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C#判断字符串中是否包含指定字符串,contains与indexof方法效率问题

TIME:2018-10-14

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 #方法一:使用string.Contains方法

string.Contains是大小写敏感的,如果要用该方法来判断一个string是否包含某个关键字keyword,需要把这个string和这个keyword都转成小写或大写再调用Contains方法;

1 string key = "bbb"; 2 string temp = "aaaBBBcccDDD"; 3 bool isContains= temp.ToLower().Contains(key.ToLower());//true
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#方法二:使用sring.Index方法

使用string.Index方法,然后通过StringComparison.OrdinalIgnoreCase指定查找过程忽略大小写;

1 string key = "bbb"; 2 string temp = "aaaBBBcccDDD"; 3 bool isContains = temp.IndexOf(key,StringComparison.OrdinalIgnoreCase)>=0;//true
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 #那什么时候使用Contains方法,什么时候使用Index方法,哪个效率高?

1、测试代码:

注:此测试针对的是拥有大量英文的情况下,并且指定的字符串为英文

每个方法测试1千万次,输出所用时间;

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 1 class Program  2  {  3 private const int N = 10000000;  4 private static Stopwatch watch = new Stopwatch();  5 static void Main(string[] args)  6  {  7  8 string source = "aaabbbcccdddeeefffggghhhiiijjjkkklllmmmnnnooopppqqq";  9 string target = "AAA"; 10 Console.WriteLine("目标在开头部分时:"); 11 Console.WriteLine("不区分大小写:"); 12 TestContains(source, target,true); 13 TestIndexOf(source, target,true); 14 Console.WriteLine("区分大小写:"); 15 target = "aaa"; 16 TestContains(source, target,false); 17 TestIndexOf(source, target,false); 18  Console.WriteLine(); 19 20 Console.WriteLine("目标在中部时:"); 21 Console.WriteLine("不区分大小写:"); 22 target = "HHH"; 23 TestContains(source, target, true); 24 TestIndexOf(source, target, true); 25 Console.WriteLine("区分大小写:"); 26 target = "hhh"; 27 TestContains(source, target, false); 28 TestIndexOf(source, target, false); 29  Console.WriteLine(); 30 31 Console.WriteLine("目标在结尾时:"); 32 Console.WriteLine("不区分大小写:"); 33 target = "QQQ"; 34 TestContains(source, target,true); 35 TestIndexOf(source, target,true); 36 Console.WriteLine("区分大小写:"); 37 target = "qqq"; 38 TestContains(source, target,false); 39 TestIndexOf(source, target,false); 40 41 Console.WriteLine("执行完毕,按任意键退出..."); 42  Console.ReadKey(); 43 44  } 45 private static void TestIndexOf(string source, string target,bool isIgnoreCase) 46  { 47  watch.Reset(); 48  watch.Start(); 49 for (int i = 0; i < N; i++) 50  { 51 if (isIgnoreCase) 52  source.IndexOf(target, StringComparison.OrdinalIgnoreCase); 53 else 54  source.IndexOf(target); 55  } 56  watch.Stop(); 57 Console.WriteLine("IndexOf: " + watch.ElapsedMilliseconds.ToString() + "ms"); 58 return; 59  } 60 61 private static void TestContains(string source, string target,bool isIgnoreCase) 62  { 63  watch.Reset(); 64  watch.Start(); 65 for (int i = 0; i < N; i++) 66  { 67 if (isIgnoreCase) 68  source.ToLower().Contains(target.ToLower()); 69 else 70  source.Contains(target); 71  } 72  watch.Stop(); 73 Console.WriteLine("Contains: " + watch.ElapsedMilliseconds.ToString() + "ms"); 74 return; 75  } 76 }
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2、测试结果:

3、总结

1、从测试结果(大量测试)中能明显看出,当拥有大量英文的字符串中:

*当不区分大小写时,string.IndexOf方法的效率明显高于string.Contains方法;

*当区分大小写时,string.Contains方法的效率明显高于string.IndexOf方法;

*如果判断的是中文,没有大小写之分,还是string.Contains方法的效率高;

2、综合上述总结,定义了一个String扩展方法,该方法包含一个StringComparison参数,返回值为是否包含子字符串:

参考:https://docs.microsoft.com/zh-cn/dotnet/api/system.string.contains?redirectedfrom=MSDN&view=netframework-4.5#System_String_Contains_System_String_

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 1 using System;  2  3 public static class StringExtensions  4 {  5 public static bool Contains(this String str, String substring,  6  StringComparison comp)  7  {  8 if (substring == null)  9 throw new ArgumentNullException("substring", 10 "substring cannot be null."); 11 else if (! Enum.IsDefined(typeof(StringComparison), comp)) 12 throw new ArgumentException("comp is not a member of StringComparison", 13 "comp"); 14 15 return str.IndexOf(substring, comp) >= 0; 16  } 17 }
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 1 using System;  2  3 public class Example  4 {  5 public static void Main()  6  {  7 String s = "This is a string.";  8 String sub1 = "this";  9 Console.WriteLine("Does '{0}' contain '{1}'?", s, sub1); 10 StringComparison comp = StringComparison.Ordinal; 11 Console.WriteLine(" {0:G}: {1}", comp, s.Contains(sub1, comp)); 12 13 comp = StringComparison.OrdinalIgnoreCase; 14 Console.WriteLine(" {0:G}: {1}", comp, s.Contains(sub1, comp)); 15  } 16 } 17 // The example displays the following output: 18 // Does 'This is a string.' contain 'this'? 19 // Ordinal: False 20 // OrdinalIgnoreCase: True
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