Matrix dimensions in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a 3x3 matrix named mat filled with numbers 1 to 9.

R Programming

mat <- matrix(1:9, nrow = [1], ncol = 3)

Drag options to blanks, or click blank then click option'

A1

B9

C3

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Setting nrow to 9 or 1, which creates wrong matrix dimensions.

The nrow argument sets the number of rows. To make a 3x3 matrix, nrow must be 3.

2fill in blank

medium

Complete the code to get the number of rows of matrix mat.

R Programming

num_rows <- nrow([1])

Drag options to blanks, or click blank then click option'

Amatrix

Barray

Cdata

Dmat

Attempts:

3 left

💡 Hint

Common Mistakes

Using matrix or data which are not the matrix variable.

The function nrow() returns the number of rows of the matrix passed as argument. Here, the matrix is mat.

3fill in blank

hard

Fix the error in the code to get the number of columns of matrix mat.

R Programming

num_cols <- [1](mat)

Drag options to blanks, or click blank then click option'

Ancol

Blength

Cnrow

Ddim

Attempts:

3 left

💡 Hint

Common Mistakes

Using nrow() which returns rows, not columns.

Using length() which returns total elements.

The function ncol() returns the number of columns of a matrix. Using nrow() or length() will not give the correct number of columns.

4fill in blank

hard

Fill both blanks to create a 2x4 matrix named m filled with numbers 1 to 8.

R Programming

m <- matrix(1:8, nrow = [1], ncol = [2])

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A2

B3

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping rows and columns values.

Using wrong numbers like 3 or 5.

To create a 2x4 matrix, nrow must be 2 and ncol must be 4.

5fill in blank

hard

Fill all three blanks to create a matrix mat with 3 rows, 2 columns, and get its dimensions.

R Programming

mat <- matrix(1:6, nrow = [1], ncol = [2])
dims <- [3](mat)

Drag options to blanks, or click blank then click option'

A2

Bdim

C3

Dncol

Attempts:

3 left

💡 Hint

Common Mistakes

Using ncol instead of dim to get dimensions.

Swapping rows and columns values.

Set nrow = 3 and ncol = 2 to create a 3x2 matrix. Use dim() to get both dimensions as a vector.

Practice

1. What does the dim() function return when used on a matrix in R?

easy

A. The first element of the matrix

B. The sum of all elements in the matrix

C. A vector with the number of rows and columns

D. The matrix converted to a vector

Solution

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

   The dim() function in R returns the dimensions of an object, typically a matrix or array.

2. Step 2: What does it return for a matrix?

   For a matrix, dim() returns a vector of length two: the number of rows and the number of columns.

3. Final Answer:

   A vector with the number of rows and columns -> Option C

4. Quick Check:

   dim(matrix) = c(rows, columns) [OK]

Hint: Remember dim() gives rows and columns together [OK]

Common Mistakes:

• Thinking dim() returns sum of elements
• Confusing dim() with length()
• Expecting dim() to return matrix elements

2. Which of the following is the correct way to create a 3x2 matrix in R?

easy

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

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

C. matrix(1:6, rows=3, cols=2)

D. matrix(1:6, 3, 2, byrow=TRUE)

Solution

1. Step 1: Check the matrix function parameters

   The matrix() function uses nrow and ncol to specify rows and columns.

2. Step 2: Identify correct parameter names and values

   matrix(1:6, nrow=3, ncol=2) uses nrow=3 and ncol=2, which correctly creates a 3-row, 2-column matrix.

3. Final Answer:

   matrix(1:6, nrow=3, ncol=2) -> Option A

4. Quick Check:

   Use nrow and ncol to set dimensions [OK]

Hint: Use nrow and ncol to set matrix size [OK]

Common Mistakes:

• Using wrong parameter names like rows or cols
• Swapping nrow and ncol values
• Omitting parameter names causing confusion

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

m <- matrix(1:12, nrow=3, ncol=4)
dim(m)

medium

A. [1] 12

B. [1] 4 3

C. [1] 3

D. [1] 3 4

Solution

1. Step 1: Understand matrix creation

   The matrix m is created with 3 rows and 4 columns using nrow=3 and ncol=4.

2. Step 2: Check the output of dim(m)

   The dim() function returns a vector with the number of rows and columns, so it returns 3 4.

3. Final Answer:

   [1] 3 4 -> Option D

4. Quick Check:

   dim(m) = c(3, 4) [OK]

Hint: dim() returns rows then columns vector [OK]

Common Mistakes:

• Confusing order of rows and columns
• Expecting dim() to return total elements
• Misreading output as a single number

4. Identify the error in this R code snippet:

m <- matrix(1:9, nrow=3, ncol=3)
dim(m) <- c(2, 5)

medium

A. The code is correct and runs without error

B. You cannot assign dimensions that don't match the number of elements

C. dim() cannot be used on matrices

D. The matrix function is missing the byrow argument

Solution

1. Step 1: Check the original matrix size

   The matrix m has 9 elements (3 rows x 3 columns).

2. Step 2: Analyze the dimension assignment

   Assigning dim(m) <- c(2, 5) tries to reshape m to 2 rows and 5 columns, which requires 10 elements, not 9.

3. Final Answer:

   You cannot assign dimensions that don't match the number of elements -> Option B

4. Quick Check:

   New dims must match total elements [OK]

Hint: New dimensions must multiply to total elements [OK]

Common Mistakes:

• Ignoring element count mismatch
• Thinking dim() cannot be assigned
• Assuming byrow affects dimension assignment

5. You have a matrix m with dimensions 4x5. You want to reshape it into a matrix with 5 rows. Which of the following commands correctly changes the dimensions without losing data?

hard

A. dim(m) <- c(5, 4)

B. dim(m) <- c(5, 5)

C. dim(m) <- c(5, 3)

D. dim(m) <- c(5, 2)

Solution

1. Step 1: Calculate total elements in original matrix

   The original matrix has 4 rows x 5 columns = 20 elements.

2. Step 2: Check which new dimension matches total elements

   dim(m) <- c(5, 4) sets dimensions to 5 rows and 4 columns, which is 20 elements, matching original size.

3. Step 3: Verify other options

   Options B, C, and D have total elements 25, 15, and 10 respectively, which do not match 20, causing errors or data loss.

4. Final Answer:

   dim(m) <- c(5, 4) -> Option A

5. Quick Check:

   New dims must multiply to 20 [OK]

Hint: New rows x new cols = total elements [OK]

Common Mistakes:

• Choosing dimensions that don't match total elements
• Swapping rows and columns incorrectly
• Forgetting to check total element count