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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
正文
#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
#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
#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；
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 }
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：
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 }
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