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

Concept Flow - While loop

Initialize variable

↓

Check condition

|Yes↓

Execute loop body

↓

Update variable

Back to Check condition↓

Exit loop

↩Back to Check condition

The while loop starts by checking a condition. If true, it runs the loop body and updates variables, then checks again. It stops when the condition is false.

Execution Sample

R Programming

x <- 1
while (x <= 3) {
  print(x)
  x <- x + 1
}

This code prints numbers 1 to 3 by increasing x until the condition x <= 3 is false.

Execution Table

Step	x value	Condition (x <= 3)	Action	Output
1	1	TRUE	Print 1, x <- 2	1
2	2	TRUE	Print 2, x <- 3	2
3	3	TRUE	Print 3, x <- 4	3
4	4	FALSE	Exit loop

💡 x reaches 4, condition 4 <= 3 is FALSE, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
x	1	2	3	4	4

Key Moments - 2 Insights

Why does the loop stop when x is 4 even though the loop body updates x after printing?

The condition is checked before each loop iteration (see Step 4 in execution_table). When x becomes 4, the condition x <= 3 is FALSE, so the loop exits before printing or updating again.

What happens if the initial value of x is greater than 3?

The condition is FALSE at the start, so the loop body never runs (no output). This is shown by the condition check before the first iteration.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x at Step 3?

A 3

B 2

C 4

D 1

💡 Hint

Check the 'x value' column in row for Step 3 in execution_table.

At which step does the condition become FALSE and the loop stops?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table to find when it is FALSE.

If we start with x <- 5, what will happen to the loop execution?

A Loop runs once and prints 5

B Loop runs infinitely

C Loop does not run at all

D Loop runs twice

💡 Hint

Refer to key_moments about initial condition check before loop body execution.

Concept Snapshot

while (condition) {
  # code to repeat
}

- Checks condition before each loop
- Runs code block if TRUE
- Stops when condition is FALSE
- Useful for repeating until a condition changes

Full Transcript

This visual trace shows how a while loop works in R. We start with x = 1. The loop checks if x is less or equal to 3. If yes, it prints x and adds 1 to x. This repeats until x becomes 4, when the condition fails and the loop stops. The variable tracker shows x changing from 1 to 4 step by step. Key moments explain why the loop stops before printing 4 and what happens if x starts bigger than 3. The quiz tests understanding of variable values and loop stopping conditions.

Practice

1. What does a while loop do in R?

easy

A. Executes code only once

B. Repeats code a fixed number of times

C. Repeats code as long as a condition is TRUE

D. Stops the program immediately

Solution

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

   A while loop runs repeatedly while its condition remains TRUE.

2. Step 2: Compare options

   Only Repeats code as long as a condition is TRUE correctly describes this behavior; others describe different concepts.

3. Final Answer:

   Repeats code as long as a condition is TRUE -> Option C

4. Quick Check:

   while loop = repeat while TRUE [OK]

Hint: While loops run while condition is TRUE, not fixed times [OK]

Common Mistakes:

• Confusing while loop with for loop
• Thinking it runs only once
• Believing it stops the program

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

easy

A. while (i <= 5) { print(i); i <- i + 1 }

B. while i <= 5 { print(i); i <- i + 1 }

C. while i <= 5: print(i); i <- i + 1

D. while (i <= 5) print(i) i <- i + 1

Solution

1. Step 1: Check R while loop syntax

   R requires parentheses around the condition and braces for the loop body.

2. Step 2: Validate each option

   Only while (i <= 5) { print(i); i <- i + 1 } uses correct parentheses and braces syntax.

3. Final Answer:

   while (i <= 5) { print(i); i <- i + 1 } -> Option A

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

Hint: Use parentheses for condition and braces for loop body [OK]

Common Mistakes:

• Omitting parentheses around condition
• Missing braces for multiple statements
• Using colon or wrong punctuation

3. What is the output of this R code?

i <- 1
while (i <= 3) {
  print(i)
  i <- i + 1
}

medium

A. Error: object 'i' not found

B. [1] 1 [1] 2

C. [1] 3

D. [1] 1 [1] 2 [1] 3

Solution

1. Step 1: Trace the loop iterations

   i starts at 1, loop runs while i <= 3, printing i each time and increasing i by 1.

2. Step 2: List printed values

   Values printed are 1, then 2, then 3 before loop stops.

3. Final Answer:

   [1] 1 [1] 2 [1] 3 -> Option D

4. Quick Check:

   Prints 1, 2, 3 in order [OK]

Hint: Increment loop variable inside while to avoid infinite loop [OK]

Common Mistakes:

• Forgetting to increment i causing infinite loop
• Expecting only last value printed
• Confusing output format

4. Identify the error in this R while loop code:

i <- 1
while (i < 5) {
  print(i)
}

medium

A. Missing increment of i inside the loop

B. Incorrect condition syntax

C. print() function is invalid

D. Variable i is not initialized

Solution

1. Step 1: Analyze loop body

   The loop prints i but never changes i, so i stays 1 forever.

2. Step 2: Understand consequence

   Without increment, the condition i < 5 is always TRUE, causing infinite loop.

3. Final Answer:

   Missing increment of i inside the loop -> Option A

4. Quick Check:

   Update loop variable to stop loop [OK]

Hint: Always update loop variable inside while to avoid infinite loop [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Assuming print causes increment
• Misreading condition syntax

5. How can you use a while loop in R to calculate the factorial of 5?
Choose the correct code snippet.

hard

A. n <- 5
fact <- 0
i <- 1
while (i < n) {
fact <- fact * i
i <- i + 1
}
fact

B. n <- 5
fact <- 1
i <- 1
while (i <= n) {
fact <- fact * i
i <- i + 1
}
fact

C. n <- 5
fact <- 1
while (n > 0) {
fact <- fact + n
n <- n - 1
}
fact

D. n <- 5
fact <- 1
while (fact <= n) {
fact <- fact * n
n <- n - 1
}
fact

Solution

1. Step 1: Understand factorial calculation

   Factorial multiplies numbers from 1 to n, so loop must multiply fact by i from 1 to n.

2. Step 2: Check each code snippet

   n <- 5 fact <- 1 i <- 1 while (i <= n) { fact <- fact * i i <- i + 1 } fact correctly initializes fact=1, loops i from 1 to n, multiplying fact by i each time.

3. Final Answer:

   n <- 5 fact <- 1 i <- 1 while (i <= n) { fact <- fact * i i <- i + 1 } fact -> Option B

4. Quick Check:

   Multiply 1 to 5 sequentially for factorial [OK]

Hint: Multiply from 1 to n inside while loop for factorial [OK]

Common Mistakes:

• Using addition instead of multiplication
• Incorrect loop condition
• Starting fact at 0 causing zero result