How to check that a string starts with (prefix) or ends (suffix) in Swift

Question

I am trying to check if a Swift line starts or ends with a specific value. These methods do not exist:

var str = "Hello, playground" str.startsWith("Hello") // error str.endsWith("ground") // error 

I would also like to get the prefix and suffix lines. I could find a substring as it answers here and here , but ranges are such a pain in Swift.

Is there an easier way to do this?

(I came across an answer when I read the documentation , and since the SO answer did not appear in my search terms, I am adding my Q & A here.)

Answer 1

Updated for Swift 4

Checking the beginning of the line and ends with

You can use the hasPrefix(_:) and hasSuffix(_:) methods to check for equality with another string.

 let str = "Hello, playground" if str.hasPrefix("Hello") { // true print("Prefix exists") } if str.hasSuffix("ground") { // true print("Suffix exists") } 

Getting the actual prefix and suffix substrings

To get the actual prefix or suffix substring, you can use one of the following methods. I recommend the first method of simplicity for him. All methods use str as

 let str = "Hello, playground" 

Method 1: (recommended) prefix(Int) and suffix(Int)

 let prefix = String(str.prefix(5)) // Hello let suffix = String(str.suffix(6)) // ground 

This is the best method in my opinion. Unlike methods 2 and 3 below, this method will not crash if the indexes go beyond. It will simply return all characters in the string.

 let prefix = String(str.prefix(225)) // Hello, playground let suffix = String(str.suffix(623)) // Hello, playground 

Of course, sometimes crashes are good because they tell you that there is a problem with your code. Therefore, consider the second method below. It throws an error if the index goes out of bounds.

Method 2: prefix(upto:) and suffix(from:)

Swift String indices are complex because they must consider special characters (e.g. emoji). However, once you get the index, it's easy to get a prefix or suffix. (See my other answer on String.Index .)

 let prefixIndex = str.index(str.startIndex, offsetBy: 5) let prefix = String(str.prefix(upTo: prefixIndex)) // Hello let suffixIndex = str.index(str.endIndex, offsetBy: -6) let suffix = String(str.suffix(from: suffixIndex)) // ground 

If you want to protect against going beyond, you can make an index using limitedBy (again, see this answer ).

Method 3: Indexes

Since String is a collection, you can use indexes to get the prefix and suffix.

 let prefixIndex = str.index(str.startIndex, offsetBy: 5) let prefix = String(str[..<prefixIndex]) // Hello let suffixIndex = str.index(str.endIndex, offsetBy: -6) let suffix = String(str[suffixIndex...]) // ground 

additional literature

• String and character documentation

Answer 2

Prefix and suffix equality

To check whether a string has a specific string prefix or suffix, call the hasPrefix (:) and hasSuffix (:) methods, both of which take a single argument of type String and return a Boolean value.

Apple doc