For loop in R Programming - Step-by-Step Execution

Concept Flow - For loop

Initialize loop variable i = 1

↓

Check condition: i <= 5?

No→EXIT loop

Yes↓

Execute loop body

↓

Update i: i = i + 1

↩Back to condition check

The loop starts by setting a variable, checks if it meets a condition, runs the code inside if yes, then updates the variable and repeats until the condition is false.

Execution Sample

R Programming

for (i in 1:5) {
  print(i)
}

This code prints numbers from 1 to 5, one number per line.

Execution Table

Iteration	i	Condition (i in 1:5)	Action	Output
1	1	TRUE	print(i)	1
2	2	TRUE	print(i)	2
3	3	TRUE	print(i)	3
4	4	TRUE	print(i)	4
5	5	TRUE	print(i)	5

💡 After iteration 5, the sequence 1:5 is exhausted (no more values), so the loop stops. i remains 5.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
i	NA	1	2	3	4	5	5

Key Moments - 2 Insights

Why does the loop stop after printing 5?

Because the loop variable i goes from 1 to 5. After i=5, there are no more values in the sequence 1:5, so the loop ends (see execution_table).

Is the loop variable i available after the loop ends?

Yes, in R the variable i still exists after the loop and has the last value it took, which is 5 here (see variable_tracker final value).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i during the 3rd iteration?

A 3

B 2

C 4

D 5

💡 Hint

Check the 'i' column in the execution_table row for iteration 3.

How many times does the loop body run?

A 6

B 1

C 5

D 7

💡 Hint

Count the number of rows where Condition is TRUE or Action is 'print(i)' in the execution_table.

If the loop was changed to for(i in 1:3), how many times would the loop body run?

A 5 times

B 3 times

C 6 times

D 1 time

💡 Hint

The loop runs once for each number in the sequence 1:3, check variable_tracker for similar pattern.

Concept Snapshot

for (variable in sequence) {
  # code to repeat
}

- Runs code for each value in sequence
- variable takes each value in order
- Stops after last value
- Loop variable keeps last value after loop

Full Transcript

A for loop in R repeats a block of code for each value in a sequence. It starts by setting a variable to the first value, checks if it is in the sequence, runs the code, then moves to the next value. This continues until all values are used. In the example, the loop prints numbers 1 to 5. The variable i changes from 1 to 5 during the loop and remains 5 after the loop ends because the sequence is exhausted. This shows how the loop controls repetition clearly and simply.

Practice

1. What does a for loop do in R programming?

easy

A. Stops the program immediately

B. Creates a new variable

C. Defines a new function

D. Repeats a block of code for each item in a sequence

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is used to repeat actions multiple times, once for each item in a sequence.

2. Step 2: Match the description to the options

   Repeats a block of code for each item in a sequence correctly describes this behavior, while others describe unrelated actions.

3. Final Answer:

   Repeats a block of code for each item in a sequence -> Option D

4. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops repeat code for each item in a list or sequence [OK]

Common Mistakes:

• Confusing for loop with function definition
• Thinking for loop stops the program
• Believing for loop creates variables only

2. Which of the following is the correct syntax for a for loop in R?

easy

A. for (i; i < 5; i++) { print(i) }

B. for (i in 1:5) { print(i) }

C. for i = 1 to 5 { print(i) }

D. loop (i in 1 to 5) { print(i) }

Solution

1. Step 1: Recall R for loop syntax

   In R, the for loop uses the format: for (variable in sequence) { code }

2. Step 2: Compare options to correct syntax

   for (i in 1:5) { print(i) } matches the correct syntax. Options A, B, and D use syntax from other languages or incorrect forms.

3. Final Answer:

   for (i in 1:5) { print(i) } -> Option B

4. Quick Check:

   R for loop = for (var in sequence) [OK]

Hint: Use 'for (var in sequence)' syntax in R [OK]

Common Mistakes:

• Using 'for i = 1 to 5' like other languages
• Using C-style for loop syntax
• Writing 'loop' instead of 'for'

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

for (i in 1:3) {
  print(i * 2)
}

medium

A. [1] 2 [1] 4 [1] 6

B. [1] 1 [1] 2 [1] 3

C. [1] 3 [1] 6 [1] 9

D. Error: object 'i' not found

Solution

1. Step 1: Understand the loop iterations

   The loop runs with i = 1, 2, 3.

2. Step 2: Calculate print output for each i

   For i=1: print(2), i=2: print(4), i=3: print(6).

3. Final Answer:

   [1] 2 [1] 4 [1] 6 -> Option A

4. Quick Check:

   Multiply i by 2 each loop [OK]

Hint: Multiply loop variable by 2 inside print [OK]

Common Mistakes:

• Printing i instead of i*2
• Confusing output format
• Expecting a single combined print

4. Identify the error in this R for loop:

for i in 1:4 {
  print(i)
}

medium

A. Loop variable should be a string

B. Using curly braces instead of parentheses

C. Missing parentheses around the loop variable and sequence

D. Incorrect print function usage

Solution

1. Step 1: Check for correct for loop syntax in R

   R requires parentheses around the loop variable and sequence: for (i in 1:4) { ... }

2. Step 2: Identify the error in given code

   The code misses parentheses around 'i in 1:4', causing a syntax error.

3. Final Answer:

   Missing parentheses around the loop variable and sequence -> Option C

4. Quick Check:

   For loop needs parentheses ( ) [OK]

Hint: Always put parentheses after 'for' in R [OK]

Common Mistakes:

• Omitting parentheses around 'i in 1:4'
• Confusing braces and parentheses
• Thinking print syntax is wrong

5. You want to create a vector of squares of numbers 1 to 5 using a for loop in R. Which code correctly does this?

hard

A. squares <- c()
for (i in 1:5) {
squares[i] <- i^2
}
squares

B. for (i in 1:5) {
squares <- i^2
}
squares

C. squares <- 1:5
for (i in squares) {
squares[i] = i * 2
}
squares

D. squares <- c()
for i in 1:5 {
squares[i] <- i^2
}
squares

Solution

1. Step 1: Initialize an empty vector to store results

   squares <- c() for (i in 1:5) { squares[i] <- i^2 } squares initializes 'squares' as an empty vector to store squares.

2. Step 2: Use a for loop to assign squares to each position

   Inside the loop, squares[i] <- i^2 correctly stores the square of i at position i.

3. Step 3: Check other options for errors

   for (i in 1:5) { squares <- i^2 } squares overwrites 'squares' each time, losing previous values. squares <- 1:5 for (i in squares) { squares[i] = i * 2 } squares multiplies by 2, not square. squares <- c() for i in 1:5 { squares[i] <- i^2 } squares misses parentheses in for loop syntax.

4. Final Answer:

   squares <- c() for (i in 1:5) { squares[i] <- i^2 } squares -> Option A

5. Quick Check:

   Initialize vector, assign squares in loop [OK]

Hint: Initialize vector before loop, assign squares by index [OK]

Common Mistakes:

• Not initializing vector before loop
• Overwriting vector inside loop
• Wrong loop syntax without parentheses