我正在做一些事情，我意识到我想要在一个字符串中找到多少个/s，然后我突然想到，有几种方法可以做到这一点，但不能决定哪种是最好的(或最简单的)。
目前我想说的是:
string source = "/once/upon/a/time/";
int count = source.Length - source.Replace("/", "").Length;
但我一点都不喜欢，有人愿意吗?
我并不想为此挖掘出正则表达式，对吧?
我知道我的字符串将包含我要搜索的项，所以你可以假设…
当然对于长度为> 1的字符串，
string haystack = "/once/upon/a/time";
string needle = "/";
int needleCount = ( haystack.Length - haystack.Replace(needle,"").Length ) / needle.Length;
当前回答
string source = "/once/upon/a/time/";
int count = 0;
int n = 0;
while ((n = source.IndexOf('/', n)) != -1)
{
n++;
count++;
}
在我的电脑上，这比5000万次迭代的每个字符解决方案快2秒左右。
2013年修订:
将字符串更改为char[]并遍历该字符串。将5000万次迭代的总时间进一步缩短一到两秒!
char[] testchars = source.ToCharArray();
foreach (char c in testchars)
{
if (c == '/')
count++;
}
这个更快:
char[] testchars = source.ToCharArray();
int length = testchars.Length;
for (int n = 0; n < length; n++)
{
if (testchars[n] == '/')
count++;
}
为了更好地衡量，从数组的末尾迭代到0似乎是最快的，大约5%。
int length = testchars.Length;
for (int n = length-1; n >= 0; n--)
{
if (testchars[n] == '/')
count++;
}
我想知道为什么这可能是谷歌周围(我记得一些关于反向迭代更快)，并遇到了这个SO问题，烦人地使用字符串char[]技术。不过，我认为在这种情况下，反转技巧是新的。
在c#中迭代字符串中单个字符的最快方法是什么?
其他回答
查找字符计数与查找字符串计数有很大不同。另外，这也取决于你是否想要检查不止一个。如果你想检查各种不同的字符计数，像这样的东西可以工作:
var charCounts =
haystack
.GroupBy(c => c)
.ToDictionary(g => g.Key, g => g.Count());
var needleCount = charCounts.ContainsKey(needle) ? charCounts[needle] : 0;
注1:分组到字典中非常有用，因此为它编写GroupToDictionary扩展方法非常有意义。
注意2:拥有自己的字典实现也很有用，它允许默认值，然后您可以自动为不存在的键获取0。
我觉得我们缺少某些类型的子字符串计数，比如不安全的逐字节比较。我把原始海报的方法和我能想到的任何方法结合在一起。
这些是我做的字符串扩展。
namespace Example
{
using System;
using System.Text;
public static class StringExtensions
{
public static int CountSubstr(this string str, string substr)
{
return (str.Length - str.Replace(substr, "").Length) / substr.Length;
}
public static int CountSubstr(this string str, char substr)
{
return (str.Length - str.Replace(substr.ToString(), "").Length);
}
public static int CountSubstr2(this string str, string substr)
{
int substrlen = substr.Length;
int lastIndex = str.IndexOf(substr, 0, StringComparison.Ordinal);
int count = 0;
while (lastIndex != -1)
{
++count;
lastIndex = str.IndexOf(substr, lastIndex + substrlen, StringComparison.Ordinal);
}
return count;
}
public static int CountSubstr2(this string str, char substr)
{
int lastIndex = str.IndexOf(substr, 0);
int count = 0;
while (lastIndex != -1)
{
++count;
lastIndex = str.IndexOf(substr, lastIndex + 1);
}
return count;
}
public static int CountChar(this string str, char substr)
{
int length = str.Length;
int count = 0;
for (int i = 0; i < length; ++i)
if (str[i] == substr)
++count;
return count;
}
public static int CountChar2(this string str, char substr)
{
int count = 0;
foreach (var c in str)
if (c == substr)
++count;
return count;
}
public static unsafe int CountChar3(this string str, char substr)
{
int length = str.Length;
int count = 0;
fixed (char* chars = str)
{
for (int i = 0; i < length; ++i)
if (*(chars + i) == substr)
++count;
}
return count;
}
public static unsafe int CountChar4(this string str, char substr)
{
int length = str.Length;
int count = 0;
fixed (char* chars = str)
{
for (int i = length - 1; i >= 0; --i)
if (*(chars + i) == substr)
++count;
}
return count;
}
public static unsafe int CountSubstr3(this string str, string substr)
{
int length = str.Length;
int substrlen = substr.Length;
int count = 0;
fixed (char* strc = str)
