Special operators (%in%, %*%) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of %in% operator with factors

What is the output of this R code?

f1 <- factor(c("apple", "banana", "cherry"))
f2 <- factor(c("banana", "date", "apple"))
f1 %in% f2

R Programming

f1 <- factor(c("apple", "banana", "cherry"))
f2 <- factor(c("banana", "date", "apple"))
f1 %in% f2

A [1] TRUE TRUE FALSE

B [1] FALSE TRUE TRUE

C [1] TRUE FALSE TRUE

D [1] FALSE FALSE FALSE

💡 Hint

Remember that %in% checks if elements of the left vector are present in the right vector, comparing levels for factors.

The %in% operator returns a logical vector indicating if each element of f1 is found in f2. 'apple' and 'banana' are in f2, but 'cherry' is not.

❓ Predict Output

intermediate

Matrix multiplication with %*%

What is the result of this R code?

A <- matrix(c(1, 2, 3, 4), nrow=2)
B <- matrix(c(5, 6, 7, 8), nrow=2)
A %*% B

R Programming

A <- matrix(c(1, 2, 3, 4), nrow=2)
B <- matrix(c(5, 6, 7, 8), nrow=2)
A %*% B

A

     [,1] [,2]
[1,]   23   31
[2,]   34   46

B

     [,1] [,2]
[1,]   23   34
[2,]   31   46

C

     [,1] [,2]
[1,]   26   30
[2,]   38   44

D

     [,1] [,2]
[1,]   17   20
[2,]   39   46

💡 Hint

Recall matrix multiplication rules: multiply rows of A by columns of B and sum.

The product matrix is calculated as: element (1,1) = 1*5 + 3*6 = 23, element (1,2) = 1*7 + 3*8 = 31, element (2,1) = 2*5 + 4*6 = 34, element (2,2) = 2*7 + 4*8 = 46.

❓ Predict Output

advanced

Behavior of %in% with NA values

What is the output of this R code?

x <- c(1, 2, NA, 4)
y <- c(2, NA, 5)
x %in% y

R Programming

x <- c(1, 2, NA, 4)
y <- c(2, NA, 5)
x %in% y

A [1] FALSE TRUE FALSE FALSE

B [1] FALSE TRUE TRUE FALSE

C [1] FALSE TRUE NA FALSE

D [1] FALSE TRUE NA NA

💡 Hint

Think about how NA values are treated in logical comparisons with %in%.

The %in% operator returns NA when comparing NA to NA because NA means unknown, so it cannot confirm presence or absence. So the third element is NA.

❓ Predict Output

advanced

Matrix multiplication dimension mismatch error

What error does this R code produce?

A <- matrix(1:6, nrow=2)
B <- matrix(1:6, nrow=2)
A %*% B

R Programming

A <- matrix(1:6, nrow=2)
B <- matrix(1:6, nrow=2)
A %*% B

A Error in A %*% B : missing values in matrix

B Error in A %*% B : object 'B' not found

C Error in A %*% B : invalid matrix multiplication

D Error in A %*% B : non-conformable arguments

💡 Hint

Check the dimensions of matrices for multiplication compatibility.

Matrix multiplication requires the number of columns in A to equal the number of rows in B. A is 2x3, B is 2x3, so columns of A (3) does not match rows of B (2), producing 'Error in A %*% B : non-conformable arguments'.

❓ Predict Output

expert

Complex use of %in% with lists and vectors

What is the output of this R code?

lst <- list(a = 1:3, b = 4:6)
v <- c(2, 5, 7)
sapply(lst, function(x) v %in% x)

R Programming

lst <- list(a = 1:3, b = 4:6)
v <- c(2, 5, 7)
sapply(lst, function(x) v %in% x)

A

       a     b
[1,] TRUE TRUE
[2,] FALSE TRUE
[3,] FALSE FALSE

B

       a     b
[1,] TRUE FALSE
[2,] FALSE TRUE
[3,] FALSE FALSE

C

       a     b
[1,] TRUE FALSE
[2,] TRUE TRUE
[3,] FALSE FALSE

D

       a     b
[1,] TRUE FALSE
[2,] FALSE TRUE
[3,] TRUE FALSE

💡 Hint

Check each element of v against each list element using %in%, then sapply returns a matrix.

For list element 'a' (1:3), 2 is in it (TRUE), 5 and 7 are not (FALSE). For 'b' (4:6), 5 is in it (TRUE), 2 and 7 are not (FALSE). So the matrix shows TRUE/FALSE accordingly.

