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.