Swift Functions and Argument Labels

Before you start

Swift basics.

Use default values and argument labels to make calls readable.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A Swift function declares each parameter with an optional argument label, a name, and a type. The underscore in _ name removes the label for the first parameter, while times keeps its label, so callers write greet("Bo", times: 2).

A default value such as times: Int = 1 makes the argument optional. The function builds an array with Array(repeating:count:) and joins it with a separator, so no trailing space is left over.

Code example

Swift

func greet(_ name: String, times: Int = 1) -> String {
    return Array(repeating: "Hi \(name)!", count: times).joined(separator: " ")
}

print(greet("Ada"))
print(greet("Bo", times: 2))

Output

Hi Ada!
Hi Bo! Hi Bo!

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

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1func greet(_ name: String, times: Int = 1) -> String {

2 return Array(repeating: "Hi \(name)!", count: times).joined(separator: " ")

3}

5print(greet("Ada"))

6print(greet("Bo", times: 2))

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
The first argument has no label; times defaults to 1.	`—`	—
times is 1, so one greeting is produced.	`name = "Ada", times = 1`	Hi Ada!
Two copies are joined by a space.	`name = "Bo", times = 2`	Hi Ada! Hi Bo! Hi Bo!

Behind the scenes

How Swift processes this code

Swift compiles the program; its top-level statements run in source order.

Cost and common mistakes

O(times) time and space.

Omitting a required argument label, as in greet("Bo", 2), is a compile error.
A negative count passed to Array(repeating:count:) crashes at runtime.

Try it yourself

Predict greet("Cy", times: 0).

Reveal answer and reasoning

It returns an empty string, so print writes an empty line.