Practice

1. What does the %in% operator do in R?

easy

A. Calculates the sum of two vectors

B. Performs element-wise multiplication of two vectors

C. Performs matrix multiplication

D. Checks if elements of one vector are present in another vector

Solution

1. Step 1: Understand the purpose of %in%

   The %in% operator is used to check membership, meaning it tests if elements of one vector exist in another.

2. Step 2: Compare with other operations

   Other options describe arithmetic or matrix operations, which are not the function of %in%.

3. Final Answer:

   Checks if elements of one vector are present in another vector -> Option D

4. Quick Check:

   %in% = membership check [OK]

Hint: Remember: %in% means 'is in' for membership [OK]

Common Mistakes:

• Confusing %in% with arithmetic operators
• Thinking %in% does matrix multiplication
• Assuming %in% returns numeric results

2. Which of the following is the correct syntax for matrix multiplication in R?

easy

A. A * B

B. A %*% B

C. A %in% B

D. A + B

Solution

1. Step 1: Identify matrix multiplication operator

   In R, matrix multiplication is done using %*%, so A %*% B is correct syntax.

2. Step 2: Differentiate from other operators

   * is element-wise multiplication, %in% is membership, and + is addition.

3. Final Answer:

   A %*% B -> Option B

4. Quick Check:

   Matrix multiply = %*% [OK]

Hint: Use %*% for matrix multiply, * for element-wise [OK]

Common Mistakes:

• Using * instead of %*% for matrix multiplication
• Confusing %in% with multiplication
• Using + for multiplication

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

x <- c(3, 1, 5)
y <- c(3, 4, 5, 6)
x %in% y

medium

A. [TRUE, FALSE, TRUE]

B. [FALSE, TRUE, FALSE]

C. [TRUE, TRUE, TRUE]

D. [FALSE, FALSE, FALSE]

Solution

1. Step 1: Check each element of x in y

   3 is in y, so TRUE; 1 is not in y, so FALSE; 5 is in y, TRUE.

2. Step 2: Form the logical vector

   The result is [TRUE, FALSE, TRUE].

3. Final Answer:

   [TRUE, FALSE, TRUE] -> Option A

4. Quick Check:

   %in% returns logical vector [OK]

Hint: Check each element presence one by one [OK]

Common Mistakes:

• Mixing order of TRUE/FALSE in output
• Assuming %in% returns indices
• Confusing %in% with element-wise comparison

4. Identify the error in the following R code:

A <- matrix(c(1,2,3,4), nrow=2)
B <- matrix(c(5,6,7,8), nrow=2)
C <- A *%* B

medium

A. Matrices A and B have incompatible dimensions

B. Missing library for matrix multiplication

C. Incorrect operator *%* used for matrix multiplication

D. Matrix A is not defined properly

Solution

1. Step 1: Check operator syntax

   The operator *%* is invalid in R; correct operator is %*%.

2. Step 2: Verify matrix dimensions and definitions

   Both matrices are 2x2, so dimensions are compatible and matrices are defined correctly.

3. Final Answer:

   Incorrect operator *%* used for matrix multiplication -> Option C

4. Quick Check:

   Matrix multiply operator is %*% [OK]

Hint: Use %*% exactly for matrix multiplication [OK]

Common Mistakes:

• Typing operators in wrong order
• Assuming *%* is valid
• Confusing element-wise * with matrix multiply

5. Given two matrices:

M1 <- matrix(c(1, 0, 2, 3), nrow=2)
M2 <- matrix(c(4, 1, 0, 2), nrow=2)

Which R expression correctly computes the matrix product and then checks if the number 5 is in the resulting matrix?

hard

A. 5 %in% (M1 %*% M2)

B. M1 *%*% M2 %in% 5

C. M1 %in% 5 %*% M2

D. M1 %*% M2 == 5

Solution

1. Step 1: Perform matrix multiplication correctly

   Use %*% to multiply M1 and M2: M1 %*% M2.

2. Step 2: Check membership of 5 in the product matrix

   Use %in% to check if 5 is present in the resulting matrix: 5 %in% (M1 %*% M2).

3. Final Answer:

   5 %in% (M1 %*% M2) -> Option A

4. Quick Check:

   Matrix multiply then membership check = 5 %in% (M1 %*% M2) [OK]

Hint: Chain %*% then %in% with parentheses [OK]

Common Mistakes:

• Using wrong operator order
• Typing *%*% instead of %*%
• Using == instead of %in% for membership