Omitting argument labels with _ in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code?

Consider the following Swift function and call:

func greet(_ name: String) {
    print("Hello, \(name)!")
}
greet("Alice")

What will this print?

Swift

func greet(_ name: String) {
    print("Hello, \(name)!")
}
greet("Alice")

A Hello, _Alice!

B Error: Missing argument label

C Hello, Alice!

D Hello, name!

💡 Hint

Check how the underscore (_) affects the argument label when calling the function.

The underscore (_) before the parameter name means the argument label is omitted when calling the function. So, greet("Alice") works without specifying a label.

❓ Predict Output

intermediate

What error does this code produce?

Given this Swift function:

func multiply(_ a: Int, b: Int) -> Int {
    return a * b
}
multiply(a: 3, b: 4)

What error will occur when compiling?

Swift

func multiply(_ a: Int, b: Int) -> Int {
    return a * b
}
multiply(a: 3, b: 4)

A Error: Extraneous argument label 'a:' in call

B Error: Missing argument label 'a:' in call

C 12

D Error: Missing argument label 'b:' in call

💡 Hint

Look at which parameters have labels and which don't.

The first parameter uses an underscore (_) so it has no argument label. Calling with a: 3 is invalid and causes an 'Extraneous argument label' error.

🔧 Debug

advanced

Why does this function call fail?

Examine this Swift code:

func divide(_ numerator: Int, _ denominator: Int) -> Int {
    return numerator / denominator
}
divide(numerator: 10, denominator: 2)

Why does the call to divide fail?

Swift

func divide(_ numerator: Int, _ denominator: Int) -> Int {
    return numerator / denominator
}
divide(numerator: 10, denominator: 2)

A Because the function parameters are constants and cannot be used in division

B Because the function parameters require argument labels, but the call omits them

C Because the function returns an Int but the call expects a Double

D Because the function parameters omit argument labels, but the call uses labels

💡 Hint

Check the use of underscores (_) in the function parameters and how the function is called.

Using underscores means no argument labels are expected when calling the function. The call incorrectly uses labels, causing a compile error.

🧠 Conceptual

advanced

How does the underscore (_) affect argument labels in Swift functions?

Which statement best describes the effect of using an underscore (_) before a parameter name in a Swift function?

A It omits the argument label when calling the function, so only the value is passed.

B It makes the argument label mandatory when calling the function.

C It makes the parameter optional and allows nil values.

D It automatically assigns a default value to the parameter.

💡 Hint

Think about how you call functions with and without argument labels.

The underscore (_) before a parameter name means you do not write the argument label when calling the function. You just pass the value.

❓ Predict Output

expert

What is the output of this Swift code with multiple underscores?

Analyze this Swift code:

func calculate(_ x: Int, _ y: Int, multiplier: Int) -> Int {
    return (x + y) * multiplier
}

let result = calculate(2, 3, multiplier: 4)
print(result)

What will be printed?

Swift

func calculate(_ x: Int, _ y: Int, multiplier: Int) -> Int {
    return (x + y) * multiplier
}

let result = calculate(2, 3, multiplier: 4)
print(result)

A Error: Missing argument label for multiplier

B 20

C Error: Missing argument label for first or second parameter

D 10

💡 Hint

Check which parameters require labels and which do not.

The first two parameters use underscores, so no labels are needed. The third parameter requires the label multiplier:. The call matches this, so the output is (2 + 3) * 4 = 20.