Summary of seven ways to get string prefix in swift

We thought"Hello World"This string is used as an example to determine whether it starts with Hello.

1. Use the hasPrefix(_:) method

Can use stringshasPrefix(_:)Method check whether the string has a specified prefix:

let str = "Hello World"
if ("Hello") { // true
    print("\(str) by Hello beginning")
} 

This method directly returns aBoolTo determine whether it starts with a certain string.

2. The prefix function gets the prefix substring

Can be usedprefix(_:)To get the prefix substring:

let str = "Hello World"
let prefix = (5)
if prefix == "Hello" { 
    print("\(str) by Hello beginning")
} 

This method usesprefixThe function gets the first 5 characters and then compares it with "Hello".

3. The prefix(upTo:) function gets the prefix substring

Can be usedprefix(upTo:)To get the prefix substring:

let str = "Hello World"
let index = (, offsetBy: 5)
let prefix = (upTo: index)
if prefix == "Hello" {
    print("\(str) by Hello beginning")
}

This method is first usedindex(_:, offsetBy:)Get the subscript of the first five characters and use itprefix(upTo:)The method of function obtaining the first 5 characters and finally comparing it with "Hello" is suitable for obtaining the first n characters of the string.

4. Use string interval index

First get the subscript of the first 5 characters, then get the values ​​of the first 5 characters according to the subscript interval, and finally compare it with the corresponding string:

let str = "Hello World"
let index = (, offsetBy: 5)
let prefix = str[..<index]
if prefix == "Hello" {
    print("\(str) by Hello beginning")
}

5. Use conditions to obtain

Can be usedprefix(while:)Get the prefix that meets the conditions:

let str = "Hello World"
let prefix =  { c in
    !
}
if prefix == "Hello" {
    print("\(str) by Hello beginning")
}

This method usesprefix(while:)The function gets the string that specifies the specified condition (before the first space), and then compares it with "Hello" to get the result.

6. Use firstIndex/lastIndex

Can be combinedfirstIndex(of:)orlastIndex(of:)Gets the index of a specific character, thus obtaining the prefix:

let str = "Hello World"
if let end = (of: " "),
   str[..<end] == "Hello" {
    print("\(str) by Hello beginning")
}

Use firstfirstIndex(of:)The method gets the position where the first space is located, and then obtains the specified prefix based on the subscript interval.

7. Use the prefix(through:) function

prefix(through:)You can obtain a subset from the beginning to the specified position, which is similar to the second method above, except that the parameters here are passed in the subscript type:

let str = "Hello World"
let index = (, offsetBy: 4)
let prefix = (through: index)
if prefix == "Hello" {
    print("\(str) by Hello beginning")
}

The above are 7 common methods to obtain string prefixes, and you can choose the most suitable method according to your needs.

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