Array iteration and enumerated in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enumerated array iteration

What is the output of this Swift code?

Swift

let fruits = ["Apple", "Banana", "Cherry"]
for (index, fruit) in fruits.enumerated() {
    print("\(index): \(fruit)")
}

A

Apple: 0
Banana: 1
Cherry: 2

B

1: Apple
2: Banana
3: Cherry

C

0: Apple
1: Banana
2: Cherry

D

Apple
Banana
Cherry

💡 Hint

Remember that enumerated() gives pairs of (index, element) starting at 0.

The enumerated() method returns a sequence of pairs (index, element) starting from 0. The loop prints the index and fruit separated by a colon.

❓ Predict Output

intermediate

Enumerated index starting value

What will be printed by this Swift code?

Swift

let numbers = [10, 20, 30]
for (index, number) in numbers.enumerated() {
    print(index + 1, number)
}

A

1 20
2 30
3 40

B

0 10
1 20
2 30

C

10 1
20 2
30 3

D

1 10
2 20
3 30

💡 Hint

The index from enumerated() starts at 0, but the code adds 1 before printing.

The enumerated() method starts index at 0, but the code prints index + 1, so indexes start at 1 in output.

🔧 Debug

advanced

Why does this enumerated loop cause an error?

This Swift code causes a compile error. What is the reason?

Swift

let items = ["a", "b", "c"]
for index, item in items.enumerated() {
    print(index, item)
}

A The array 'items' is empty

B Missing parentheses around (index, item) in the for loop

C The variable 'item' is not declared

D Cannot use enumerated() on arrays

💡 Hint

Check the syntax for tuple unpacking in for loops.

In Swift, when unpacking tuples in a for loop, parentheses are required around the variables: for (index, item) in ...

❓ Predict Output

advanced

Output of modifying array during enumerated iteration

What is the output of this Swift code?

Swift

var letters = ["x", "y", "z"]
for (i, letter) in letters.enumerated() {
    letters[i] = letter.uppercased()
}
print(letters)

A ["X", "Y", "Z"]

B ["x", "y", "z"]

C ["X", "y", "z"]

D Runtime error: Concurrent modification

💡 Hint

Think about whether modifying elements by index during iteration is allowed.

The code modifies each element by index during iteration, which is allowed. The letters become uppercase.

🧠 Conceptual

expert

Count of elements after filtering with enumerated

Given this Swift code, how many elements will be in the filtered array?

Swift

let values = [5, 10, 15, 20, 25]
let filtered = values.enumerated().filter { index, value in
    index % 2 == 0 && value > 10
}.map { $0.element }
print(filtered.count)

A 2

B 1

C 3

D 4

💡 Hint

Check which elements have even index and value greater than 10.

Indexes 0, 2, 4 are even. Values at these indexes are 5, 15, 25. Only 15 and 25 are > 10, so 2 elements. But 5 is not > 10, so filtered elements are 15 and 25. Wait, 15 at index 2 and 25 at index 4. So count is 2.

Practice

1. What does the enumerated() method do when used on an array in Swift?

easy

A. It provides both the index and the value of each element during iteration.

B. It reverses the order of the array elements.

C. It filters out duplicate elements from the array.

D. It sorts the array in ascending order.

Solution

1. Step 1: Understand the purpose of enumerated()

   The enumerated() method returns a sequence of pairs (index, value) for each element in the array.

2. Step 2: Identify what is returned during iteration

   Each iteration gives you the position (index) and the element itself (value), allowing you to use both.

3. Final Answer:

   It provides both the index and the value of each element during iteration. -> Option A

4. Quick Check:

   enumerated() = index + value [OK]

Hint: Remember: enumerated() gives index and item together [OK]

Common Mistakes:

• Thinking enumerated() changes array order
• Believing enumerated() filters or sorts elements
• Confusing enumerated() with map or filter

2. Which of the following is the correct syntax to iterate over an array fruits using enumerated() in Swift?

easy

A. for (index, fruit) in fruits.enumerated() { print(index, fruit) }

B. for index, fruit in fruits { print(index, fruit) }

C. for index, fruit in fruits.enumerated { print(index, fruit) }

D. for (fruit, index) in fruits.enumerated() { print(index, fruit) }

Solution

1. Step 1: Check the correct use of enumerated()

   The method enumerated() must be called with parentheses to return the sequence.

2. Step 2: Verify tuple unpacking syntax

   The for loop must unpack the tuple as (index, value) in parentheses.

3. Final Answer:

   for (index, fruit) in fruits.enumerated() { print(index, fruit) } -> Option A

4. Quick Check:

   Correct syntax = for (index, fruit) in fruits.enumerated() { print(index, fruit) } [OK]

Hint: Use parentheses with enumerated() and tuple unpacking [OK]

Common Mistakes:

• Omitting parentheses after enumerated()
• Swapping index and value order in tuple
• Trying to unpack without parentheses

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

let animals = ["cat", "dog", "bird"]
for (i, animal) in animals.enumerated() {
    print("\(i): \(animal)")
}

medium

A. cat: 0
dog: 1
bird: 2

B. 1: cat
2: dog
3: bird

C. 0: cat
1: dog
2: bird

D. cat
dog
bird

Solution

1. Step 1: Understand enumerated() output

   enumerated() gives index starting at 0 and the element value.

2. Step 2: Trace the print statements

   Each line prints index, colon, space, then the animal name.

3. Final Answer:

   0: cat 1: dog 2: bird -> Option C

4. Quick Check:

   Index starts at 0, prints index: value [OK]

Hint: Indexes start at 0 unless adjusted [OK]

Common Mistakes:

• Assuming index starts at 1
• Printing value before index
• Ignoring the colon and space formatting

4. Identify the error in the following Swift code snippet:

let numbers = [10, 20, 30]
for index, value in numbers.enumerated {
    print(index, value)
}

medium

A. print statement syntax is invalid.

B. Incorrect variable names in the for loop.

C. Array declaration syntax is wrong.

D. Missing parentheses after enumerated() method call.

Solution

1. Step 1: Check method call syntax

   The method enumerated must be called with parentheses: enumerated().

2. Step 2: Verify other parts of the code

   Variable names and array declaration are correct; print syntax is valid.

3. Final Answer:

   Missing parentheses after enumerated() method call. -> Option D

4. Quick Check:

   enumerated() needs () [OK]

Hint: Always add () after enumerated [OK]

Common Mistakes:

• Forgetting parentheses after enumerated
• Swapping index and value order
• Using wrong print syntax

5. Given an array scores = [85, 92, 78, 90], write Swift code using enumerated() to print each score with its position starting from 1, like "Player 1: 85". Which code snippet correctly does this?

hard

A. for score in scores { print("Player \(score): \(score)") }

B. for (index, score) in scores.enumerated() { print("Player \(index + 1): \(score)") }

C. for (index, score) in scores.enumerated() { print("Player \(index): \(score)") }

D. for (score, index) in scores.enumerated() { print("Player \(index + 1): \(score)") }

Solution

1. Step 1: Use enumerated() to get index and score

   enumerated() returns (index, value) tuples; index starts at 0.

2. Step 2: Adjust index to start from 1 in print

   Add 1 to index when printing to start counting players from 1.

3. Final Answer:

   for (index, score) in scores.enumerated() { print("Player \(index + 1): \(score)") } -> Option B

4. Quick Check:

   Index + 1 for counting players [OK]

Hint: Add 1 to index to start counting from 1 [OK]

Common Mistakes:

• Printing index without adding 1
• Swapping index and value in tuple
• Using wrong tuple unpacking order