Map, filter, reduce patterns in Swift - Mini Project: Build & Apply

Map, filter, reduce patterns

📖 Scenario: You work at a small fruit store. You have a list of fruit prices and want to do some simple calculations to help with pricing and sales.

🎯 Goal: Learn how to use map, filter, and reduce in Swift to transform, select, and combine data from a list.

📋 What You'll Learn

Create an array of fruit prices

Create a discount rate variable

Use map to apply the discount to all prices

Use filter to find prices above a threshold

Use reduce to calculate the total of filtered prices

Print the final total

💡 Why This Matters

🌍 Real World

Stores and shops often need to update prices, apply discounts, and calculate totals quickly and clearly.

💼 Career

Knowing how to use map, filter, and reduce is useful for data processing tasks in many programming jobs, especially in apps that handle lists of items.

1

Create the fruit prices array

Create an array called fruitPrices with these exact values: 2.5, 3.0, 4.75, 1.25, and 5.0.

Swift

# Create the fruitPrices array with the given values
# Your code here

Hint

Use square brackets [] to create an array and separate values with commas.

2

Add a discount rate variable

Create a variable called discountRate and set it to 0.1 to represent a 10% discount.

Swift

let fruitPrices = [2.5, 3.0, 4.75, 1.25, 5.0]
# Create the discountRate variable and set it to 0.1
# Your code here

Hint

Use let to create a constant variable for the discount rate.

3

Apply discount and filter prices

Use map on fruitPrices to create a new array called discountedPrices where each price is reduced by the discountRate. Then use filter on discountedPrices to create an array called expensivePrices that only contains prices greater than 3.0.

Swift

let fruitPrices = [2.5, 3.0, 4.75, 1.25, 5.0]
let discountRate = 0.1
# Use map to create discountedPrices by reducing each price by discountRate
# Use filter on discountedPrices to create expensivePrices with prices > 3.0
# Your code here

Hint

Use map with a closure to multiply each price by 1 - discountRate. Use filter with a closure to keep prices greater than 3.0.

4

Calculate total and print

Use reduce on expensivePrices to calculate the total sum and store it in a variable called totalPrice. Then print totalPrice.

Swift

let fruitPrices = [2.5, 3.0, 4.75, 1.25, 5.0]
let discountRate = 0.1
let discountedPrices = fruitPrices.map { price in price * (1 - discountRate) }
let expensivePrices = discountedPrices.filter { price in price > 3.0 }
# Use reduce to sum expensivePrices into totalPrice
# Print totalPrice
# Your code here

Hint

Use reduce(0) with a closure that adds each price to the sum. Then print the total.

Practice

1. What does the map function do in Swift when applied to an array?

easy

A. It changes each item in the array based on a rule.

B. It removes items that don't match a condition.

C. It combines all items into a single value.

D. It sorts the array in ascending order.

Solution

1. Step 1: Understand the purpose of map

   The map function applies a transformation to each element in the array, creating a new array with the changed items.

2. Step 2: Compare with other functions

   Unlike filter which removes items, or reduce which combines items, map changes each item individually.

3. Final Answer:

   It changes each item in the array based on a rule. -> Option A

4. Quick Check:

   map = change each item [OK]

Hint: Map transforms every item, filter picks, reduce combines [OK]

Common Mistakes:

• Confusing map with filter or reduce
• Thinking map removes items
• Assuming map combines items into one

2. Which of the following is the correct syntax to use filter on an array numbers to keep only even numbers?

easy

A. let evens = numbers.map { $0 % 2 == 0 }

B. let evens = numbers.filter($0 % 2 == 0)

C. let evens = numbers.reduce(0) { $0 % 2 == 0 }

D. let evens = numbers.filter { $0 % 2 == 0 }

Solution

1. Step 1: Identify correct filter syntax

   The filter method uses a closure with a condition inside curly braces, like { $0 % 2 == 0 }, to keep items matching the condition.

2. Step 2: Check each option

   let evens = numbers.filter { $0 % 2 == 0 } correctly uses filter with the closure. let evens = numbers.map { $0 % 2 == 0 } uses map which changes items, not filters. let evens = numbers.reduce(0) { $0 % 2 == 0 } misuses reduce. let evens = numbers.filter($0 % 2 == 0) misses the closure braces.

3. Final Answer:

   let evens = numbers.filter { $0 % 2 == 0 } -> Option D

4. Quick Check:

   filter syntax = filter { condition } [OK]

Hint: Filter needs braces and a condition inside { } [OK]

Common Mistakes:

• Using map instead of filter
• Omitting braces in closure
• Using reduce incorrectly for filtering

3. What is the output of this Swift code?

let numbers = [1, 2, 3, 4]
let result = numbers.reduce(0) { $0 + $1 }
print(result)

medium

A. 0

B. [1, 2, 3, 4]

C. 10

D. Error

Solution

1. Step 1: Understand reduce with initial value 0

   The reduce starts with 0 and adds each number in the array: 0 + 1 + 2 + 3 + 4.

2. Step 2: Calculate the sum

   Adding all numbers: 0 + 1 = 1, 1 + 2 = 3, 3 + 3 = 6, 6 + 4 = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Sum of [1,2,3,4] = 10 [OK]

Hint: Reduce with + sums all items starting from initial value [OK]

Common Mistakes:

• Confusing reduce with map or filter output
• Forgetting initial value in reduce
• Expecting array output instead of single value

4. Identify the error in this Swift code snippet:

let words = ["apple", "banana", "cherry"]
let filtered = words.filter { word in
    word.count > 5
}
print(filtered)

medium

A. No error, code runs correctly

B. Incorrect use of filter instead of map

C. Using word.count instead of word.length

D. Missing return keyword inside filter closure

Solution

1. Step 1: Check filter closure syntax

   The closure uses implicit return because it is a single expression, so no explicit return is needed.

2. Step 2: Verify property usage

   word.count is the correct way to get string length in Swift, so no error there.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Single-expression closures return implicitly [OK]

Hint: Single-expression closures return automatically, no return needed [OK]

Common Mistakes:

• Adding unnecessary return keyword
• Confusing count with length property
• Thinking filter needs map syntax

5. Given an array of integers, how would you use map, filter, and reduce together to find the sum of squares of only the odd numbers? Choose the correct Swift code.

hard

A. let sum = numbers.map { $0 * $0 }.filter { $0 % 2 != 0 }.reduce(0, +)

B. let sum = numbers.filter { $0 % 2 != 0 }.map { $0 * $0 }.reduce(0, +)

C. let sum = numbers.reduce(0) { $0 + $1 * $1 }

D. let sum = numbers.filter { $0 % 2 == 0 }.map { $0 * $0 }.reduce(0, +)

Solution

1. Step 1: Filter odd numbers first

   We want only odd numbers, so use filter { $0 % 2 != 0 } to keep them.

2. Step 2: Map to square each odd number

   Use map { $0 * $0 } to get squares of the filtered odd numbers.

3. Step 3: Reduce to sum all squares

   Use reduce(0, +) to add all squared values into one sum.

4. Final Answer:

   let sum = numbers.filter { $0 % 2 != 0 }.map { $0 * $0 }.reduce(0, +) -> Option B

5. Quick Check:

   Filter odds, map squares, reduce sum [OK]

Hint: Filter odds first, then map squares, finally reduce sum [OK]

Common Mistakes:

• Mapping before filtering odd numbers
• Filtering even numbers by mistake
• Using reduce without mapping squares