While loop in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in R?

A while loop in R repeats a block of code as long as a given condition is TRUE. It checks the condition before each repetition.

beginner

How do you write a basic while loop in R?

Use the syntax:

while(condition) {
  # code to repeat
}

The code inside runs while the condition is TRUE.

beginner

What happens if the condition in a while loop is FALSE at the start?

The code inside the while loop does not run at all because the condition is checked before the first iteration.

intermediate

How can you avoid an infinite loop with a while loop?

Make sure the condition will eventually become FALSE by changing variables inside the loop. Otherwise, the loop runs forever.

beginner

Example: What does this R code print?

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

It prints:
1
2
3
Because i starts at 1 and increases by 1 each time until it is greater than 3.

What does a while loop do in R?

A Repeats code a fixed number of times

B Repeats code while a condition is TRUE

C Runs code once regardless of condition

D Stops the program immediately

A while loop repeats the code block as long as the condition remains TRUE.

When is the condition in a while loop checked?

A Only once at the start

B After each repetition

C Before each repetition

D Never

The condition is checked before each time the loop runs.

What happens if the while loop condition is FALSE at the start?

A The loop code does not run

B The loop runs once

C The loop runs infinitely

D The program crashes

If the condition is FALSE initially, the loop body is skipped.

How can you prevent an infinite while loop?

A Use a for loop instead

B Do nothing; infinite loops are normal

C Add a break statement outside the loop

D Change variables inside the loop to eventually make the condition FALSE

You must update variables so the condition becomes FALSE to stop the loop.

What will this code print?
i <- 2
while(i < 5) {
print(i)
i <- i + 2
}

A 2 and 4

B 2, 3, 4

C 2, 4, 6

D Nothing

It prints 2 and 4 because i increases by 2 each time and stops before 5.

Explain how a while loop works in R and give a simple example.

Think about how you repeat a task until something changes.

What is an infinite loop and how can you avoid it when using a while loop?

Imagine a clock that never stops ticking.

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