Modifying and adding elements in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code modifying a vector element?

Consider the following R code that modifies an element in a vector. What will be printed?

R Programming

vec <- c(10, 20, 30, 40)
vec[2] <- 25
print(vec)

A [1] 10 20 30 40

B [1] 10 25 30 40

C [1] 25 20 30 40

D [1] 10 25 25 40

💡 Hint

Remember that indexing in R starts at 1 and you can assign a new value to a specific position.

The code changes the second element of the vector from 20 to 25. So the vector becomes 10, 25, 30, 40.

❓ Predict Output

intermediate

What happens when adding a new element to a list in R?

What will be the output of this R code that adds a new element to a list?

R Programming

my_list <- list(a=1, b=2)
my_list$c <- 3
print(my_list)

A Error in my_list$c <- 3 : object is not a list

B [1] 1 2 3

C

$a
[1] 1

$b
[1] 2

$c
[1] 3

D list(a=1, b=2, c=3)

💡 Hint

In R, you can add named elements to a list using the $ operator.

The code adds a new element named 'c' with value 3 to the list. Printing the list shows all elements with their names.

🔧 Debug

advanced

Why does this R code produce an error when modifying a vector element?

Identify the error in this R code and explain why it happens.

R Programming

vec <- c(1, 2, 3)
vec[4] <- 5
print(vec)

A No error; output is [1] 1 2 3 5

B Warning: NAs introduced by coercion

C Error: subscript out of bounds

D Error: object 'vec' not found

💡 Hint

In R, vectors can be extended by assigning to an index beyond their current length.

Assigning to vec[4] adds a new element 5 at position 4. The vector becomes 1, 2, 3, 5 without error.

❓ Predict Output

advanced

What is the output after modifying a data frame column in R?

Given this R code that modifies a data frame column, what will be printed?

R Programming

df <- data.frame(x = 1:3, y = c('a', 'b', 'c'))
df$x[2] <- 10
print(df)

A

  x y
1  1 a
2 10 b
3  3 c

B

  x y
1  1 a
2  2 b
3  3 c

C Error: replacement has length zero

D

  x y
1 10 a
2 10 b
3 10 c

💡 Hint

You can modify specific elements of a data frame column by indexing.

The second element of column x is changed from 2 to 10. Other values remain unchanged.

🧠 Conceptual

expert

How does R handle adding elements beyond current vector length?

In R, what happens when you assign a value to an index beyond the current length of a vector?

A R truncates the vector to the assigned index and assigns the value

B R throws an error: subscript out of bounds

C R ignores the assignment silently without changing the vector

D R extends the vector, filling intermediate positions with NA, then assigns the value

💡 Hint

Think about what happens if you assign to an index that does not yet exist in the vector.

R automatically extends the vector to the new length, filling any new positions with NA before assigning the new value.

Practice

1. In R, how do you change the second element of a vector v to 10?

easy

A. v[2] = 10()

B. v(2) = 10

C. v{2} <- 10

D. v[2] <- 10

Solution

1. Step 1: Understand element access in R vectors

   Elements in R vectors are accessed using square brackets [ ] with the position number inside.

2. Step 2: Assign new value to the element

   Use the assignment operator <- to set the new value at the specified position.

3. Final Answer:

   v[2] <- 10 -> Option D

4. Quick Check:

   Use [ ] and <- to modify elements [OK]

Hint: Use square brackets and <- to change elements [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Adding parentheses after the value
• Using curly braces for indexing

2. Which of the following is the correct syntax to add a new element 5 to the end of vector v in R?

easy

A. v + 5

B. v <- c(v, 5)

C. v[+5]

D. v <- v + c(5)

Solution

1. Step 1: Understand how to add elements to a vector

   In R, use the c() function to combine vectors or add elements.

2. Step 2: Combine existing vector with new element

   Use v <- c(v, 5) to add 5 at the end of vector v.

3. Final Answer:

   v <- c(v, 5) -> Option B

4. Quick Check:

   Use c() to add elements [OK]

Hint: Use c() to add elements to vectors [OK]

Common Mistakes:

• Using + operator to add elements
• Incorrect indexing syntax
• Adding vectors without assignment

3. What will be the output of the following R code?

v <- c(1, 2, 3)
v[1] <- 10
v <- c(v, 4)
v

medium

A. [10, 2, 3]

B. [1, 2, 3, 4]

C. [10, 2, 3, 4]

D. [1, 2, 3]

Solution

1. Step 1: Modify first element of vector

   The first line changes the first element from 1 to 10, so vector becomes (10, 2, 3).

2. Step 2: Add new element 4 at the end

   Using c(v, 4) adds 4 to the end, resulting in (10, 2, 3, 4).

3. Final Answer:

   [10, 2, 3, 4] -> Option C

4. Quick Check:

   Modify then add element = [10, 2, 3, 4] [OK]

Hint: Modify first, then add with c() to get final vector [OK]

Common Mistakes:

• Forgetting to assign after adding element
• Not updating the first element correctly
• Confusing vector output format

4. Identify the error in the following R code that tries to add 7 to vector v:

v <- c(1, 2, 3)
v + 7

medium

A. The code adds 7 to each element, not appends 7

B. Using + operator is correct to add elements

C. v + 7 appends 7 correctly to vector

D. Syntax error due to missing assignment operator

Solution

1. Step 1: Understand vector addition vs adding elements

   Using v + 7 adds 7 to each element, but does not append a new element.

2. Step 2: Check for assignment to update vector

   The code does not assign the result back to v, so vector remains unchanged. Also, to add an element, c() should be used.

3. Final Answer:

   The code adds 7 to each element, not appends 7 -> Option A

4. Quick Check:

   Adding elements needs c() and assignment [OK]

Hint: Use c() and <- to add elements, not + alone [OK]

Common Mistakes:

• Using + to append elements
• Not assigning result back to vector
• Confusing element-wise addition with adding elements

5. Given a vector v <- c(2, 4, 6), which code correctly replaces the last element with 8 and then adds 10 at the end?

hard

A. v[length(v)] <- 8; v <- c(v, 10)

B. v[3] = 8; v + 10

C. v[-1] <- 8; v <- c(v, 10)

D. v[length(v)+1] <- 8; v <- c(v, 10)

Solution

1. Step 1: Replace last element using length()

   Use v[length(v)] <- 8 to change the last element (position 3) to 8.

2. Step 2: Add 10 at the end with c()

   Use v <- c(v, 10) to append 10 to the vector.

3. Final Answer:

   v[length(v)] <- 8; v <- c(v, 10) -> Option A

4. Quick Check:

   Use length() to index last element, c() to add [OK]

Hint: Use length(v) to find last element, c() to add new [OK]

Common Mistakes:

• Using negative index incorrectly
• Adding element with + operator
• Assigning to length(v)+1 without c()