String.Join和StringBuilder:哪个更快？
String.Join和StringBuilder:哪个更快？
提问于 2009-02-25 12:49:22
在关于将double[][]格式化为CSV格式的previous question中，it was suggested表示使用StringBuilder将比String.Join更快。这是真的吗？
回答 3
Stack Overflow用户
发布于 2009-02-25 13:05:42
这是我的测试平台，为了简单起见，使用int[][]；结果优先：
Join: 9420ms (chk: 210710000
OneBuilder: 9021ms (chk: 210710000( double结果更新：)
Join: 11635ms (chk: 210710000
OneBuilder: 11385ms (chk: 210710000(更新re 2048 * 64 * 150)
Join: 11620ms (chk: 206409600
OneBuilder: 11132ms (chk: 206409600并且启用了OptimizeForTesting：
Join: 11180ms (chk: 206409600
OneBuilder: 10784ms (chk: 206409600所以更快，但不是很快；rig (在控制台运行，在发布模式下运行，等等)：
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
namespace ConsoleApplication2
{
class Program
{
static void Collect()
{
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
GC.WaitForPendingFinalizers();
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
GC.WaitForPendingFinalizers();
}
static void Main(string[] args)
{
const int ROWS = 500, COLS = 20, LOOPS = 2000;
int[][] data = new int[ROWS][];
Random rand = new Random(123456);
for (int row = 0; row < ROWS; row++)
{
int[] cells = new int[COLS];
for (int col = 0; col < COLS; col++)
{
cells[col] = rand.Next();
}
data[row] = cells;
}
Collect();
int chksum = 0;
Stopwatch watch = Stopwatch.StartNew();
for (int i = 0; i < LOOPS; i++)
{
chksum += Join(data).Length;
}
watch.Stop();
Console.WriteLine("Join: {0}ms (chk: {1}", watch.ElapsedMilliseconds, chksum);
Collect();
chksum = 0;
watch = Stopwatch.StartNew();
for (int i = 0; i < LOOPS; i++)
{
chksum += OneBuilder(data).Length;
}
watch.Stop();
Console.WriteLine("OneBuilder: {0}ms (chk: {1}", watch.ElapsedMilliseconds, chksum);
Console.WriteLine("done");
Console.ReadLine();
}
public static string Join(int[][] array)
{
return String.Join(Environment.NewLine,
Array.ConvertAll(array,
row => String.Join(",",
Array.ConvertAll(row, x => x.ToString()))));
}
public static string OneBuilder(IEnumerable<int[]> source)
{
StringBuilder sb = new StringBuilder();
bool firstRow = true;
foreach (var row in source)
{
if (firstRow)
{
firstRow = false;
}
else
{
sb.AppendLine();
}
if (row.Length > 0)
{
sb.Append(row[0]);
for (int i = 1; i < row.Length; i++)
{
sb.Append(',').Append(row[i]);
}
}
}
return sb.ToString();
}
}
}Stack Overflow用户
发布于 2009-02-25 13:03:19
我不这样认为。通过反射器观察，String.Join的实现看起来非常优化。它还有一个额外的好处，那就是提前知道要创建的字符串的总大小，因此不需要任何重新分配。
我创建了两个测试方法来比较它们：
public static string TestStringJoin(double[][] array)
{
return String.Join(Environment.NewLine,
Array.ConvertAll(array,
row => String.Join(",",
Array.ConvertAll(row, x => x.ToString()))));
}
public static string TestStringBuilder(double[][] source)
{
// based on Marc Gravell's code
StringBuilder sb = new StringBuilder();
foreach (var row in source)
{
if (row.Length > 0)
{
sb.Append(row[0]);
for (int i = 1; i < row.Length; i++)
{
sb.Append(',').Append(row[i]);
}
}
}
return sb.ToString();
}我将每个方法运行了50次，传入了一个大小为[2048][64]的数组。我对两个数组这样做:一个数组填充零，另一个数组填充随机值。我在我的机器上得到了以下结果(P4 3.0 GHz，单核，无HT，从CMD运行释放模式)：
// with zeros:
TestStringJoin took 00:00:02.2755280
TestStringBuilder took 00:00:02.3536041
// with random values:
TestStringJoin took 00:00:05.6412147
TestStringBuilder took 00:00:05.8394650将数组的大小增加到[2048][512]，同时将迭代次数减少到10，得到了以下结果：
// with zeros:
TestStringJoin took 00:00:03.7146628
TestStringBuilder took 00:00:03.8886978
// with random values:
TestStringJoin took 00:00:09.4991765
TestStringBuilder took 00:00:09.3033365结果是可重复的(几乎是；由不同的随机值引起的小波动)。显然，String.Join在大多数情况下都要快一点(尽管只有很小的差距)。
这是我用来测试的代码：
const int Iterations = 50;
const int Rows = 2048;
const int Cols = 64; // 512
static void Main()
{
OptimizeForTesting(); // set process priority to RealTime
// test 1: zeros
double[][] array = new double[Rows][];
for (int i = 0; i < array.Length; ++i)
array[i] = new double[Cols];
CompareMethods(array);
// test 2: random values
Random random = new Random();
double[] template = new double[Cols];
for (int i = 0; i < template.Length; ++i)
template[i] = random.NextDouble();
for (int i = 0; i < array.Length; ++i)
array[i] = template;
CompareMethods(array);
}
static void CompareMethods(double[][] array)
{
Stopwatch stopwatch = Stopwatch.StartNew();
for (int i = 0; i < Iterations; ++i)
TestStringJoin(array);
stopwatch.Stop();
Console.WriteLine("TestStringJoin took " + stopwatch.Elapsed);
stopwatch.Reset(); stopwatch.Start();
for (int i = 0; i < Iterations; ++i)
TestStringBuilder(array);
stopwatch.Stop();
Console.WriteLine("TestStringBuilder took " + stopwatch.Elapsed);
}
static void OptimizeForTesting()
{
Thread.CurrentThread.Priority = ThreadPriority.Highest;
Process currentProcess = Process.GetCurrentProcess();
currentProcess.PriorityClass = ProcessPriorityClass.RealTime;
if (Environment.ProcessorCount > 1) {
// use last core only
currentProcess.ProcessorAffinity
= new IntPtr(1 << (Environment.ProcessorCount - 1));
}
}Stack Overflow用户
发布于 2009-02-25 13:18:25
除非1%的差异在整个程序的运行时间方面变得很重要，否则这看起来就像是微优化。我会写出最具可读性/可理解性的代码，不用担心1%的性能差异。
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原文链接：
https://stackoverflow.com/questions/585860
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