Question

我想找到用转义版本替换字符串中所有保留字符的最快方法。

我自发地想到了两种天真的方式（注意保留字符集就是一个例子）：

答：使用查找字典和String.Replace

private Dictionary<string, string> _someEscapeTokens = new Dictionary<string, string>()
{
    {"\t", @"\t"},
    {"\n", @"\n"},
    {"\r", @"\r"}
};

public string GetEscapedStringByNaiveLookUp(string s)
{
    foreach (KeyValuePair<string, string> escapeToken in _someEscapeTokens.Where(kvp => s.Contains(kvp.Key)))
    {
        s = s.Replace(escapeToken.Key, escapeToken.Value);
    }
    return s;
}

B：遍历字符串中的每个字符

public string GetEscapedStringByTraversingCharArray(string s)
{
    char[] chars = s.ToCharArray();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < chars.Length; i++)
    {
        switch (chars[i])
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            default:
                sb.Append(chars[i]); break;
         }
    }
    return sb.ToString();
}

正如我已经测试的那样，B版很容易胜过第一个。

注意：我已经考虑了Regex.Escape，但由于字符集与我的字符集不匹配，因此不适合。

然而，还有其他方法可以解决这个问题（考虑到性能）吗？

更新：测试

我已经做了一些测试，想分享结果。请参阅下面的代码。

针对 .Net Framework 4.0 的两个不同系统进行了测试。无论如何，结果几乎相同：

Char Array (short string) average time: 38600 ns
Foreach (short string) average time: 26680 ns
Char Array (long string) average time: 48,1 ms
Foreach (long string) average time: 64,2 ms
Char Array (escaping only) average time: 13,6 ms
Foreach (escaping only) average time: 17,3 ms

这让我得出这样的结论：对于短字符串，foreach版本似乎稍微快一点，但对于较长的字符串，某种程度上会“下降”。然而，我们在这里谈论的是非常小的差异。

测试代码：

private static void Main(string[] args)
{
    //around 700 characters
    string shortString = new StackTrace().ToString();
    string longString;
    string pureEscape;
    //loading from a file with 1000000 words http://loremipsum.de/
    using (StreamReader sr = new StreamReader(@"C:\users\ekrueger\desktop\LoremIpsum.txt"))
    {
        longString = sr.ReadToEnd();
    }
    //text file containing only escapable characters (length ~1000000)
    using (StreamReader sr = new StreamReader(@"C:\users\ekrueger\desktop\PureEscape.txt"))
    {
        pureEscape = sr.ReadToEnd();
    }
    List<double> timesCharArrayShortString = new List<double>();
    List<double> timesForeachShortString = new List<double>();
    List<long> timesCharArrayLongString = new List<long>();
    List<long> timesForeachLongString = new List<long>();
    List<long> timesCharArrayPureEscape = new List<long>();
    List<long> timesForeachPureEscape = new List<long>();
    Stopwatch sw = new Stopwatch();

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(shortString);
        sw.Stop();
        timesCharArrayShortString.Add(sw.Elapsed.TotalMilliseconds * 1000000);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(shortString);
        sw.Stop();
        timesForeachShortString.Add(sw.Elapsed.TotalMilliseconds * 1000000);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(longString);
        sw.Stop();
        timesCharArrayLongString.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(longString);
        sw.Stop();
        timesForeachLongString.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(pureEscape);
        sw.Stop();
        timesCharArrayPureEscape.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(pureEscape);
        sw.Stop();
        timesForeachPureEscape.Add(sw.ElapsedMilliseconds);
    }

    Console.WriteLine("Char Array (short string) average time: {0} ns", timesCharArrayShortString.Average());
    Console.WriteLine("Foreach (short string) average time: {0} ns", timesForeachShortString.Average());
    Console.WriteLine("Char Array (long string) average time: {0} ms", timesCharArrayLongString.Average());
    Console.WriteLine("Foreach (long string) average time: {0} ms", timesForeachLongString.Average());
    Console.WriteLine("Char Array (escaping only) average time: {0} ms", timesCharArrayPureEscape.Average());
    Console.WriteLine("Foreach (escaping only) average time: {0} ms", timesForeachPureEscape.Average());

    Console.Read();
}

private static string GetEscapedStringByTraversingCharArray(string s)
{
    if (String.IsNullOrEmpty(s))
        return s;

    char[] chars = s.ToCharArray();
    StringBuilder sb = new StringBuilder(s.Length);
    for (int i = 0; i < chars.Length; i++)
    {
        switch (chars[i])
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            case '\f':
                sb.Append(@"\f"); break;
            default:
                sb.Append(chars[i]); break;
        }
    }
    return sb.ToString();
}

public static string GetEscapedStringForeach(string s)
{
    if (String.IsNullOrEmpty(s))
        return s;

    StringBuilder sb = new StringBuilder(s.Length);
    foreach (Char ch in s)
    {
        switch (ch)
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            default:
                sb.Append(ch); break;
        }
    }
    return sb.ToString();
}

Answer 1

有意义的是，第一个选项较慢，因为您使用以下命令创建了许多字符串对象：

s = s.Replace(escapeToken.Key, escapeToken.Value);

在第二种方法中，不需要创建char []，因为string也有索引器。也许你唯一可以做的就是提高性能是用容量初始化StringBuilder，所以不需要调整大小。您仍然可以在第二种方法中使用Dictionary。

Answer 2

您无需将string转换为Char[]，因此可以略微改进解决方案：

public string GetEscapedStringByTraversingCharArray(string s) {
  // do not forget about null...
  if (String.IsNullOrEmpty(s))
    return s;

  // we can be sure, that it requires at least s.Length symbols, let's allocate then
  // in case we want to trade memory for speed we can even put
  //   StringBuilder sb = new StringBuilder(2 * s.Length);
  // for the worst case when all symbols should be escaped
  StringBuilder sb = new StringBuilder(s.Length);

  foreach(Char ch in s) {
    switch (ch) {
      case '\t':
        sb.Append(@"\t"); 

        break;
      case '\n':
        sb.Append(@"\n"); 

        break;
      case '\r':
        sb.Append(@"\r"); 

        break;
      default:
        sb.Append(ch); 

        break;
    }
  }

  return sb.ToString();
}

在C＃中转义字符串中字符的最快方法

2 个答案: