sums elements

2. Which of the following is the correct syntax to create a 2x3 matrix filled by rows with numbers 1 to 6 in R?

easy

A. matrix(1:6, nrow=3, ncol=2, byrow=TRUE)

B. matrix(1:6, nrow=2, ncol=3, byrow=TRUE)

C. matrix(1:6, nrow=2, ncol=3, byrow=FALSE)

D. matrix(1:6, nrow=3, ncol=2, byrow=FALSE)

Solution

1. Step 1: Identify matrix dimensions and filling order

   The question asks for a 2 rows by 3 columns matrix filled by rows with numbers 1 to 6.

2. Step 2: Match syntax to requirements

   matrix(1:6, nrow=2, ncol=3, byrow=TRUE) uses nrow=2, ncol=3, and byrow=TRUE, which matches the question exactly.

3. Final Answer:

   matrix(1:6, nrow=2, ncol=3, byrow=TRUE) -> Option B

4. Quick Check:

   2 rows, 3 cols, byrow=TRUE = A [OK]

Hint: Match nrow, ncol, and byrow exactly [OK]

Common Mistakes:

• Swapping nrow and ncol values
• Forgetting to set byrow=TRUE for row-wise filling
• Using byrow=FALSE when row-wise fill is needed

3. What is the output of the following R code?

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

medium

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

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

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

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

Solution

1. Step 1: Understand matrix filling by columns

   With byrow=FALSE, the matrix fills column-wise: first column gets 1 and 2, second gets 3 and 4, third gets 5 and 6.

2. Step 2: Write matrix layout

   Row 1: 1, 3, 5
   Row 2: 2, 4, 6

3. Final Answer:

   [,1] [,2] [,3] [1,] 1 3 5 [2,] 2 4 6 -> Option D

4. Quick Check:

   byrow=FALSE fills columns = A [OK]

Hint: byrow=FALSE fills columns top to bottom [OK]

Common Mistakes:

• Assuming byrow=FALSE fills rows instead of columns
• Mixing up row and column indices in output
• Confusing output format with list notation

4. Identify the error in this R code that tries to create a 3x2 matrix:

matrix(1:6, nrow=3, ncol=2, byrow=TRUE, fill=TRUE)

medium

A. The argument 'fill' is not valid in matrix()

B. nrow and ncol values are swapped

C. byrow must be FALSE for this matrix size

D. 1:6 cannot fill a 3x2 matrix

Solution

1. Step 1: Check matrix() arguments

   The matrix() function does not have a fill argument; this causes an error.

2. Step 2: Validate other arguments

   nrow=3 and ncol=2 are valid, byrow=TRUE is allowed, and 1:6 perfectly fills a 3x2 matrix.

3. Final Answer:

   The argument 'fill' is not valid in matrix() -> Option A

4. Quick Check:

   Invalid argument 'fill' causes error = C [OK]

Hint: matrix() has no 'fill' argument [OK]

Common Mistakes:

• Assuming 'fill' is a valid argument
• Swapping nrow and ncol unnecessarily
• Thinking byrow must be FALSE for certain sizes

5. You want to create a 4x4 matrix in R where the numbers 1 to 16 fill the matrix by rows, but every even number should be replaced by 0. Which code correctly achieves this?

hard

A. matrix(ifelse((1:16) %% 2 == 0, 0, 1:16), nrow=4, ncol=4, byrow=FALSE)

B. matrix(ifelse((1:16) %% 2 == 1, 0, 1:16), nrow=4, ncol=4, byrow=TRUE)

C. matrix(ifelse((1:16) %% 2 == 0, 0, 1:16), nrow=4, ncol=4, byrow=TRUE)

D. matrix(ifelse((1:16) %% 2 == 1, 0, 1:16), nrow=4, ncol=4, byrow=FALSE)

Solution

1. Step 1: Replace even numbers with 0

   The expression ifelse((1:16) %% 2 == 0, 0, 1:16) replaces even numbers with 0 and keeps odd numbers as is.

2. Step 2: Create 4x4 matrix filled by rows

   Using nrow=4, ncol=4, and byrow=TRUE fills the matrix row-wise as required.

3. Final Answer:

   matrix(ifelse((1:16) %% 2 == 0, 0, 1:16), nrow=4, ncol=4, byrow=TRUE) -> Option C

4. Quick Check:

   Replace evens with 0, fill by rows = B [OK]

Hint: Use ifelse() and byrow=TRUE for row-wise conditional fill [OK]

Common Mistakes:

• Using byrow=FALSE when row-wise fill is needed
• Replacing odd numbers instead of even
• Not using ifelse() for conditional replacement