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.

Practice

1. In R, what does the apply() function do when used on a matrix?

easy

A. It creates a new matrix with doubled values.

B. It converts the matrix into a vector.

C. It sorts the matrix elements in ascending order.

D. It applies a function to each row or column of the matrix.

Solution

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

   The apply() function is designed to run a function on margins of an array or matrix.

2. Step 2: Identify what margins mean in a matrix

   Margin 1 means rows, margin 2 means columns. So apply() runs the function on each row or column.

3. Final Answer:

   It applies a function to each row or column of the matrix. -> Option D

4. Quick Check:

   apply() = function on rows/columns [OK]

Hint: Remember: MARGIN=1 rows, MARGIN=2 columns in apply() [OK]

Common Mistakes:

• Thinking apply() changes matrix shape automatically
• Confusing apply() with sorting or vector conversion
• Using apply() without specifying margin

2. Which of the following is the correct syntax to calculate the sum of each column in a matrix m using apply() in R?

easy

A. apply(m, 2, sum)

B. apply(m, sum, 2)

C. apply(m, 1, sum)

D. apply(sum, m, 2)

Solution

1. Step 1: Identify margin for columns

   In apply(), margin 2 means columns.

2. Step 2: Check function placement

   The function to apply (sum) is the third argument, so syntax is apply(m, 2, sum).

3. Final Answer:

   apply(m, 2, sum) -> Option A

4. Quick Check:

   sum columns = apply(m, 2, sum) [OK]

Hint: Use margin=2 for columns in apply() [OK]

Common Mistakes:

• Using margin=1 for columns instead of 2
• Swapping function and margin arguments
• Missing the function argument

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

m <- matrix(1:6, nrow=2)
apply(m, 1, max)

medium

A. [1] 3 6

B. [1] 4 5

C. [1] 5 6

D. [1] 2 4

Solution

1. Step 1: Understand the matrix structure

   The matrix m is created with values 1 to 6, 2 rows: Row 1: 1, 3, 5 Row 2: 2, 4, 6

2. Step 2: Apply max function on rows (margin=1)

   Max of row 1 is 5, max of row 2 is 6.

3. Final Answer:

   [1] 5 6 -> Option C

4. Quick Check:

   max per row = 5, 6 [OK]

Hint: Check matrix layout before applying functions [OK]

Common Mistakes:

• Confusing rows and columns in margin
• Misreading matrix values
• Assuming apply returns matrix instead of vector

4. Identify the error in this R code snippet:

m <- matrix(1:9, nrow=3)
apply(m, 3, mean)

medium

A. apply() cannot be used with mean.

B. Margin 3 is invalid for a 2D matrix.

C. Matrix m is not created properly.

D. The function mean is not defined.

Solution

1. Step 1: Check matrix dimensions

   The matrix m has 2 dimensions (rows and columns).

2. Step 2: Validate margin argument

   Margin 3 refers to a third dimension, which does not exist in a 2D matrix.

3. Final Answer:

   Margin 3 is invalid for a 2D matrix. -> Option B

4. Quick Check:

   Margin must be 1 or 2 for matrices [OK]

Hint: Use only 1 or 2 as margin for 2D matrices [OK]

Common Mistakes:

• Using margin > 2 for 2D matrices
• Assuming apply works on non-existent dimensions
• Confusing apply() with array functions

5. You have a matrix m of numeric values. You want to create a new vector that contains the mean of each row but only for rows where the sum is greater than 10. Which R code correctly does this using apply()?

hard

A. apply(m, 1, function(x) if(sum(x) > 10) mean(x) else NA)[!is.na(.)]

B. apply(m, 1, function(x) if(sum(x) > 10) mean(x))

C. apply(m, 1, function(x) mean(x[sum(x) > 10]))

D. apply(m, 2, function(x) if(sum(x) > 10) mean(x))

Solution

1. Step 1: Understand the requirement

   We want mean of each row only if that row's sum is > 10, else exclude it.

2. Step 2: Analyze each option

   A returns NA for rows not >10 but does not remove them. C incorrectly filters inside mean. D uses margin=2 (columns) instead of rows.

3. Step 3: Correct approach

   apply(m, 1, function(x) if(sum(x) > 10) mean(x) else NA)[!is.na(.)] returns NA for rows with sum ≤ 10, then removes NAs with [!is.na(.)], giving only desired means.

4. Final Answer:

   apply(m, 1, function(x) if(sum(x) > 10) mean(x) else NA)[!is.na(.)] -> Option A

5. Quick Check:

   Filter rows by sum > 10, then mean [OK]

Hint: Use if-else inside apply and remove NA after [OK]

Common Mistakes:

• Using margin=2 instead of 1 for rows
• Not handling rows with sum ≤ 10 properly
• Trying to filter inside mean() incorrectly