{
fixed (char* substrc = substr)
{
int n = 0;
for (int i = 0; i < length; ++i)
{
if (*(strc + i) == *(substrc + n))
{
++n;
if (n == substrlen)
{
++count;
n = 0;
}
}
else
n = 0;
}
}
}
return count;
}
public static int CountSubstr3(this string str, char substr)
{
return CountSubstr3(str, substr.ToString());
}
public static unsafe int CountSubstr4(this string str, string substr)
{
int length = str.Length;
int substrLastIndex = substr.Length - 1;
int count = 0;
fixed (char* strc = str)
{
fixed (char* substrc = substr)
{
int n = substrLastIndex;
for (int i = length - 1; i >= 0; --i)
{
if (*(strc + i) == *(substrc + n))
{
if (--n == -1)
{
++count;
n = substrLastIndex;
}
}
else
n = substrLastIndex;
}
}
}
return count;
}
public static int CountSubstr4(this string str, char substr)
{
return CountSubstr4(str, substr.ToString());
}
}
}
接下来是测试代码…
static void Main()
{
const char matchA = '_';
const string matchB = "and";
const string matchC = "muchlongerword";
const string testStrA = "_and_d_e_banna_i_o___pfasd__and_d_e_banna_i_o___pfasd_";
const string testStrB = "and sdf and ans andeians andano ip and and sdf and ans andeians andano ip and";
const string testStrC =
"muchlongerword amuchlongerworsdfmuchlongerwordsdf jmuchlongerworijv muchlongerword sdmuchlongerword dsmuchlongerword";
const int testSize = 1000000;
Console.WriteLine(testStrA.CountSubstr('_'));
Console.WriteLine(testStrA.CountSubstr2('_'));
Console.WriteLine(testStrA.CountSubstr3('_'));
Console.WriteLine(testStrA.CountSubstr4('_'));
Console.WriteLine(testStrA.CountChar('_'));
Console.WriteLine(testStrA.CountChar2('_'));
Console.WriteLine(testStrA.CountChar3('_'));
Console.WriteLine(testStrA.CountChar4('_'));
Console.WriteLine(testStrB.CountSubstr("and"));
Console.WriteLine(testStrB.CountSubstr2("and"));
Console.WriteLine(testStrB.CountSubstr3("and"));
Console.WriteLine(testStrB.CountSubstr4("and"));
Console.WriteLine(testStrC.CountSubstr("muchlongerword"));
Console.WriteLine(testStrC.CountSubstr2("muchlongerword"));
Console.WriteLine(testStrC.CountSubstr3("muchlongerword"));
Console.WriteLine(testStrC.CountSubstr4("muchlongerword"));
var timer = new Stopwatch();
timer.Start();
for (int i = 0; i < testSize; ++i)
testStrA.CountSubstr(matchA);
timer.Stop();
Console.WriteLine("CS1 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrB.CountSubstr(matchB);
timer.Stop();
Console.WriteLine("CS1 and: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrC.CountSubstr(matchC);
timer.Stop();
Console.WriteLine("CS1 mlw: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountSubstr2(matchA);
timer.Stop();
Console.WriteLine("CS2 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrB.CountSubstr2(matchB);
timer.Stop();
Console.WriteLine("CS2 and: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrC.CountSubstr2(matchC);
timer.Stop();
Console.WriteLine("CS2 mlw: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountSubstr3(matchA);
timer.Stop();
Console.WriteLine("CS3 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrB.CountSubstr3(matchB);
timer.Stop();
Console.WriteLine("CS3 and: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrC.CountSubstr3(matchC);
timer.Stop();
Console.WriteLine("CS3 mlw: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountSubstr4(matchA);
timer.Stop();
