Matrix creation in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used to create a matrix in R?

The matrix() function is used to create a matrix in R.

beginner

How do you specify the number of rows and columns when creating a matrix?

Use the arguments nrow for number of rows and ncol for number of columns inside the matrix() function.

intermediate

What does the argument byrow = TRUE do in matrix()?

It fills the matrix by rows instead of the default column-wise filling.

beginner

How can you create a 3x3 matrix with numbers 1 to 9 filled by rows?

Use matrix(1:9, nrow = 3, ncol = 3, byrow = TRUE).

intermediate

What happens if you provide fewer elements than needed for the matrix?

R recycles the elements to fill the matrix completely.

Which function creates a matrix in R?

A data.frame()

B matrix()

C array()

D list()

The matrix() function is specifically designed to create matrices.

What does byrow = TRUE do in matrix()?

A Fills matrix by rows

B Fills matrix by columns

C Creates a row vector

D Creates a column vector

Setting byrow = TRUE fills the matrix row-wise instead of the default column-wise.

How do you specify a matrix with 4 rows and 2 columns?

A matrix(data, rows=2, cols=4)

B matrix(data, nrow=2, ncol=4)

C matrix(data, rows=4, cols=2)

D matrix(data, nrow=4, ncol=2)

Use nrow and ncol to set rows and columns respectively.

What happens if you give fewer elements than needed in matrix()?

A R recycles elements to fill matrix

B R throws an error

C Matrix is partially filled with NA

D Matrix is created empty

R repeats the given elements to fill the entire matrix.

Which of these creates a 2x3 matrix filled by columns?

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

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

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

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

By default, matrix() fills by columns, so this creates a 2x3 matrix filled column-wise.

Explain how to create a matrix in R with 3 rows and 4 columns filled by rows.

Think about the function and its arguments for size and filling order.

What happens if the data vector is shorter than the matrix size when creating a matrix in R?

Consider how R handles insufficient data length.

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