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.