Vector indexing (1-based) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code using 1-based indexing?

Consider the vector v <- c(10, 20, 30, 40, 50). What is the output of v[3]?

R Programming

v <- c(10, 20, 30, 40, 50)
v[3]

A [1] 30

B [1] 20

C [1] 40

D [1] 10

💡 Hint

Remember R vectors start counting at 1, not 0.

In R, vector indexing starts at 1, so v[3] returns the third element, which is 30.

❓ Predict Output

intermediate

What does this R code output when indexing with a vector?

Given v <- c(5, 10, 15, 20, 25), what is the output of v[c(2, 4)]?

R Programming

v <- c(5, 10, 15, 20, 25)
v[c(2, 4)]

A [1] 10 20

B [1] 5 15

C [1] 15 25

D [1] 20 25

💡 Hint

Indexing with a vector returns elements at those positions.

v[c(2,4)] returns the elements at positions 2 and 4, which are 10 and 20.

🧠 Conceptual

advanced

What error does this R code raise when indexing with zero?

What happens when you run v <- c(1,2,3); v[0] in R?

R Programming

v <- c(1,2,3)
v[0]

A Throws an error: subscript out of bounds

B Returns the first element of v

C Returns an empty vector with no elements

D Returns NULL

💡 Hint

In R, zero is a special index that means no elements.

Indexing with 0 returns an empty vector of the same type, no error is raised.

❓ Predict Output

advanced

What is the output of negative indexing in R?

Given v <- c(2, 4, 6, 8, 10), what does v[-c(1,3)] return?

R Programming

v <- c(2, 4, 6, 8, 10)
v[-c(1,3)]

A [1] 2 6 8 10

B [1] 4 8 10

C [1] 2 4 6

D [1] 4 6 8 10

💡 Hint

Negative indices exclude those positions from the result.

v[-c(1,3)] excludes elements at positions 1 and 3, returning elements at positions 2,4,5.

❓ Predict Output

expert

What is the output of this complex vector indexing in R?

Given v <- c(3, 6, 9, 12, 15), what is the output of v[c(TRUE, FALSE, TRUE, FALSE, TRUE)]?

R Programming

v <- c(3, 6, 9, 12, 15)
v[c(TRUE, FALSE, TRUE, FALSE, TRUE)]

A [1] 6 12

B [1] 15

C [1] 3 6 9

D [1] 3 9 15

💡 Hint

Logical vectors select elements where TRUE appears.

The logical vector selects elements at positions 1,3,5 which are 3, 9, and 15.

Practice

1. In R, what is the position of the first element in a vector v when using indexing like v[1]?

easy

A. The first element (position 1)

B. The zeroth element (position 0)

C. The last element

D. An error because indexing starts at 0

Solution

1. Step 1: Understand R vector indexing

   R vectors start indexing at 1, not 0 like some other languages.

2. Step 2: Apply indexing to vector v

   Using v[1] selects the first element of the vector.

3. Final Answer:

   The first element (position 1) -> Option A

4. Quick Check:

   R indexing starts at 1 = first element [OK]

Hint: Remember: R indexing always starts at 1, never 0 [OK]

Common Mistakes:

• Thinking indexing starts at 0 like in Python or C
• Confusing position 1 with last element
• Assuming indexing causes an error if 0 is used

2. Which of the following is the correct syntax to get the third element from a vector vec in R?

easy

A. vec[3]

B. vec<3>

C. vec{3}

D. vec(3)

Solution

1. Step 1: Recall R vector indexing syntax

   In R, square brackets [] are used to access elements by position.

2. Step 2: Identify correct syntax for third element

   Using vec[3] correctly accesses the third element.

3. Final Answer:

   vec[3] -> Option A

4. Quick Check:

   Use square brackets for indexing in R [OK]

Hint: Use square brackets [] to index vectors in R [OK]

Common Mistakes:

• Using parentheses () instead of square brackets
• Using curly braces {} which are invalid for indexing
• Using angle brackets <> which are not valid syntax

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

nums <- c(10, 20, 30, 40, 50)
nums[2:4]

medium

A. [10 20 30]

B. [20 30 40]

C. [20 30 40 50]

D. [30 40 50]

Solution

1. Step 1: Understand the vector and indexing range

   The vector nums has elements 10, 20, 30, 40, 50 at positions 1 to 5.

2. Step 2: Extract elements from position 2 to 4

   Using nums[2:4] selects elements at positions 2, 3, and 4 which are 20, 30, and 40.

3. Final Answer:

   [20 30 40] -> Option B

4. Quick Check:

   Positions 2 to 4 = 20, 30, 40 [OK]

Hint: Use colon : to select a range of positions [OK]

Common Mistakes:

• Including position 5 by mistake
• Starting range at 1 instead of 2
• Confusing vector values with positions

4. Consider the R code below. What is the error and how to fix it?

vals <- c(5, 10, 15, 20)
print(vals[0])

medium

A. Error: Use parentheses instead of brackets

B. No error; prints 5

C. Error: Index 0 is invalid; use index starting at 1

D. Error: Vector must be numeric

Solution

1. Step 1: Identify the indexing error

   In R, vector indexing starts at 1; index 0 is invalid and returns an empty vector.

2. Step 2: Correct the index to start at 1

   Replace vals[0] with vals[1] to access the first element.

3. Final Answer:

   Error: Index 0 is invalid; use index starting at 1 -> Option C

4. Quick Check:

   R indexing starts at 1, not 0 [OK]

Hint: Never use 0 as index in R vectors [OK]

Common Mistakes:

• Assuming 0 is a valid index like in other languages
• Trying to use parentheses for indexing
• Thinking vector type causes the error

5. You have a vector data <- c(3, 6, 9, 12, 15). How do you create a new vector containing only the 1st, 3rd, and 5th elements using vector indexing in R?

hard

A. data[1, 3, 5]

B. data[1:3:5]

C. data[c(0, 2, 4)]

D. data[c(1, 3, 5)]

Solution

1. Step 1: Understand how to select multiple elements

   In R, use c() inside square brackets to select multiple positions.

2. Step 2: Apply correct indexing for positions 1, 3, and 5

   Using data[c(1, 3, 5)] selects elements at these positions: 3, 9, and 15.

3. Final Answer:

   data[c(1, 3, 5)] -> Option D

4. Quick Check:

   Use c() inside [] to pick multiple positions [OK]

Hint: Use c() inside [] to select multiple indices [OK]

Common Mistakes:

• Using colon with multiple steps incorrectly
• Using commas inside [] without c()
• Using zero-based indices like 0, 2, 4