While loop in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in Swift?

A while loop repeats a block of code as long as a condition is true. It checks the condition before running the code each time.

intermediate

How does a while loop differ from a repeat-while loop in Swift?

A while loop checks the condition before running the code, so it might not run at all. A repeat-while loop runs the code first, then checks the condition, so it runs at least once.

beginner

Write a simple Swift while loop that counts from 1 to 5.

var count = 1 while count <= 5 { print(count) count += 1 }

beginner

What happens if the condition in a while loop never becomes false?

The loop runs forever, causing an infinite loop. This can freeze or crash your program.

beginner

Why is it important to update variables inside a while loop?

Updating variables changes the condition so the loop can eventually stop. Without updates, the loop might run forever.

What does a while loop do in Swift?

A Stops the program immediately

B Runs code only once

C Runs code after checking the condition once

D Repeats code while a condition is true

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

When does a while loop check its condition?

A Only once at the start

B After running the loop code

C Before running the loop code

D It never checks

The while loop checks the condition before each iteration.

What will happen if the condition in a while loop is always true?

A The loop runs forever (infinite loop)

B The loop runs once

C The loop never runs

D The program crashes immediately

If the condition never becomes false, the loop never stops.

Which of these is a correct way to stop a while loop?

A Change the condition variable inside the loop

B Do nothing

C Use a for loop instead

D Write code outside the loop

Updating the variable that controls the condition lets the loop end.

What is the output of this code? var i = 3 while i > 0 { print(i) i -= 1 }

A 1 2 3

B 3 2 1

C 0 1 2

D No output

The loop prints 3, then 2, then 1, then stops when i becomes 0.

Explain how a while loop works in Swift and why updating the condition variable inside the loop is important.

Think about what happens if the condition never changes.

Write a simple Swift while loop that prints numbers from 1 to 5 and explain each step.

Start with a variable set to 1 and increase it each time.

Practice

1. What does a while loop do in Swift?

easy

A. Repeats code while a condition is true

B. Runs code only once

C. Runs code a fixed number of times

D. Stops the program immediately

Solution

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

   A while loop runs repeatedly as long as its condition stays true.

2. Step 2: Compare options with the definition

   Only Repeats code while a condition is true correctly describes repeating code while a condition is true.

3. Final Answer:

   Repeats code while a condition is true -> Option A

4. Quick Check:

   While loop = repeat while true condition [OK]

Hint: While loops run as long as condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statements
• Assuming while loops run only once

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

easy

A. while condition: { /* code */ }

B. while (condition) do { /* code */ }

C. while { condition } /* code */

D. while condition { /* code */ }

Solution

1. Step 1: Recall Swift while loop syntax

   Swift uses: while condition { code } without parentheses or extra keywords.

2. Step 2: Check each option

   while condition { /* code */ } matches Swift syntax exactly; others have extra keywords or wrong punctuation.

3. Final Answer:

   while condition { /* code */ } -> Option D

4. Quick Check:

   Swift while syntax = while condition { } [OK]

Hint: Swift while loops use no parentheses around condition [OK]

Common Mistakes:

• Adding parentheses around the condition
• Using 'do' keyword incorrectly
• Using colon instead of braces

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

var count = 3
while count > 0 {
    print(count)
    count -= 1
}

medium

A. 1 2 3

B. 3 2 1

C. 3 2 1 0

D. 0 1 2 3

Solution

1. Step 1: Analyze the loop condition and variable

   Variable count starts at 3 and loop runs while count > 0.

2. Step 2: Trace each iteration

   Prints 3, then count becomes 2; prints 2, count 1; prints 1, count 0; loop stops.

3. Final Answer:

   3 2 1 -> Option B

4. Quick Check:

   Loop prints count down from 3 to 1 [OK]

Hint: Count down from 3 to 1, stops before 0 [OK]

Common Mistakes:

• Including 0 in output
• Printing in ascending order
• Not decreasing count inside loop

4. Identify the error in this Swift code:

var i = 1
while i < 5 {
    print(i)
}

medium

A. Variable i should be a constant

B. Incorrect while syntax

C. Missing increment of i inside the loop

D. print statement is invalid

Solution

1. Step 1: Check loop structure and variable changes

   The loop condition depends on i, but i never changes inside the loop.

2. Step 2: Understand effect of missing increment

   Without increasing i, the condition i < 5 stays true forever, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the loop -> Option C

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Always update loop variable inside while loop [OK]

Common Mistakes:

• Forgetting to increment or change loop variable
• Thinking print causes error
• Assuming syntax is wrong

5. How can you use a while loop to sum all even numbers from 2 to 10 inclusive in Swift?

hard

A. Use a variable starting at 2, add it to sum, increment by 2 until 10

B. Start at 1, add all numbers, then multiply by 2

C. Use a while loop with condition i <= 10, increment i by 2, add i

D. Use a while loop starting at 10, increment 2 each time, add to sum

Solution

1. Step 1: Plan loop to add even numbers from 2 to 10

   Start with i = 2, add i to sum, then increase i by 2 to get next even number.

2. Step 2: Check options for correct approach

   Use a variable starting at 2, add it to sum, increment by 2 until 10 correctly describes starting at 2, adding to sum, incrementing by 2 until 10.

3. Final Answer:

   Use a variable starting at 2, add it to sum, increment by 2 until 10 -> Option A

4. Quick Check:

   Sum evens by stepping 2 from 2 to 10 [OK]

Hint: Increment by 2 to sum even numbers quickly [OK]

Common Mistakes:

• Incrementing by 2 without starting at 2
• Starting from 10 and incrementing
• Multiplying sum incorrectly