How to join an Array of strings into a Single string in Swift

There are two easy ways to join an array of strings into a single string.

1. Use the joined method
2. Use ListFormatter

joined method

The joined(separator:) method is an Array instance method which combine all element together into a single string.

You can also specify a separator that will insert between each element.

In this example, we combine three names and separate them with a comma (", ").

let names = ["John", "Alice", "Bob"]

let joinedNames = names.joined(separator: ", ")
print(joinedNames)
// John, Alice, Bob

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ListFormatter

Swift also has a dedicated formatter, ListFormatter, that provides formatting for a list of items.

ListFormatter will format a list using the separator and conjunction that match the provided Locale.

let names = ["John", "Alice", "Bob"]

let listFormatter = ListFormatter()
let joinedNames = listFormatter.string(from: names)
print(joinedNames)
// John, Alice, and Bob

As you can see, ListFormatter add both separators (,) and conjunction (and) for us.

It is also locale-aware, so if you set a locale that doesn't need separators or conjunction, it won't use that in the resulting string.

Here is an example of using ListFormatter with a Thai locale.

let listFormatter = ListFormatter()
listFormatter.locale = Locale(identifier: "th_TH")
let joinedNames = listFormatter.string(from: names)
print(joinedNames)
// John Alice และ Bob

The Thai language doesn't use separators when presenting a list of items.

ListFormatter knows that and presents a string without separators.

John Alice และ Bob
// และ is conjunction.

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Which method should I use

Both joined and ListFormatter can join an Array of strings into a single string, but it is suitable in different situations.

ListFormatter is suitable for user-facing UI. Since it is locale-aware, you can ensure it is present in the best way possible in a specified locale.

On the other hand, the joined method gives you more control, and the resulting string is predictable. The string is separated by what you specified without taking a locale into consideration.

This makes the joined method suitable for a string that you want to control the format, e.g., build a file name from an array of component strings.

Here is an example of building a filename string using joined.

let fileName = "foo"
let createdDate = "2022-11-03"
let nameComponents = [fileName, createdDate]

let name = nameComponents.joined(separator: "_").appending(".png")
print(name)
// foo_2022-11-03.png