Vector indexing (1-based) in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the first element of the vector.

R Programming

vec <- c(10, 20, 30)
first_element <- vec[[1]]

Drag options to blanks, or click blank then click option'

A0

B1

C-1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as an index, which returns an empty vector.

Using negative indices incorrectly to exclude elements.

In R, vector indexing starts at 1, so the first element is accessed with index 1.

2fill in blank

medium

Complete the code to access the third element of the vector.

R Programming

numbers <- c(5, 15, 25, 35)
third <- numbers[[1]]

Drag options to blanks, or click blank then click option'

A2

B0

C4

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 4 to access the third element.

Confusing zero-based indexing from other languages.

The third element is accessed with index 3 because R uses 1-based indexing.

3fill in blank

hard

Fix the error in the code to correctly access the second element.

R Programming

values <- c(100, 200, 300)
second_value <- values[[1]]

Drag options to blanks, or click blank then click option'

A2

B3

C-2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or negative numbers as indices.

Using index 3 which accesses the third element instead.

Index 2 accesses the second element in R vectors.

4fill in blank

hard

Fill both blanks to create a vector of squares for elements greater than 2.

R Programming

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

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A>

B<

C**

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for filtering.

Using ** which is not the exponent operator in R.

We select elements greater than 2 using > and square them using the ^ operator.

5fill in blank

hard

Complete the code to access the last element of the vector.

R Programming

vec <- c(10, 20, 30, 40, 50)
last_element <- vec[[1]]

Drag options to blanks, or click blank then click option'

A-1

B5

Clength(vec)

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1, which excludes the first element in R.

Hardcoding the value 5.

Using 0 as an index.

In R, the last element can be accessed using length(vec) as the index since indexing is 1-based.

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