While loop in Swift - Mini Project: Build & Apply

Counting Down with a While Loop

📖 Scenario: Imagine you have a rocket that needs to count down before launch. You want to create a simple program that counts down from a number to zero.

🎯 Goal: Build a Swift program that uses a while loop to count down from 5 to 0 and print each number.

📋 What You'll Learn

Create a variable called count with the value 5

Create a variable called zero with the value 0

Use a while loop that runs as long as count is greater than or equal to zero

Inside the loop, print the current value of count

Inside the loop, decrease count by 1 each time

💡 Why This Matters

🌍 Real World

Counting down is used in many real-world situations like rocket launches, game timers, or cooking timers.

💼 Career

Understanding loops is essential for programming tasks that require repetition, such as processing data, animations, or user interactions.

1

Create the starting number

Create a variable called count and set it to 5.

Swift

# Your code here

Hint

Use var to create a variable and assign the number 5 to count.

2

Create the zero value

Create a variable called zero and set it to 0.

Swift

var count = 5
# Your code here

Hint

This variable will be used to compare in the loop condition.

3

Write the while loop

Write a while loop that runs as long as count is greater than or equal to zero. Inside the loop, print count and then subtract 1 from count.

Swift

var count = 5
var zero = 0
# Your code here

Hint

Use while count >= zero { ... } and inside the braces print and decrease count.

4

Run and see the countdown

Run the program to print the countdown from 5 to 0.

Swift

var count = 5
var zero = 0
while count >= zero {
    print(count)
    count -= 1
}
# Your code here

Hint

The program should print numbers from 5 down to 0, each on its own line.

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