Nested functions in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested function call

What is the output of this Swift code with nested functions?

Swift

func outer() -> String {
    func inner() -> String {
        return "Hello"
    }
    return inner() + " World"
}
print(outer())

A Hello World

B World

C Hello

D Compilation error

💡 Hint

Look at how the inner function is called and combined with a string.

The inner function returns "Hello". The outer function calls inner() and adds " World", so the output is "Hello World".

❓ Predict Output

intermediate

Value returned by nested function with parameter

What value does the function call outer(3) return in this Swift code?

Swift

func outer(_ x: Int) -> Int {
    func inner(_ y: Int) -> Int {
        return y * 2
    }
    return inner(x) + 1
}
let result = outer(3)

A 7

B 6

C 8

D Compilation error

💡 Hint

Calculate inner(3) first, then add 1.

inner(3) returns 3 * 2 = 6. Adding 1 gives 7, so outer(3) returns 7.

🔧 Debug

advanced

Identify the error in nested function usage

What error does this Swift code produce when compiled?

Swift

func outer() {
    func inner() {
        print("Inside inner")
    }
}
inner()

A Error: Missing return in function expected to return value

B Error: Use of unresolved identifier 'inner'

C No error, prints "Inside inner"

D Error: Function 'outer' must be called before 'inner'

💡 Hint

Check the scope of the inner function.

The inner function is defined inside outer and is not visible outside. Calling inner() outside causes an unresolved identifier error.

❓ Predict Output

advanced

Output of nested function modifying outer variable

What is printed by this Swift code?

Swift

func outer() -> Int {
    var count = 0
    func inner() {
        count += 1
    }
    inner()
    inner()
    return count
}
print(outer())

A Compilation error

B 0

C 1

D 2

💡 Hint

Consider how many times inner() changes count.

inner() increments count by 1 each call. Called twice, count becomes 2, which is returned and printed.

🧠 Conceptual

expert

Behavior of nested functions and variable capture

Consider this Swift code. What is the output when calling outer()?

Swift

func outer() -> () -> Int {
    var x = 0
    func inner() -> Int {
        x += 1
        return x
    }
    return inner
}
let f = outer()
print(f())
print(f())

A

0
0

B

1
1

C

1
2

D Compilation error

💡 Hint

Think about how the inner function remembers the variable x.

The inner function captures x by reference. Each call to f() increments x, so outputs are 1 then 2.