在c# / VB.NET/。哪个循环运行得更快,for还是foreach?
自从很久以前我读到for循环比foreach循环工作得快,我就认为它适用于所有集合、泛型集合、所有数组等。
我搜索了谷歌,找到了几篇文章,但大多数都是不确定的(阅读文章评论),而且是开放式的。
理想的情况是列出每种情况以及最佳解决方案。
例如(这只是一个例子):
用于迭代1000+的数组
字符串- for比foreach好
对于迭代IList(非泛型)字符串- foreach更好
比
在网上找到了一些相同的参考资料:
由Emmanuel Schanzer撰写的原创文章
CodeProject FOREACH Vs. FOR
博客——去博客还是不去博客,这是个问题
ASP。NET论坛- NET 1.1 c# for vs foreach
(编辑)
除了可读性之外,我对事实和数据真的很感兴趣。在某些应用中,最后一英里的性能优化确实很重要。
首先是对德米特里的回答(现已删除)的反诉。对于数组,c#编译器为foreach生成的代码与等效的For循环生成的代码大致相同。这就解释了为什么这个基准测试的结果基本相同:
using System;
using System.Diagnostics;
using System.Linq;
class Test
{
const int Size = 1000000;
const int Iterations = 10000;
static void Main()
{
double[] data = new double[Size];
Random rng = new Random();
for (int i=0; i < data.Length; i++)
{
data[i] = rng.NextDouble();
}
double correctSum = data.Sum();
Stopwatch sw = Stopwatch.StartNew();
for (int i=0; i < Iterations; i++)
{
double sum = 0;
for (int j=0; j < data.Length; j++)
{
sum += data[j];
}
if (Math.Abs(sum-correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("For loop: {0}", sw.ElapsedMilliseconds);
sw = Stopwatch.StartNew();
for (int i=0; i < Iterations; i++)
{
double sum = 0;
foreach (double d in data)
{
sum += d;
}
if (Math.Abs(sum-correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("Foreach loop: {0}", sw.ElapsedMilliseconds);
}
}
结果:
For loop: 16638
Foreach loop: 16529
接下来,验证Greg关于集合类型的观点是重要的——在上面将数组更改为List<double>,您将得到完全不同的结果。它不仅在一般情况下要慢得多,而且foreach也比通过索引访问慢得多。话虽如此,我仍然几乎总是喜欢foreach而不是for循环,因为它使代码更简单——因为可读性几乎总是重要的,而微优化很少是。
我遇到了一个案子,foreach比For快得多
为什么foreach在读取richtextbox行时比for循环快
我有一个类似于那个问题中的OP的案例。
A textbox reading about 72K lines, and I was accessling the Lines property(which is actually a getter method). (And apparently often in winforms there are getter methods that aren't O(1). I suppose it's O(n), so the larger the textbox the longer it takes to get a value from that 'property'. And in the for loop I had as the OP there had for(int i=0;i<textBox1.lines.length;i++) str=textBox1.Lines[i] , and it was really quite slow as it was reading the entire textbox each time it read a line plus it was reading the entire textbox each time it checked the condition.
Jon Skeet演示了您可以只访问一次Lines属性(甚至不是每次迭代一次,只是一次)。而不是每次迭代两次(这是大量的次数)。Do string[] strarlines = textBox1.Lines;然后在星线间循环。
但是一个直观形式的for循环访问Lines属性是非常低效的
for (int i = 0; i < richTextBox.Lines.Length; i++)
{
s = richTextBox.Lines[i];
}
对于文本框,或者富文本框,它非常慢。
OP在一个富文本框上测试了这个循环,发现“有15000行。For循环花了8分钟才循环到15000行。而foreach只花了不到一秒钟的时间来列举它。”
那个链接的OP发现这个foreach比上面提到的他(同一个OP)的for循环要有效得多。就像我一样。
String s=String.Empty;
foreach(string str in txtText.Lines)
{
s=str;
}
我发现foreach循环迭代列表更快。下面是我的测试结果。在下面的代码中,我分别迭代一个大小为100、10000和100000的数组,使用for和foreach循环来测量时间。
private static void MeasureTime()
{
var array = new int[10000];
var list = array.ToList();
Console.WriteLine("Array size: {0}", array.Length);
Console.WriteLine("Array For loop ......");
var stopWatch = Stopwatch.StartNew();
for (int i = 0; i < array.Length; i++)
{
Thread.Sleep(1);
}
stopWatch.Stop();
Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("Array Foreach loop ......");
var stopWatch1 = Stopwatch.StartNew();
foreach (var item in array)
{
Thread.Sleep(1);
}
stopWatch1.Stop();
Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch1.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("List For loop ......");
var stopWatch2 = Stopwatch.StartNew();
for (int i = 0; i < list.Count; i++)
{
Thread.Sleep(1);
}
stopWatch2.Stop();
Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch2.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("List Foreach loop ......");
var stopWatch3 = Stopwatch.StartNew();
foreach (var item in list)
{
Thread.Sleep(1);
}
stopWatch3.Stop();
Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch3.ElapsedMilliseconds);
}
更新
在@jgauffin建议后,我使用了@johnskeet代码,发现使用数组的for循环比下面的更快,
Foreach循环与数组。
For带列表的循环。
Foreach循环与列表。
请看下面我的测试结果和代码,
private static void MeasureNewTime()
{
var data = new double[Size];
var rng = new Random();
for (int i = 0; i < data.Length; i++)
{
data[i] = rng.NextDouble();
}
Console.WriteLine("Lenght of array: {0}", data.Length);
Console.WriteLine("No. of iteration: {0}", Iterations);
Console.WriteLine(" ");
double correctSum = data.Sum();
Stopwatch sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
double sum = 0;
for (int j = 0; j < data.Length; j++)
{
sum += data[j];
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("For loop with Array: {0}", sw.ElapsedMilliseconds);
sw = Stopwatch.StartNew();
for (var i = 0; i < Iterations; i++)
{
double sum = 0;
foreach (double d in data)
{
sum += d;
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("Foreach loop with Array: {0}", sw.ElapsedMilliseconds);
Console.WriteLine(" ");
var dataList = data.ToList();
sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
double sum = 0;
for (int j = 0; j < dataList.Count; j++)
{
sum += data[j];
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("For loop with List: {0}", sw.ElapsedMilliseconds);
sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
double sum = 0;
foreach (double d in dataList)
{
sum += d;
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("Foreach loop with List: {0}", sw.ElapsedMilliseconds);
}