Console.WriteLine("CS4 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrB.CountSubstr4(matchB);
timer.Stop();
Console.WriteLine("CS4 and: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrC.CountSubstr4(matchC);
timer.Stop();
Console.WriteLine("CS4 mlw: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountChar(matchA);
timer.Stop();
Console.WriteLine("CC1 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountChar2(matchA);
timer.Stop();
Console.WriteLine("CC2 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountChar3(matchA);
timer.Stop();
Console.WriteLine("CC3 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
timer.Restart();
for (int i = 0; i < testSize; ++i)
testStrA.CountChar4(matchA);
timer.Stop();
Console.WriteLine("CC4 chr: " + timer.Elapsed.TotalMilliseconds + "ms");
}
结果:CSX与CountSubstrX对应，CCX与CountCharX对应。“chr”搜索字符串中的“_”，“and”搜索字符串中的“and”，“mlw”搜索字符串中的“muchlongerword”
CS1 chr: 824.123ms
CS1 and: 586.1893ms
CS1 mlw: 486.5414ms
CS2 chr: 127.8941ms
CS2 and: 806.3918ms
CS2 mlw: 497.318ms
CS3 chr: 201.8896ms
CS3 and: 124.0675ms
CS3 mlw: 212.8341ms
CS4 chr: 81.5183ms
CS4 and: 92.0615ms
CS4 mlw: 116.2197ms
CC1 chr: 66.4078ms
CC2 chr: 64.0161ms
CC3 chr: 65.9013ms
CC4 chr: 65.8206ms
最后，我有了一个包含360万个字符的文件。“derp adfderdserp dfaerpderp deasderp”重复了10万次。我用上述方法在文件中搜索“derp”100次，得到这些结果。
CS1Derp: 1501.3444ms
CS2Derp: 1585.797ms
CS3Derp: 376.0937ms
CS4Derp: 271.1663ms
所以我的第四种方法肯定是赢家，但是，实际上，如果一个360万个字符的文件100次只需要1586ms作为最坏的情况，那么所有这些都是可以忽略不计的。
顺便说一下，我还用100次CountSubstr和CountChar方法在360万个字符的文件中扫描了'd'字符。结果……
CS1 d : 2606.9513ms
CS2 d : 339.7942ms
CS3 d : 960.281ms
CS4 d : 233.3442ms
CC1 d : 302.4122ms
CC2 d : 280.7719ms
CC3 d : 299.1125ms
CC4 d : 292.9365ms
原来海报的方法是非常糟糕的单个字符针在一个大草堆根据这一点。
注:所有值更新为发布版本输出。在我第一次发布这篇文章时，我不小心忘记了建立发布模式。我的一些声明已经被修改了。
string source = "/once/upon/a/time/";
int count = 0;
foreach (char c in source)
if (c == '/') count++;
必须比source.Replace()本身更快。
var conditionalStatement = conditionSetting.Value;
//order of replace matters, remove == before =, incase of ===
conditionalStatement = conditionalStatement.Replace("==", "~").Replace("!=", "~").Replace('=', '~').Replace('!', '~').Replace('>', '~').Replace('<', '~').Replace(">=", "~").Replace("<=", "~");
var listOfValidConditions = new List<string>() { "!=", "==", ">", "<", ">=", "<=" };
if (conditionalStatement.Count(x => x == '~') != 1)
{
result.InvalidFieldList.Add(new KeyFieldData(batch.DECurrentField, "The IsDoubleKeyCondition does not contain a supported conditional statement. Contact System Administrator."));
result.Status = ValidatorStatus.Fail;
return result;
}
需要做一些类似于从字符串测试条件语句的事情。
用单个字符替换我正在寻找的内容，并计算单个字符的实例数。
显然，在发生这种情况之前，您需要检查您正在使用的单个字符是否存在于字符串中，以避免错误计数。
从。net 7开始，我们就有了不需要分配(并且高度优化)的Regex api。计数尤其容易和有效。
var input = "abcd abcabc ababc";
var result = Regex.Count(input: input, pattern: "abc"); // 4
当匹配动态模式时，记得转义它们:
public static int CountOccurences(string input, string pattern)
{
pattern = Regex.Escape(pattern); // Aww, no way to avoid heap allocations here
var result = Regex.Count(input: input, pattern: pattern);
return result;
}
而且，作为固定模式的额外奖励，. net 7引入了分析器，帮助将正则表达式字符串转换为源代码生成的代码。这不仅避免了regex的运行时编译开销，而且还提供了非常可读的代码，展示了它是如何实现的。事实上，该代码通常至少与手动编写的任何替代代码一样有效。
如果您的正则表达式调用是合格的，分析程序将给出提示。简单地选择“转换为'GeneratedRegexAttribute '”并享受结果:
[GeneratedRegex("abc")]
private static partial Regex MyRegex(); // Go To Definition to see the generated code
