Different ways to check for String prefix in Swift

Checking whether a String starts with a string

You can use the hasPrefix(_:) method to test whether a string begins with the specified prefix.

let str = "Hello! Swift"
str.hasPrefix("Hello") // true

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Getting a substring of prefix

There are many ways and variations to do this.

Using length

You use use prefix(_ maxLength:) to get substrings up to the specified length.

let str = "Hello! Swift"
str.prefix(6) 
// "Hello!"

Since the parameter is maxLength, you can specify an argument which larger than a string without crashing. This method will return the whole string if the number specified is larger than the string length.

let str = "Hello! Swift"
str.prefix(100)
// "Hello! Swift"

Using String.Index

String.Index is like a cursor to tell the position of a character in a string. In my example, I will point our String.Index to ! in Hello! Swift string.

To do that, we get the string startIndex and add the offset of 5.

let str = "Hello! Swift"
let index = str.index(str.startIndex, offsetBy: 5)
// index point to !

After we have the index, we have two options to get a substring of a prefix, prefix(through:) and prefix(upTo:).

The only difference between these two methods is prefix(upTo:) will return a prefix up to the string index specified, but not include the specified position.

let str = "Hello! Swift"
let index = str.index(str.startIndex, offsetBy: 5)

str.prefix(through: index) // "Hello!"
str.prefix(upTo: index)    // "Hello", not include index specified (!)

Subscripts

We can achieve the same result of prefix(through:) and prefix(upTo:) with a string subscript with a range of string index.

let str = "Hello! Swift"
let index = str.index(str.startIndex, offsetBy: 5)

str[...index] // "Hello!", same as str.prefix(through: index)
str[..<index] // "Hello", same as str.prefix(upTo: index)

Advanced

All of the examples above are getting prefix by using a length and index of characters. If you have specific needs, you might want to use the following methods.

Condition

You can use prefix(while:) to get a prefix up to the point where the predicate return false.

let str = "Hello! Swift"
str.prefix(while: { (character) -> Bool in
    return character != " "
})
// Hello!

In the above example, we get a prefix until the first empty character. The result is everything before empty space, Hello!.

String.Index

In the previous String.Index example, we use a simple index offset, but String.Index has many more ways to get an index of a character.

Here are some examples:

firstIndex(of:), this method will return the first index where the specified value appears.

let str = "Hello! Swift"
if let index = str.firstIndex(of: "l") {
    str.prefix(through: index) // "Hel"
}

.lastIndex(of:), this method will return the last index where the specified value appears.

let str = "Hello! Swift"
if let index = str.lastIndex(of: "l") {
    str.prefix(through: index) // "Hell"
}

After you get an index you want, you can use it as a argument for the methods we learn to get a prefix you want.

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Related Resources

• Different ways to check for String suffix in Swift
• Swift Strings and Characters
• Apple Documentation
  • prefix(_:)
  • prefix(upTo:)
  • prefix(through:)
  • prefix(while:)
  • firstIndex(of:)
  • firstIndex(where:)
  • lastIndex(of:)
  • lastIndex(where:)