Vector indexing (1-based) in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 1-based indexing mean in R vectors?

It means the first element of a vector is accessed with index 1, not 0 like in some other languages.

beginner

How do you access the third element of a vector v in R?

Use v[3] to get the third element because R starts counting at 1.

intermediate

What happens if you try to access an index that is out of range in an R vector?

R returns NA (missing value) instead of an error.

beginner

How can you access multiple elements at once in an R vector?

Use a vector of indices inside the square brackets, e.g., v[c(1,3,5)] to get elements 1, 3, and 5.

intermediate

Why is it important to remember R uses 1-based indexing when translating code from languages like Python?

Because Python uses 0-based indexing, so the same index number refers to different elements. Forgetting this can cause bugs.

In R, what index accesses the first element of a vector?

A 0

B 1

C -1

D None of the above

R uses 1-based indexing, so the first element is at index 1.

What does v[0] return if v is a vector in R?

A The first element

B An error

C NA

D An empty vector

Index 0 returns an empty vector in R, not an element or error.

How do you get the 2nd and 4th elements of vector v in R?

A v[2,4]

B v(2,4)

C v[c(2,4)]

D v[2:4]

Use v[c(2,4)] to select specific elements by index.

What does R return if you access an index larger than the vector length?

A NA

B An error

C The last element

D NULL

R returns NA for out-of-range indices.

Why might code using 0-based indexing cause bugs when ported to R?

A R uses 1-based indexing, so indices shift by one

B R does not support indexing

C R only supports negative indices

D R vectors are unordered

Because R starts counting at 1, using 0-based indices will access wrong elements or empty results.

Explain how vector indexing works in R and how it differs from 0-based indexing languages.

Think about how you count items in a list starting from one.

Describe what happens when you try to access an index outside the range of an R vector.

Consider what R does when you ask for something that does not exist.

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