Omitting argument labels with _ in Swift

Introduction

Sometimes you want to call a function without writing the argument label every time. Using _ lets you skip the label for cleaner calls.

When the argument label is obvious and repeating it feels unnecessary.

When you want to make function calls shorter and easier to read.

When working with functions that have many parameters and you want simpler calls.

When you want to match a function signature that requires no labels for arguments.

Syntax

Swift

func functionName(_ parameterName: Type) {
    // function body
}

The underscore _ before a parameter name means you don't have to write the label when calling the function.

This only affects how you call the function, not how you use the parameter inside the function.

Examples

You call greet without writing the label name: because of the _.

Swift

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

Both parameters omit labels, so you just pass values separated by commas.

Swift

func add(_ a: Int, _ b: Int) -> Int {
    return a + b
}
let sum = add(3, 5)
print(sum)

The first parameter has a label factor1, the second omits it with _.

Swift

func multiply(factor1 a: Int, _ b: Int) -> Int {
    return a * b
}
let result = multiply(factor1: 4, 5)
print(result)

Sample Program

This program defines a function that greets a person. The argument label is omitted, so you just pass the name directly.

Swift

func sayHello(_ person: String) {
    print("Hi, \(person)!")
}
sayHello("Bob")

Important Notes

Using _ can make your code cleaner but be careful not to lose clarity.

Omitting labels is common in Swift's standard library functions like print().

Summary

Use _ to skip argument labels when calling functions.

This makes function calls shorter and sometimes easier to read.

Inside the function, you still use the parameter name as usual.

Practice

1. What does the underscore _ do when used before a function parameter in Swift?

easy

A. It changes the parameter type to Any.

B. It makes the parameter optional.

C. It makes the parameter a constant inside the function.

D. It allows you to call the function without writing the argument label.

Solution

1. Step 1: Understand the role of underscore in parameters

   The underscore before a parameter name tells Swift to omit the argument label when calling the function.

2. Step 2: Effect on function call

   This means you can pass the argument directly without naming it, making the call shorter and simpler.

3. Final Answer:

   It allows you to call the function without writing the argument label. -> Option D

4. Quick Check:

   Underscore = omit argument label [OK]

Hint: Underscore means skip argument label when calling [OK]

Common Mistakes:

• Thinking underscore makes parameter optional
• Confusing underscore with type changes
• Assuming underscore affects parameter mutability

2. Which of the following function declarations correctly omits the argument label for the first parameter?

easy

A. func greet(_name: String) { print("Hello, \(_name)!") }

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

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

D. func greet(name: String) { print("Hello, \(name)!") }

Solution

1. Step 1: Identify correct underscore usage

   The underscore must appear before the parameter name with a space, like _ name: String.

2. Step 2: Check each option

   func greet(_ name: String) { print("Hello, \(name)!") } uses _ name: String correctly. func greet(name: String) { print("Hello, \(name)!") } uses a label. func greet(name _ : String) { print("Hello, \(name)!") } has wrong syntax. func greet(_name: String) { print("Hello, \(_name)!") } uses an invalid parameter name.

3. Final Answer:

   func greet(_ name: String) { print("Hello, \(name)!") } -> Option B

4. Quick Check:

   Correct underscore syntax = func greet(_ name: String) { print("Hello, \(name)!") } [OK]

Hint: Underscore must be alone before parameter name [OK]

Common Mistakes:

• Putting underscore after parameter name
• Adding spaces between underscore and name incorrectly
• Using underscore as part of parameter name

3. What is the output of this Swift code?

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

print(multiply(3, 4))

medium

A. 12

B. multiply(3, 4)

C. Error: Missing argument labels

D. 7

Solution

1. Step 1: Understand function parameters with underscore

   Both parameters have underscores, so argument labels are omitted when calling.

2. Step 2: Calculate the function result

   Calling multiply(3, 4) returns 3 * 4 = 12, which is printed.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Underscore means call without labels, so multiply(3,4) works [OK]

Common Mistakes:

• Expecting argument labels in call
• Confusing output with string of function call
• Adding parameters incorrectly

4. Identify the error in this Swift function call:

func divide(_ numerator: Double, denominator: Double) -> Double {
    return numerator / denominator
}

divide(10, denominator: 2)

medium

A. Cannot mix omitted and required labels in parameters

B. Missing argument label for denominator

C. No error, code runs fine

D. Missing argument label for numerator

Solution

1. Step 1: Analyze parameter labels

   The first parameter uses underscore, so no label needed. The second requires label denominator.

2. Step 2: Check function call

   The call uses divide(10, denominator: 2), which matches the parameter labels correctly.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   Underscore omits first label, second label required [OK]

Hint: First param no label, second param label required [OK]

Common Mistakes:

• Expecting both labels omitted
• Trying to omit second label incorrectly
• Confusing parameter order

5. You want to write a Swift function formatDate that takes a Date and a String format. You want to call it like formatDate(myDate, "yyyy-MM-dd") without argument labels. Which function declaration is correct?

hard

A. func formatDate(_ date: Date, _ format: String) -> String { /* implementation */ }

B. func formatDate(date: Date, format: String) -> String { /* implementation */ }

C. func formatDate(_ date: Date, format: String) -> String { /* implementation */ }

D. func formatDate(date: Date, _ format: String) -> String { /* implementation */ }

Solution

1. Step 1: Understand call without labels

   Calling formatDate(myDate, "yyyy-MM-dd") means both parameters must omit labels.

2. Step 2: Match function declaration

   Only func formatDate(_ date: Date, _ format: String) -> String { /* implementation */ } uses underscore for both parameters, allowing call without labels.

3. Final Answer:

   func formatDate(_ date: Date, _ format: String) -> String { /* implementation */ } -> Option A

4. Quick Check:

   Both params underscore = no labels in call [OK]

Hint: Use underscore for all params to omit all labels [OK]

Common Mistakes:

• Omitting only one label but calling without both
• Mixing labeled and unlabeled parameters incorrectly
• Forgetting underscore syntax