Vector indexing (1-based) in R Programming - Step-by-Step Execution

Concept Flow - Vector indexing (1-based)

Start with vector

↓

Choose index i (1-based)

↓

Access element at position i

↓

Return element value

↓

End

In R, vectors start counting at 1. You pick an index starting from 1 to get the element at that position.

Execution Sample

R Programming

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

This code picks the second element from the vector and prints it.

Execution Table

Step	Variable	Value	Action	Output
1	vec	c(10, 20, 30, 40)	Create vector
2	index	2	Set index to 2
3	value	vec[2]	Access element at position 2	20
4	print(value)		Print the value	20
5			End of execution

💡 Index 2 is valid, so element 20 is returned and printed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
vec	undefined	c(10, 20, 30, 40)	c(10, 20, 30, 40)	c(10, 20, 30, 40)	c(10, 20, 30, 40)
index	undefined	undefined	2	2	2
value	undefined	undefined	undefined	20	20

Key Moments - 3 Insights

Why does R start counting vector positions from 1, not 0?

R uses 1-based indexing, so the first element is at position 1. This is shown in the execution_table where index 2 accesses the second element.

What happens if you try to access vec[0] in R?

Accessing vec[0] returns an empty vector because R does not use zero-based indexing. This differs from some other languages.

Why does vec[2] return 20 and not 10?

Because indexing starts at 1, vec[1] is 10 and vec[2] is 20, as shown in the execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'value' after step 3?

A 30

B 20

C 10

D undefined

💡 Hint

Check the 'value' variable in the execution_table row for step 3.

At which step is the vector 'vec' created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in the execution_table for vector creation.

If index was set to 3 instead of 2, what would 'value' be after step 3?

A 20

B 40

C 30

D 10

💡 Hint

Refer to the vector elements in variable_tracker and the meaning of index.

Concept Snapshot

Vector indexing in R starts at 1.
Use vec[i] to get the element at position i.
Index 1 is the first element.
Accessing index 0 returns empty.
Always count positions starting from 1.

Full Transcript

This visual trace shows how vector indexing works in R, which uses 1-based counting. We create a vector with four numbers, then pick an index starting at 1. Accessing vec[2] returns the second element, 20. The variable tracker shows how variables change step by step. Key moments clarify why R starts at 1 and what happens if you use zero. The quiz tests understanding of variable values and indexing steps.

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