string中Insert与Format效率对比、String与List中Contains与IndexOf的效率对比

  关于string的效率,众所周知的恐怕是“+”和StringBuilder了,这些本文就不在赘述了。关于本文,请先回答以下问题(假设都是基于多次循环反复调用的情况下):
1.使用Insert与Format方法,哪个效率更高?
2.Contains(value)与IndexOf(value)谁效率更高?

假如您对此2问不感兴趣或已非常了解,请忽略此文。另外本文将不对文中代码的实际用途做任何解释。

<一> 首先看以下的使用场景

            string str1 = "abc";
string str2 = "";
str1 = string.Format("{0}:{1}", str1, str2);
str1 = str1.Insert(, str2);

接下来开始我们的对比之旅(不包含上述代码),编写如下代码用来向控制台输出结果

        static void WriteTime(string title, long time)
{
Console.WriteLine("{1}用时:{0} ms", time, title);
}

再添加一个方法对字符串进行循环操作

        static long LoopCalc(Action<string, string> action)
{
string[] array = new string[];
for (int i = ; i < array.Length; i++)
{
array[i] = i.ToString();
} Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = ; i < array.Length; i++)
{
action(array[i], "bc");
} sw.Stop();
return sw.ElapsedMilliseconds;
}

添加对string进行Insert与Format的效率对比代码并在Main方法中调用

        static void StringPlusDemo()
{
long inserTime = LoopCalc((x, y) => x.Insert(, y));
long formatTime = LoopCalc((x, y) => string.Format("{0}{1}", y, x));
WriteTime("Insert", inserTime);
WriteTime("Format", formatTime);
}

运行结果如下string中Insert与Format效率对比、String与List中Contains与IndexOf的效率对比

明显看到Insert效率更高,但是这种结果有局限性,如果字符串很长,那么经过我亲测他们效率相差无几。

注:我这个只是将Format用于字符串拼接的场景,更高的效率应该仍然是StringBuilder,当然Format的其他不可替代用途太多了,Insert和StringBuilder根本无法替代它,这里就不罗嗦了。

<二> 依然是先看下Contains与IndexOf的使用场景

            string str1 = "abcd";
string str2 = "bc";
if (str1.Contains(str2)) { }
if (str1.IndexOf(str2) > -) { }

在这里仍然使用了上述的LoopCalc方法,并增加如下方法,然后在Main方法中调用其

        static void StringContainsDemo()
{
long indexOfTime = LoopCalc((x, y) => { if (x.IndexOf(y) >= ) { } });
long containersTime = LoopCalc((x, y) => { if (x.Contains(y)) { } });
WriteTime("Contains", containersTime);
WriteTime("IndexOf", indexOfTime);
}

结果string中Insert与Format效率对比、String与List中Contains与IndexOf的效率对比

显然Contains效率更高,为什么呢?我之前也不懂为什么,现在来看下String类的源码(关于.NET自带类库的源码可以谷歌搜到官方的下载地址,我忘了地址了),代码很多,我就贴出以下string类中的方法给各位看官

        // Determines the position within this string of the first occurence of the specified
// string, according to the specified search criteria. The search begins at
// the first character of this string, it is case-sensitive and culture-sensitive,
// and the default culture is used.
//
public int IndexOf(String value) {
return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this,value);
} // Determines the position within this string of the first occurence of the specified
// string, according to the specified search criteria. The search begins at
// startIndex, it is case-sensitive and culture-sensitve, and the default culture is used.
//
public int IndexOf(String value, int startIndex) {
return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this,value,startIndex);
}
        public bool Contains( string value ) {
return ( IndexOf(value, StringComparison.Ordinal) >= );
}

以下是有关Contains调用的IndexOf的重载

        public int IndexOf(String value, StringComparison comparisonType) {
return IndexOf(value, , this.Length, comparisonType);
} public int IndexOf(String value, int startIndex, StringComparison comparisonType) {
return IndexOf(value, startIndex, this.Length - startIndex, comparisonType);
} public int IndexOf(String value, int startIndex, int count, StringComparison comparisonType) {
// Validate inputs
if (value == null)
throw new ArgumentNullException("value"); if (startIndex < || startIndex > this.Length)
throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index")); if (count < || startIndex > this.Length - count)
throw new ArgumentOutOfRangeException("count",Environment.GetResourceString("ArgumentOutOfRange_Count")); switch (comparisonType) {
case StringComparison.CurrentCulture:
return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.None); case StringComparison.CurrentCultureIgnoreCase:
return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.IgnoreCase); case StringComparison.InvariantCulture:
return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.None); case StringComparison.InvariantCultureIgnoreCase:
return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.IgnoreCase); case StringComparison.Ordinal:
return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.Ordinal); case StringComparison.OrdinalIgnoreCase:
return TextInfo.IndexOfStringOrdinalIgnoreCase(this, value, startIndex, count); default:
throw new ArgumentException(Environment.GetResourceString("NotSupported_StringComparison"), "comparisonType");
}
}

