Zip for combining collections in Kotlin - Step-by-Step Execution

Concept Flow - Zip for combining collections

Start with two lists

↓

Take first element from each list

↓

Pair elements together

↓

Add pair to result list

↓

Move to next elements

↓

Repeat until one list ends

↓

Return list of pairs

Zip takes two collections and pairs their elements one by one until one runs out.

Execution Sample

Kotlin

val list1 = listOf("a", "b", "c")
val list2 = listOf(1, 2, 3)
val zipped = list1.zip(list2)
println(zipped)

This code pairs elements from two lists into a list of pairs.

Execution Table

Step	list1 element	list2 element	Pair created	Result list so far
1	"a"	1	("a", 1)	[("a", 1)]
2	"b"	2	("b", 2)	[("a", 1), ("b", 2)]
3	"c"	3	("c", 3)	[("a", 1), ("b", 2), ("c", 3)]
4	No more elements	No more elements	Stop	Final result: [("a", 1), ("b", 2), ("c", 3)]

💡 Stop when one list runs out of elements (both lists ended here).

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
list1 element	N/A	"a"	"b"	"c"	No more elements
list2 element	N/A	1	2	3	No more elements
Result list	[]	[("a", 1)]	[("a", 1), ("b", 2)]	[("a", 1), ("b", 2), ("c", 3)]	[("a", 1), ("b", 2), ("c", 3)]

Key Moments - 3 Insights

Why does the zip stop when one list is shorter?

Zip pairs elements only while both lists have elements. When one ends, no more pairs can be made, as shown in step 4 of the execution_table.

What happens if lists have different types of elements?

Zip can combine any types because it creates pairs of the elements as they are, like pairing a String with an Int in the example.

Does zip change the original lists?

No, zip creates a new list of pairs without modifying the original lists, as seen in variable_tracker where list1 and list2 elements remain unchanged.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pair created at step 2?

A ("a", 1)

B ("b", 2)

C ("c", 3)

D ("b", 3)

💡 Hint

Check the 'Pair created' column at step 2 in the execution_table.

At which step does the zip operation stop according to the execution_table?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look for the row where 'Stop' appears in the 'Pair created' column.

If list2 had only two elements, how would the final result list change?

A It would be empty

B It would have three pairs

C It would have two pairs only

D It would have one pair

💡 Hint

Zip stops when one list ends, so result length matches shorter list length.

Concept Snapshot

Zip combines two collections element-wise.
Pairs elements until one collection ends.
Returns a list of pairs.
Original collections stay unchanged.
Useful to combine related data simply.

Full Transcript

Zip in Kotlin takes two lists and pairs their elements one by one. It starts from the first elements of both lists, creates a pair, and adds it to a new list. Then it moves to the next elements and repeats this until one list runs out of elements. The result is a list of pairs. The original lists do not change. This is useful when you want to combine two related lists into one list of 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