blocks|key|128300|text|针对Swift+4更新|type|unstyled|depth|inlineStyleRanges|entityRanges|data|128301|检查字符串的开头和结尾|128302|可以使用hasPrefix(_:)和hasSuffix(_:)方法来测试与另一个字符串的相等性。|offset|length|style|CODE|128303|let+str+=+"Hello,+playground"

if+str.hasPrefix("Hello")+{+//+true
++++print("Prefix+exists")
}

if+str.hasSuffix("ground")+{+//+true
++++print("Suffix+exists")
}|code-block|syntax|javascript|128304|获取实际的前缀和后缀子字符串|128305|为了获得实际的前缀或后缀子字符串，您可以使用以下方法之一。我推荐第一种方法，因为它很简单。所有方法都使用str作为|128306|let+str+=+"Hello,+playground"|128307|方法1：(推荐)+prefix(Int)+suffix(Int)+|BOLD|128308|let+prefix+=+String(str.prefix(5))+//+Hello
let+suffix+=+String(str.suffix(6))+//+ground|128309|在我看来，这是更好的方法。与下面的方法2和3不同，如果索引超出界限，此方法不会崩溃。它将只返回字符串中的所有字符。|128310|let+prefix+=+String(str.prefix(225))+//+Hello,+playground
let+suffix+=+String(str.suffix(623))+//+Hello,+playground|128311|当然，有时崩溃是好的，因为它们让你知道你的代码有问题。因此，也可以考虑下面的第二种方法。如果索引超出范围，它将抛出错误。|128312|方法2：prefix(upto:)+suffix(from:)+|128313|Swift字符串索引很棘手，因为它们必须考虑特殊字符(如表情符号)。但是，一旦获得索引，就很容易获得前缀或后缀。(请参阅String.Index上的my+other+answer。)|128314|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|128315|如果您想防止越界，可以使用limitedBy+(同样，请参阅this+answer)创建索引。|128316|方法3:下标|128317|因为String是一个集合，所以您可以使用下标来获取前缀和后缀。|128318|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|128319|进一步阅读|128320|128321|Strings+and+Characters+documentation|unordered-list-item|128322|128323|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/39676940/3681880|1|2|https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html^0|0|0|4|D|I|D|0|0|0|1G|3|0|0|9|B|L|B|9|B|L|B|0|0|0|0|0|4|D|I|D|4|D|I|D|0|1O|C|22|F|0|0|0|D|9|U|B|1|0|0|6|0|0|0|0|0|0|10|2|0|0^^$0|@$1|2|3|4|5|6|7|21|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|22|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|23|8|@$F|24|G|25|H|I]|$F|26|G|27|H|I]]|9|@]|A|$]]|$1|J|3|K|5|L|7|28|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|29|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|2A|8|@$F|2B|G|2C|H|I]]|9|@]|A|$]]|$1|S|3|T|5|L|7|2D|8|@]|9|@]|A|$M|N]]|$1|U|3|V|5|6|7|2E|8|@$F|2F|G|2G|H|W]|$F|2H|G|2I|H|W]|$F|2J|G|2K|H|I]|$F|2L|G|2M|H|I]]|9|@]|A|$]]|$1|X|3|Y|5|L|7|2N|8|@]|9|@]|A|$M|N]]|$1|Z|3|10|5|6|7|2O|8|@]|9|@]|A|$]]|$1|11|3|12|5|L|7|2P|8|@]|9|@]|A|$M|N]]|$1|13|3|14|5|6|7|2Q|8|@]|9|@]|A|$]]|$1|15|3|16|5|6|7|2R|8|@$F|2S|G|2T|H|W]|$F|2U|G|2V|H|W]|$F|2W|G|2X|H|I]|$F|2Y|G|2Z|H|I]]|9|@]|A|$]]|$1|17|3|18|5|6|7|30|8|@$F|31|G|32|H|I]]|9|@$F|33|G|34|1|35]]|A|$]]|$1|19|3|1A|5|L|7|36|8|@]|9|@]|A|$M|N]]|$1|1B|3|1C|5|6|7|37|8|@$F|38|G|39|H|I]]|9|@$F|3A|G|3B|1|3C]]|A|$]]|$1|1D|3|1E|5|6|7|3D|8|@$F|3E|G|3F|H|W]]|9|@]|A|$]]|$1|1F|3|1G|5|6|7|3G|8|@]|9|@]|A|$]]|$1|1H|3|1I|5|L|7|3H|8|@]|9|@]|A|$M|N]]|$1|1J|3|1K|5|6|7|3I|8|@]|9|@]|A|$]]|$1|1L|3|-4|5|6|7|3J|8|@]|9|@]|A|$]]|$1|1M|3|1N|5|1O|7|3K|8|@]|9|@$F|3L|G|3M|1|3N]]|A|$]]|$1|1P|3|-4|5|6|7|3O|8|@]|9|@]|A|$]]|$1|1Q|3|-4|5|6|7|3P|8|@]|9|@]|A|$]]]|1R|$1S|$5|1T|1U|1V|A|$1W|1X]]|1Y|$5|1T|1U|1V|A|$1W|1X]]|1Z|$5|1T|1U|1V|A|$1W|20]]]]

Updated for Swift 4

<h1>Checking what a String starts with and ends with</h1>

You can use the <code>hasPrefix(_:)</code> and <code>hasSuffix(_:)</code> methods to test equality with another String.