结论差不多出来了吧,不过这里还牵扯到另一个类CultureInfo.InvariantCulture.CompareInfo,我也看过该类的代码,里头有unsafe代码,不在本文范畴,但是有个结论就是当把我的Demo里的代码的IndexOf改为“x.IndexOf(y, StringComparison.Ordinal)”,那么他们俩的相率将相差无二。

这里没有牵扯到正则匹配以及LastIndexOf,其实正则匹配有时可能效率比上述方式更高,但是要视场景使用,更通用的方式还是建议“IndexOf(value, StringComparison.Ordinal)”或“Contains”方法。

string其本身就是char数组的封装,其或多或少体现着Array的一些特点,那么接下来再来看看在List集合中的关于Contains与IndexOf的情况。

<三> List的IndexOf方法并没有StringComparison枚举作为参数的方法,直接上代码吧

/// <summary>
/// 演示string,在contain中还可延伸List类(实际上string就是char的集合)
/// </summary>
/// <param name="action"></param>
/// <returns></returns>
static long LoopCalcList(Action<List<int>, int> action)
{
List<int>[] array = new List<int>[];
for (int i = ; i < array.Length; i++)
{
array[i] = new List<int>
{
i,,,,,,
};
} Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = ; i < array.Length; i++)
{
action(array[i], );
} sw.Stop();
return sw.ElapsedMilliseconds;
} static void ListContainsDemo()
{
long indexOfTime = LoopCalcList((x, y) => { if (x.IndexOf(y) >= ) { } });
long containersTime = LoopCalcList((x, y) => { if (x.Contains(y)) { } });
WriteTime("Contains", containersTime);
WriteTime("IndexOf", indexOfTime);
}

List中的效率对比

运行结果string中Insert与Format效率对比、String与List中Contains与IndexOf的效率对比

很显然在判断是否包含时,我们应该坚定不移的使用Contains。

下面再给出List的源码,各位看官自行分析了,对此不做深入研究

        // Contains returns true if the specified element is in the List.
// It does a linear, O(n) search. Equality is determined by calling
// item.Equals().
//
public bool Contains(T item) {
if ((Object) item == null) {
for(int i=; i<_size; i++)
if ((Object) _items[i] == null)
return true;
return false;
}
else {
EqualityComparer<T> c = EqualityComparer<T>.Default;
for(int i=; i<_size; i++) {
if (c.Equals(_items[i], item)) return true;
}
return false;
}
}

List类的Contains方法

        // Returns the index of the first occurrence of a given value in a range of
// this list. The list is searched forwards from beginning to end.
// The elements of the list are compared to the given value using the
// Object.Equals method.
//
// This method uses the Array.IndexOf method to perform the
// search.
//
public int IndexOf(T item) {
return Array.IndexOf(_items, item, , _size);
} int System.Collections.IList.IndexOf(Object item)
{
if(IsCompatibleObject(item)) {
return IndexOf((T)item);
}
return -;
} // Returns the index of the first occurrence of a given value in a range of
// this list. The list is searched forwards, starting at index
// index and ending at count number of elements. The
// elements of the list are compared to the given value using the
// Object.Equals method.
//
// This method uses the Array.IndexOf method to perform the
// search.
//
public int IndexOf(T item, int index) {
if (index > _size)
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
return Array.IndexOf(_items, item, index, _size - index);
} // Returns the index of the first occurrence of a given value in a range of
// this list. The list is searched forwards, starting at index
// index and upto count number of elements. The
// elements of the list are compared to the given value using the
// Object.Equals method.
//
// This method uses the Array.IndexOf method to perform the
// search.
//
public int IndexOf(T item, int index, int count) {
if (index > _size)
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index); if (count < || index > _size - count) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count); return Array.IndexOf(_items, item, index, count);
}

List类的IndexOf方法

总结:

  1.这点效率问题对于某些人来说可能无所谓,但是我觉得更重要的是编码习惯的养成问题。

  2.能用Contains的地方还是尽量使用Contains(发现我改的代码中有不少同事直接用了"IndexOf(value)"),当然会有特殊的例外场景,这里不罗嗦。
  3.关于Insert,我编写了两个扩展方法,如下(方法虽简单,但是给代码带来了更大的优雅性)

        public static string InsertLast(this string source, string str)
{
return source.Insert(source.Length, str);
} public static string InsertFirst(this string source, string str)
{
return source.Insert(, str);
}

 

最后本人不是什么资深狼友,将不会提供任何福利图片作别,抱歉!!

上一篇:eclipse maven项目中使用tomcat插件部署项目


下一篇:Elasticsearch 5.0 —— Head插件部署指南(Head目前支持5.0了!请不要看本篇文章了)