For-in loop with collections in Swift - Mini Project: Build & Apply

For-in Loop with Collections in Swift

📖 Scenario: You work at a small bookstore. You have a list of book titles and their prices. You want to find out which books cost more than $20.

🎯 Goal: Build a Swift program that uses a for-in loop to check each book's price and collect the titles of books costing more than $20.

📋 What You'll Learn

Create a dictionary called books with these exact entries: "Swift Programming": 25, "iOS Development": 18, "Data Structures": 30, "Algorithms": 15

Create a variable called priceThreshold and set it to 20

Use a for-in loop with variables title and price to iterate over books

Inside the loop, check if price is greater than priceThreshold and if so, add title to a list called expensiveBooks

Print the expensiveBooks list at the end

💡 Why This Matters

🌍 Real World

This project shows how to filter items from a collection based on conditions, a common task in apps like shopping, inventory, or data analysis.

💼 Career

Understanding how to loop through collections and apply conditions is essential for software developers working with data structures and user data.

1

Create the books dictionary

Create a dictionary called books with these exact entries: "Swift Programming": 25, "iOS Development": 18, "Data Structures": 30, "Algorithms": 15

Swift

# Create the books dictionary with the given entries
# Your code here

Hint

Use square brackets [] to create a dictionary with key-value pairs separated by colons.

2

Set the price threshold

Create a variable called priceThreshold and set it to 20

Swift

let books = ["Swift Programming": 25, "iOS Development": 18, "Data Structures": 30, "Algorithms": 15]
# Create a variable called priceThreshold and set it to 20
# Your code here

Hint

Use let to create a constant variable with the value 20.

3

Use a for-in loop to find expensive books

Create an empty array called expensiveBooks of type [String]. Then use a for-in loop with variables title and price to iterate over books. Inside the loop, check if price is greater than priceThreshold. If yes, append title to expensiveBooks.

Swift

let books = ["Swift Programming": 25, "iOS Development": 18, "Data Structures": 30, "Algorithms": 15]
let priceThreshold = 20
// Create an empty array called expensiveBooks
// Use a for-in loop to check prices and add titles to expensiveBooks
# Your code here

Hint

Use for (title, price) in books to loop through the dictionary. Use append() to add items to the array.

4

Print the list of expensive books

Write a print statement to display the expensiveBooks array.

Swift

let books = ["Swift Programming": 25, "iOS Development": 18, "Data Structures": 30, "Algorithms": 15]
let priceThreshold = 20
var expensiveBooks: [String] = []
for (title, price) in books {
    if price > priceThreshold {
        expensiveBooks.append(title)
    }
}
# Print the expensiveBooks array
# Your code here

Hint

Use print(expensiveBooks) to show the list of books costing more than $20.

Practice

1. What does the following Swift code do?

let numbers = [1, 2, 3]
for number in numbers {
    print(number)
}

easy

A. Prints each number in the array one by one

B. Prints the entire array once

C. Causes a syntax error

D. Prints the sum of the numbers

Solution

1. Step 1: Understand the for-in loop structure

   The loop goes through each element in the array named 'numbers'.

2. Step 2: Observe the print statement inside the loop

   Each element is printed on its own line as the loop runs.

3. Final Answer:

   Prints each number in the array one by one -> Option A

4. Quick Check:

   for number in numbers prints each number [OK]

Hint: For-in loops print items one at a time [OK]

Common Mistakes:

• Thinking it prints the whole array at once
• Confusing the loop variable with the collection
• Expecting a sum instead of individual prints

2. Which of the following is the correct syntax for a for-in loop iterating over an array named fruits in Swift?

easy

A. for fruit in fruits { print(fruit) }

B. for (fruit : fruits) { print(fruit) }

C. for fruit to fruits { print(fruit) }

D. foreach fruit in fruits { print(fruit) }

Solution

1. Step 1: Recall Swift for-in loop syntax

   The correct syntax uses 'for variable in collection { }'.

2. Step 2: Compare options to syntax

   Only for fruit in fruits { print(fruit) } matches this syntax exactly.

3. Final Answer:

   for fruit in fruits { print(fruit) } -> Option A

4. Quick Check:

   Correct for-in syntax uses 'in' keyword [OK]

Hint: Remember: for item in collection { } is correct [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using parentheses incorrectly
• Using 'foreach' which is not Swift syntax

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

let names = ["Anna", "Bob", "Cara"]
for name in names {
    print(name.count)
}

medium

A. 3 3 4

B. 3 4 3

C. 4 3 3

D. 4 3 4

Solution

1. Step 1: Identify the array elements and their lengths

   "Anna" has 4 letters, "Bob" has 3, "Cara" has 4.

2. Step 2: Understand the loop prints each name's character count

   The loop prints 4, then 3, then 4 on separate lines.

3. Final Answer:

   4 3 4 -> Option D

4. Quick Check:

   name.count matches each string length [OK]

Hint: Count letters of each name inside the loop [OK]

Common Mistakes:

• Mixing up the order of counts
• Counting letters incorrectly
• Assuming print outputs on same line

4. Find the error in this Swift code snippet:

let colors = ["red", "green", "blue"]
for color in colors
    print(color)
}

medium

A. Array declaration syntax is wrong

B. Variable name 'color' is invalid

C. Missing opening brace '{' after for loop

D. print statement should be outside the loop

Solution

1. Step 1: Check for proper loop syntax

   The for-in loop requires braces '{' and '}' to enclose the loop body.

2. Step 2: Identify missing opening brace

   The code has a closing brace '}' but no opening brace '{' after the for line.

3. Final Answer:

   Missing opening brace '{' after for loop -> Option C

4. Quick Check:

   for loops need braces around their body [OK]

Hint: Always use { } to wrap for loop code block [OK]

Common Mistakes:

• Forgetting opening brace
• Misplacing braces
• Assuming print can be outside without braces

5. You have a dictionary of student names and their scores:

let scores = ["Alice": 85, "Bob": 92, "Cara": 78]

Using a for-in loop, how do you print only the names of students who scored above 80?

hard

A. for name in scores.keys {
if scores[name] > 80 {
print(name)
}
}

B. for (name, score) in scores {
if score > 80 {
print(name)
}
}

C. for score in scores.values {
if score > 80 {
print(score)
}
}

D. for name in scores {
if scores[name] > 80 {
print(name)
}
}

Solution

1. Step 1: Understand dictionary iteration with key-value pairs

   Using 'for (name, score) in scores' lets you access both keys and values directly.

2. Step 2: Apply condition to print names with score above 80

   Inside the loop, check if score > 80, then print the name.

3. Final Answer:

   for (name, score) in scores { if score > 80 { print(name) } } -> Option B

4. Quick Check:

   Use (key, value) pairs to filter dictionary [OK]

Hint: Use (key, value) in dictionary to access both [OK]

Common Mistakes:

• Trying to loop only keys or values without pairing
• Forgetting to unwrap optional values
• Printing scores instead of names