For-in loop with collections in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a for-in loop over an array

What is the output of this Swift code?

let numbers = [1, 2, 3, 4]
var result = 0
for num in numbers {
    result += num
}
print(result)

Swift

let numbers = [1, 2, 3, 4]
var result = 0
for num in numbers {
    result += num
}
print(result)

A 1234

B 24

C 10

D Error: Cannot add Int to Int

💡 Hint

Think about what adding all numbers in the array means.

The for-in loop goes through each number and adds it to result. 1+2+3+4 equals 10.

❓ Predict Output

intermediate

Iterating over a dictionary with for-in

What will this Swift code print?

let dict = ["a": 1, "b": 2, "c": 3]
var keys = ""
for key in dict.keys {
    keys += key
}
print(keys)

Swift

let dict = ["a": 1, "b": 2, "c": 3]
var keys = ""
for key in dict.keys {
    keys += key
}
print(keys)

A "cba"

B Any permutation of "abc"

C "abc"

D Error: Cannot iterate over dict.keys

💡 Hint

Dictionary keys are iterated in insertion order since Swift 5.0.

Since Swift 5.0, the keys property returns a collection of keys in insertion order. For this dictionary, the output is "abc".

🔧 Debug

advanced

Identify the error in for-in loop over a set

What error does this Swift code produce?

let fruits: Set = ["apple", "banana", "cherry"]
for fruit in fruits {
    print(fruit.uppercased())
}

Swift

let fruits: Set = ["apple", "banana", "cherry"]
for fruit in fruits {
    print(fruit.uppercased())
}

A No error; prints uppercase fruit names in any order

B Error: Cannot iterate over a Set

C Runtime error: fruit is nil

D Error: 'uppercased()' is not a method on String

💡 Hint

Sets can be iterated like arrays.

Sets in Swift conform to Sequence, so you can use for-in loops. Strings have the uppercased() method. The output order is not guaranteed.

❓ Predict Output

advanced

Output of nested for-in loops with arrays

What is the output of this Swift code?

let arr1 = [1, 2]
let arr2 = ["a", "b"]
var output = ""
for num in arr1 {
    for letter in arr2 {
        output += "\(num)\(letter) "
    }
}
print(output)

Swift

let arr1 = [1, 2]
let arr2 = ["a", "b"]
var output = ""
for num in arr1 {
    for letter in arr2 {
        output += "\(num)\(letter) "
    }
}
print(output)

A "1a 1b 2a 2b "

B "a1 b1 a2 b2 "

C "1a 2a 1b 2b "

D "Error: Cannot concatenate Int and String"

💡 Hint

The outer loop runs first, then the inner loop fully for each outer item.

The outer loop goes through 1 and 2. For each, the inner loop goes through "a" and "b". So output is "1a 1b 2a 2b ".

📝 Syntax

expert

Which option causes a syntax error in for-in loop?

Which of these Swift for-in loops will cause a syntax error?

let items = ["x", "y", "z"]

Swift

let items = ["x", "y", "z"]

A for item in items { print(item) }

B

for item in items {
print(item)
}

C

}
)meti(tnirp
{ smeti ni meti rof

D for item in items print(item)

💡 Hint

Swift requires braces {} for the body of for-in loops.

Option D misses braces and does not allow omitting them for multi-line statements, causing a syntax error.

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