Contains is faster than StartsWith?

Question

A consultant came by yesterday and somehow the topic of strings came up. He mentioned that he had noticed that for strings less than a certain length, Contains is actually faster than StartsWith. I had to see it with my own two eyes, so I wrote a little app and sure enough, Contains is faster!

How is this possible?

DateTime start = DateTime.MinValue;
DateTime end = DateTime.MinValue;
string str = "Hello there";

start = DateTime.Now;
for (int i = 0; i < 10000000; i++)
{
    str.Contains("H");
}
end = DateTime.Now;
Console.WriteLine("{0}ms using Contains", end.Subtract(start).Milliseconds);

start = DateTime.Now;
for (int i = 0; i < 10000000; i++)
{
    str.StartsWith("H");
}
end = DateTime.Now;
Console.WriteLine("{0}ms using StartsWith", end.Subtract(start).Milliseconds);

Outputs:

726ms using Contains 
865ms using StartsWith

I've tried it with longer strings too!

Answer 1

Keep in mind you're saving 139 milliseconds over 10 million iterations. Sure, if you're trying to set a world record, go for it. Otherwise, use what is going to best convey the meaning of what you're trying to do.

Also keep in mind, the rough equivalent for StartsWith would not be Contains, but rather IndexOf with a result of 0.

Answer 2

Try using StopWatch to measure the speed instead of DateTime checking.

http://stackoverflow.com/questions/2923283/stopwatch-vs-using-system-datetime-now-for-timing-events

I think the key is the following the important parts bolded:

Contains:

This method performs an ordinal (case-sensitive and culture-insensitive) comparison.

StartsWith:

This method performs a word (case-sensitive and culture-sensitive) comparison using the current culture.

I think the key is the ordinal comparison which amounts to:

An ordinal sort compares strings based on the numeric value of each Char object in the string. An ordinal comparison is automatically case-sensitive because the lowercase and uppercase versions of a character have different code points. However, if case is not important in your application, you can specify an ordinal comparison that ignores case. This is equivalent to converting the string to uppercase using the invariant culture and then performing an ordinal comparison on the result.

References:

http://msdn.microsoft.com/en-us/library/system.string.aspx

http://msdn.microsoft.com/en-us/library/dy85x1sa.aspx

http://msdn.microsoft.com/en-us/library/baketfxw.aspx

Using Reflector you can see the code for the two:

public bool Contains(string value)
{
    return (this.IndexOf(value, StringComparison.Ordinal) >= 0);
}

public bool StartsWith(string value, bool ignoreCase, CultureInfo culture)
{
    if (value == null)
    {
        throw new ArgumentNullException("value");
    }
    if (this == value)
    {
        return true;
    }
    CultureInfo info = (culture == null) ? CultureInfo.CurrentCulture : culture;
    return info.CompareInfo.IsPrefix(this, value,
        ignoreCase ? CompareOptions.IgnoreCase : CompareOptions.None);

}

Answer 3

I figured it out. It's because StartsWith is culture-sensitive, while Contains is not. That inherently means StartsWith has to do more work.

FWIW, here are my results on Mono with the below (corrected) benchmark:

1988.7906ms using Contains
10174.1019ms using StartsWith

I'd be glad to see people's results on MS, but my main point is that correctly done (and assuming similar optimizations), I think StartsWith has to be slower:

using System;
using System.Diagnostics;

public class ContainsStartsWith
{
    public static void Main()
    {
        string str = "Hello there";

        Stopwatch s = new Stopwatch();
        s.Start();
        for (int i = 0; i < 10000000; i++)
        {
            str.Contains("H");
        }
        s.Stop();
        Console.WriteLine("{0}ms using Contains", s.Elapsed.TotalMilliseconds);

        s.Reset();
        s.Start();
        for (int i = 0; i < 10000000; i++)
        {
            str.StartsWith("H");
        }
        s.Stop();
        Console.WriteLine("{0}ms using StartsWith", s.Elapsed.TotalMilliseconds);

    }
}

Answer 4

I twiddled around in Reflector and found a potential answer:

Contains:

return (this.IndexOf(value, StringComparison.Ordinal) >= 0);

StartsWith:

...
    switch (comparisonType)
    {
        case StringComparison.CurrentCulture:
            return CultureInfo.CurrentCulture.CompareInfo.IsPrefix(this, value, CompareOptions.None);

        case StringComparison.CurrentCultureIgnoreCase:
            return CultureInfo.CurrentCulture.CompareInfo.IsPrefix(this, value, CompareOptions.IgnoreCase);

        case StringComparison.InvariantCulture:
            return CultureInfo.InvariantCulture.CompareInfo.IsPrefix(this, value, CompareOptions.None);

        case StringComparison.InvariantCultureIgnoreCase:
            return CultureInfo.InvariantCulture.CompareInfo.IsPrefix(this, value, CompareOptions.IgnoreCase);

        case StringComparison.Ordinal:
            return ((this.Length >= value.Length) && (nativeCompareOrdinalEx(this, 0, value, 0, value.Length) == 0));

        case StringComparison.OrdinalIgnoreCase:
            return ((this.Length >= value.Length) && (TextInfo.CompareOrdinalIgnoreCaseEx(this, 0, value, 0, value.Length, value.Length) == 0));
    }
    throw new ArgumentException(Environment.GetResourceString("NotSupported_StringComparison"), "comparisonType");

And there are some overloads so that the default culture is CurrentCulture.

So first of all, Ordinal will be faster (if the string is close to the beginning) anyway, right? And secondly, there's more logic here which could slow things down (although so so trivial)

Answer 5

Comparing the time between two events using datetime comparisons is not really a good idea. There are so many other factors that can affect your calculations. Have you tried switching the order of execution? Is the result exactly same? Moreover, logically thinking, startswith should be faster. Isn't it?