<pre><code>let str = "Hello, playground"

if str.hasPrefix("Hello") { // true
 print("Prefix exists")
}

if str.hasSuffix("ground") { // true
 print("Suffix exists")
}
</code></pre>

<h1>Getting the Actual Prefix and Suffix Substrings</h1>

In order to get the actual prefix or suffix substring, you can use one of the following methods. I recommend the first method for it's simplicity. All methods use <code>str</code> as

<pre><code>let str = "Hello, playground"
</code></pre>

Method 1: (Recommended) <code>prefix(Int)</code> and <code>suffix(Int)</code>

<pre><code>let prefix = String(str.prefix(5)) // Hello
let suffix = String(str.suffix(6)) // ground
</code></pre>

This is the better method in my opinion. Unlike the methods 2 and 3 below, this method will not crash if the indexes go out of bounds. It will just return all the characters in the string. 

<pre><code>let prefix = String(str.prefix(225)) // Hello, playground
let suffix = String(str.suffix(623)) // Hello, playground
</code></pre>

Of course, sometimes crashes are good because they let you know there is a problem with your code. So consider the second method below as well. It will throw an error if the index goes out of bounds.

Method 2: <code>prefix(upto:)</code> and <code>suffix(from:)</code>

Swift String indexes are tricky because they have to take into account special characters (like emoji). However once you get the index it is easy to get the prefix or suffix. (See <a href="https://stackoverflow.com/a/39676940/3681880">my other answer</a> on <code>String.Index</code>.)

<pre><code>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
</code></pre>

If you want to guard against going out of bounds, you can make an index using <code>limitedBy</code> (again, see <a href="https://stackoverflow.com/a/39676940/3681880">this answer</a>).

Method 3: subscripts

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

<pre><code>let prefixIndex = str.index(str.startIndex, offsetBy: 5)
let prefix = String(str[..&lt;prefixIndex]) // Hello

let suffixIndex = str.index(str.endIndex, offsetBy: -6)
let suffix = String(str[suffixIndex...]) // ground
</code></pre>

<h1>Further Reading</h1>

<ul>
<li><a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html" rel="noreferrer">Strings and Characters documentation</a></li>
</ul>

blocks|key|127748|text|字符串前缀和后缀相等要检查字符串是否具有特定的字符串前缀或后缀，请调用该字符串的hasPrefix(:)和hasSuffix(:)方法，这两个方法都采用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|127749|类型的单个参数并返回布尔值。|blockquote|127750|127751|127752|Apple+Doc|offset|length|127753|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html^0|0|0|0|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$]]|$1|E|3|-4|5|6|7|U|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|V|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|W|8|@]|9|@$I|X|J|Y|1|Z]]|A|$]]|$1|K|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

<blockquote>
 <blockquote>
 Prefix and Suffix Equality
 </blockquote>
 
 To check whether a string has a particular string prefix or suffix,
 call the string’s hasPrefix(:) and hasSuffix(:) methods, both of
 which take a single argument of type String and return a Boolean
 value.
</blockquote>

<a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html" rel="noreferrer">Apple Doc</a>

I'm trying to test if a Swift string starts or ends with a certain value. These methods do not exist:

<pre><code>var str = "Hello, playground"
str.startsWith("Hello") // error
str.endsWith("ground") // error
</code></pre>

I would also like to get the prefix and suffix strings. I could find a substring, as is answered <a href="https://stackoverflow.com/questions/24044851/how-do-you-use-string-substringwithrange-or-how-do-ranges-work-in-swift">here</a> and <a href="https://stackoverflow.com/questions/28182441/swift-how-to-get-substring-from-start-to-last-index-of-character">here</a>, but ranges are such a pain in Swift.

Is there an easier way to do it?

(I stumbled across the answer when I was reading <a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html" rel="noreferrer">the documentation</a>, and since an SO answer didn't come up for my search terms, I am adding my Q&amp;A here.)

How to check what a String starts with (prefix) or ends with (suffix) in Swift

Swift

我正在尝试测试Swift字符串是以某个值开始还是以某个值结束。这些方法不存在：var str = "Hello, playground"str.startsWith("Hello") // errorstr.endsWith("ground") // error我还想获得前缀和后缀字符串。我可以找到一个子字符串，答案是...

问如何在Swift中检查字符串以(前缀)开头或以(后缀)结尾
EN

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