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POIs String.IndexOf(char) really slower than manual searching?
 

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  1. POIs String.IndexOf(char) really slower than manual searching?
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    <p>When doing a simple performance measurement, I was astonished to see that calling <code>String.IndexOf(char)</code> was actually slower than doing it manually! Is this really true?! Here is my test code:</p> <pre><code>const string str = @"91023m lkajsdfl;jkasdf;piou-09324\\adf \asdf\45\ 65u\ 86\ 8\\\;"; static int testIndexOf() { return str.IndexOf('\\'); } static int testManualIndexOf() { string s = str; for (int i = 0; i &lt; s.Length; ++i) if (s[i] == '\\') return i; return -1; } </code></pre> <p>I ran each method 25 million times, and measured the time using a <code>Stopwatch</code>. The manual version was consistently 25% faster than the other one.</p> <p>Any thoughts?!</p> <hr> <p><strong>Edit 2:</strong> Running the tests on a different machine with .NET 4.0 yields results very similar to those in Marc Gravell's answer. <strong><code>String.IndexOf(char)</code></strong> <em>is faster</em> than manual searching.</p> <hr> <p><strong>Edit:</strong> Today (Jan 4 '09) I wanted to check this issue in a more comprehensive way, so I created a new project, and wrote the code you'll find below. I got the following results when running release mode from cmd for 100 million iterations:</p> <pre><code> - String. IndexOf : 00:00:07.6490042 - MyString.PublicIndexOf : 00:00:05.6676471 - MyString.InternIndexOf : 00:00:06.1191796 - MyString.PublicIndexOf2: 00:00:09.1363687 - MyString.InternIndexOf2: 00:00:09.1182569 </code></pre> <p>I ran these tests at least 20 times, getting almost the same results every time. My machine is XP SP3, VS 2008 SP1, P4 3.0 GHz with no hyper-threading and 1 GB RAM. I find the results really strange. As you can see, <code>String.IndexOf</code> was about 33% slower than my <code>PublicIndexOf</code>. Even stranger, I wrote the same method as <code>internal</code>, and it was about 8% slower than the <code>public</code> one! I do not understand what's happening, and I hope you could help me understand!</p> <p>The testing code is below. (Sorry for the repeated code, but I found that using a delegate showed different timings, with the <code>public</code> and <code>internal</code> methods taking the same time.)</p> <pre><code>public static class MyString { public static int PublicIndexOf(string str, char value) { for (int i = 0; i &lt; str.Length; ++i) if (str[i] == value) return i; return -1; } internal static int InternIndexOf(string str, char value) { for (int i = 0; i &lt; str.Length; ++i) if (str[i] == value) return i; return -1; } public static int PublicIndexOf2(string str, char value, int startIndex) { if (startIndex &lt; 0 || startIndex &gt;= str.Length) throw new ArgumentOutOfRangeException("startIndex"); for (; startIndex &lt; str.Length; ++startIndex) if (str[startIndex] == value) return startIndex; return -1; } internal static int InternIndexOf2(string str, char value, int startIndex) { if (startIndex &lt; 0 || startIndex &gt;= str.Length) throw new ArgumentOutOfRangeException("startIndex"); for (; startIndex &lt; str.Length; ++startIndex) if (str[startIndex] == value) return startIndex; return -1; } } class Program { static void Main(string[] args) { int iterations = 100 * 1000 * 1000; // 100 millions char separator = '\\'; string str = @"91023m lkajsdfl;jkasdf;piou-09324\\adf \asdf\45\ 65u\ 86\ 8\\\;"; Stopwatch watch = new Stopwatch(); // test String.IndexOf int sum = 0; watch.Start(); for (int i = 0; i &lt; iterations; ++i) sum += str.IndexOf(separator); watch.Stop(); Console.WriteLine(" String. IndexOf : ({0}, {1})", watch.Elapsed, sum); // test MyString.PublicIndexOf sum = 0; watch.Reset(); watch.Start(); for (int i = 0; i &lt; iterations; ++i) sum += MyString.PublicIndexOf(str, separator); watch.Stop(); Console.WriteLine("MyString.PublicIndexOf : ({0}, {1})", watch.Elapsed, sum); // test MyString.InternIndexOf sum = 0; watch.Reset(); watch.Start(); for (int i = 0; i &lt; iterations; ++i) sum += MyString.InternIndexOf(str, separator); watch.Stop(); Console.WriteLine("MyString.InternIndexOf : ({0}, {1})", watch.Elapsed, sum); // test MyString.PublicIndexOf2 sum = 0; watch.Reset(); watch.Start(); for (int i = 0; i &lt; iterations; ++i) sum += MyString.PublicIndexOf2(str, separator,0); watch.Stop(); Console.WriteLine("MyString.PublicIndexOf2: ({0}, {1})", watch.Elapsed, sum); // test MyString.InternIndexOf2 sum = 0; watch.Reset(); watch.Start(); for (int i = 0; i &lt; iterations; ++i) sum += MyString.InternIndexOf2(str, separator,0); watch.Stop(); Console.WriteLine("MyString.InternIndexOf2: ({0}, {1})", watch.Elapsed, sum); } } </code></pre>
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    1. COEy, that's interesting. Maybe IndexOf handles some special cases.
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    2. COIt would help if you posted the actual code that ran the test.
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    3. COPlease post the test rig. I cannot reproduce (see my reply for my test rig).
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      1. USMarc Gravell
 

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