Capturing values from context in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift closure capturing example?

Consider this Swift code where a closure captures a variable from its context. What will be printed?

Swift

var count = 0
let incrementer = { count += 1; print(count) }
incrementer()
incrementer()
print(count)

A 2\n2\n2

B 1\n1\n0

C 0\n1\n2

D 1\n2\n2

💡 Hint

Closures capture variables by reference, so changes inside affect the original variable.

The closure increments count each time it is called, so the first call prints 1, the second prints 2, and the final print shows the updated count value 2.

❓ Predict Output

intermediate

What does this Swift closure print when capturing a constant?

Look at this Swift code where a closure captures a constant. What will be printed?

Swift

let number = 5
let printNumber = { print(number) }
printNumber()

A Compilation error

B 0

C 5

D nil

💡 Hint

Constants can be captured by closures and used inside them.

The closure captures the constant number and prints its value 5.

🔧 Debug

advanced

Why does this Swift closure capture cause unexpected output?

Examine this Swift code where a closure captures a loop variable. Why does it print the same number multiple times?

Swift

var closures: [() -> Void] = []
for i in 1...3 {
    closures.append { print(i) }
}
closures.forEach { $0() }

A All closures capture the same variable <code>i</code> by reference, so they print the final value 4.

B Each closure captures a different copy of <code>i</code>, so they print 1, 2, 3 respectively.

C The closures print 1, 1, 1 because <code>i</code> is reset each loop.

D The code causes a runtime error because <code>i</code> is out of scope.

💡 Hint

Think about how Swift captures loop variables in closures.

Swift captures i by reference in the loop, so all closures share the same i which ends at 4 after the loop, so all print 4.

❓ Predict Output

advanced

What is the output of this Swift closure with capture list?

What will this Swift code print? It uses a capture list to capture a variable by value.

Swift

var value = 10
let closure = { [value] in print(value) }
value = 20
closure()

A 10

B Compilation error

C 20

D nil

💡 Hint

Capture lists let you capture a variable's value at the time the closure is created.

The closure captures value as 10 when created, so it prints 10 even though value changed later.

🧠 Conceptual

expert

Which statement about Swift closure captures is true?

Choose the correct statement about how Swift closures capture values from their context.

A Closures always capture variables by value, so changes outside do not affect the closure.

B Closures capture variables by reference by default, but capture lists can force capture by value.

C Closures cannot capture constants from their surrounding context.

D Capturing variables in closures always causes a memory leak.

💡 Hint

Think about how capture lists affect variable capturing.

By default, Swift closures capture variables by reference, but you can use capture lists to capture by value.