Zip for combining collections in Kotlin - Mini Project: Build & Apply

Zip for combining collections

📖 Scenario: You are organizing a small event and have two lists: one with the names of guests and another with their favorite drinks. You want to pair each guest with their favorite drink to prepare the right refreshments.

🎯 Goal: Build a Kotlin program that uses zip to combine two lists into pairs of guest names and their favorite drinks.

📋 What You'll Learn

Create a list called guests with the exact names: "Alice", "Bob", "Charlie"

Create a list called drinks with the exact drinks: "Coffee", "Tea", "Juice"

Create a variable called guestDrinkPairs that uses zip to combine guests and drinks

Print the guestDrinkPairs list

💡 Why This Matters

🌍 Real World

Combining two related lists is common when you want to pair data, like matching names with preferences or products with prices.

💼 Career

Understanding how to combine collections efficiently is useful in data processing, UI development, and backend programming.

1

Create the guests list

Create a list called guests with these exact names: "Alice", "Bob", "Charlie"

Kotlin

# Create the list called guests with the names "Alice", "Bob", "Charlie"
# Your code here

Hint

Use listOf to create a list with the exact names.

2

Create the drinks list

Create a list called drinks with these exact drinks: "Coffee", "Tea", "Juice"

Kotlin

val guests = listOf("Alice", "Bob", "Charlie")
# Create the list called drinks with the drinks "Coffee", "Tea", "Juice"
# Your code here

Hint

Use listOf again to create the drinks list.

3

Combine guests and drinks with zip

Create a variable called guestDrinkPairs that uses zip to combine the guests and drinks lists

Kotlin

val guests = listOf("Alice", "Bob", "Charlie")
val drinks = listOf("Coffee", "Tea", "Juice")
# Use zip to combine guests and drinks into guestDrinkPairs
# Your code here

Hint

Use guests.zip(drinks) to combine the two lists.

4

Print the guest and drink pairs

Print the guestDrinkPairs list to show the pairs of guests and their favorite drinks

Kotlin

val guests = listOf("Alice", "Bob", "Charlie")
val drinks = listOf("Coffee", "Tea", "Juice")
val guestDrinkPairs = guests.zip(drinks)
# Print guestDrinkPairs below
# Your code here

Hint

Use println(guestDrinkPairs) to display the pairs.

Practice

1. What does the zip function do when used with two lists in Kotlin?

easy

A. Combines all elements from both lists into one longer list.

B. Pairs elements from both lists into a list of pairs, stopping at the shorter list length.

C. Creates a map from the first list's elements to the second list's elements without pairing.

D. Filters elements that are common to both lists.

Solution

1. Step 1: Understand the zip function behavior

   The zip function pairs elements from two collections by their positions.

2. Step 2: Consider the length of the resulting list

   The resulting list length equals the smaller of the two input lists, pairing elements until one list runs out.

3. Final Answer:

   Pairs elements from both lists into a list of pairs, stopping at the shorter list length. -> Option B

4. Quick Check:

   Zip pairs elements up to shortest list length [OK]

Hint: Zip pairs elements by index, stops at shortest list length [OK]

Common Mistakes:

• Thinking zip concatenates lists instead of pairing
• Assuming zip returns a map automatically
• Believing zip pairs all elements ignoring list length

2. Which of the following is the correct syntax to zip two lists listA and listB in Kotlin?

easy

A. val zipped = listA + listB.zip()

B. val zipped = zip(listA, listB)

C. val zipped = listA.zip(listB)

D. val zipped = listA.zipWith(listB)

Solution

1. Step 1: Recall Kotlin's zip syntax

   The zip function is called as an extension on the first list, passing the second list as argument.

2. Step 2: Check each option

   val zipped = listA.zip(listB) matches the correct syntax: listA.zip(listB). Others are invalid or incorrect method calls.

3. Final Answer:

   val zipped = listA.zip(listB) -> Option C

4. Quick Check:

   Use listA.zip(listB) to zip two lists [OK]

Hint: Use listA.zip(listB) to combine two lists [OK]

Common Mistakes:

• Trying to call zip as a standalone function
• Using incorrect method names like zipWith
• Concatenating lists instead of zipping

3. What is the output of the following Kotlin code?

val nums = listOf(1, 2, 3)
val chars = listOf('a', 'b')
val zipped = nums.zip(chars)
println(zipped)

medium

A. [(1, a), (2, b), (3, null)]

B. [(a, 1), (b, 2)]

C. [(1, a), (2, b), (3, c)]

D. [(1, a), (2, b)]

Solution

1. Step 1: Identify the input lists and their lengths

   nums has 3 elements, chars has 2 elements.

2. Step 2: Understand zip output length and pairing

   Zip pairs elements by index until the shorter list ends, so only first 2 pairs: (1, 'a') and (2, 'b').

3. Final Answer:

   [(1, a), (2, b)] -> Option D

4. Quick Check:

   Zip length = shorter list length = 2 [OK]

Hint: Zip stops at shorter list length, pairs elements by index [OK]

Common Mistakes:

• Expecting zip to fill missing elements with null
• Assuming zip pairs all elements ignoring length
• Mixing order of elements in pairs

4. The following Kotlin code throws an error. What is the problem?

val list1 = listOf(1, 2, 3)
val list2 = listOf("a", "b", "c")
val zipped = list1.zip(list2) { a, b -> a + b }
println(zipped)

medium

A. Cannot add Int and String directly with + operator.

B. zip requires lists to be the same size.

C. The lambda syntax for zip is incorrect.

D. listOf cannot contain different types.

Solution

1. Step 1: Analyze the lambda inside zip

   The lambda tries to add a (Int) and b (String) using +.

2. Step 2: Understand Kotlin type rules

   Adding Int and String directly with + is not allowed; it causes a type error.

3. Final Answer:

   Cannot add Int and String directly with + operator. -> Option A

4. Quick Check:

   Int + String causes type error in Kotlin [OK]

Hint: Int + String needs explicit conversion or concatenation [OK]

Common Mistakes:

• Assuming zip needs equal list sizes
• Thinking lambda syntax is wrong
• Believing listOf can't hold different types

5. Given two lists val names = listOf("Anna", "Bob", "Cara") and val scores = listOf(90, 85), how can you create a list of strings like ["Anna: 90", "Bob: 85"] using zip with a lambda?

hard

A. names.zip(scores) { name, score -> "$name: $score" }

B. names.zip(scores).map { name, score -> "$name: $score" }

C. names.zipWith(scores) { name, score -> "$name: $score" }

D. names.zip(scores).toString { "$name: $score" }

Solution

1. Step 1: Use zip with a transform lambda

   The zip function can take a lambda to transform pairs directly into desired output.

2. Step 2: Format each pair as a string

   Using { name, score -> "$name: $score" } creates the string format needed.

3. Final Answer:

   names.zip(scores) { name, score -> "$name: $score" } -> Option A

4. Quick Check:

   Use zip with lambda to transform pairs [OK]

Hint: Use zip with lambda to format pairs directly [OK]

Common Mistakes:

• Using zipWith which doesn't exist in Kotlin
• Trying to call toString with a lambda
• Mapping after zip instead of using lambda inside zip