Hos*_*Aly 73 .net string performance stringbuilder
在之前关于格式化double[][]为CSV格式的问题中,Marc Gravell 表示使用StringBuilder将比使用更快String.Join.这是真的？
Jon*_*eet 106
简答:这取决于.
答案很长:如果你已经有一个字符串数组连接在一起(用分隔符),这String.Join是最快的方法.
String.Join可以查看所有字符串以计算出所需的确切长度,然后再次复制并复制所有数据.这意味着不会涉及额外的复制.该唯一的缺点是,它要经过串的两倍,这意味着潜在吹内存缓存更多的时间比必要的.
如果您事先没有将字符串作为数组,那么它的使用速度可能会更快StringBuilder- 但是会出现这种情况.如果使用StringBuilder手段做很多很多副本,那么构建一个数组然后调用String.Join可能会更快.
编辑:这是String.Join对一连串调用的一次调用StringBuilder.Append.在最初的问题中,我们有两个不同级别的String.Join调用,因此每个嵌套调用都会创建一个中间字符串.换句话说,它更复杂,更难以猜测.我会惊讶地发现,与典型数据相比,(在复杂性方面)要么"获胜".
编辑:当我在家时,我会写一个尽可能痛苦的基准StringBuilder.基本上,如果你有一个数组,其中每个元素大小是前一个元素的两倍,并且你得到它恰到好处,你应该能够强制复制每个追加(元素,而不是分隔符,尽管这需要也要考虑到).那时它几乎和简单的字符串连接一样糟糕 - 但是String.Join没有问题.
Mar*_*ell 30
这是我的试验台,int[][]用于简单; 结果第一:
Join: 9420ms (chk: 210710000
OneBuilder: 9021ms (chk: 210710000
(更新double结果:)
Join: 11635ms (chk: 210710000
OneBuilder: 11385ms (chk: 210710000
(更新re 2048*64*150)
Join: 11620ms (chk: 206409600
OneBuilder: 11132ms (chk: 206409600
并启用OptimizeForTesting:
Join: 11180ms (chk: 206409600
OneBuilder: 10784ms (chk: 206409600
如此之快,但不是那么大; rig(在控制台上运行,在发布模式下运行等):
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();
}
}
}
Hos*_*Aly 18
我不这么认为.透过Reflector看,执行效果String.Join非常优化.它还具有预先知道要创建的字符串总大小的额外好处,因此不需要任何重新分配.
我创建了两种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();
}
我运行了每个方法50次,传入一个大小的数组[2048][64].我为两个阵列做了这个; 一个填充零,另一个填充随机值.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
增加数组的大小[2048][512],同时将迭代次数减少到10得到以下结果:
// 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;由不同的随机值引起的小波动).显然String.Join大部分时间都要快一点(虽然幅度非常小).
这是我用于测试的代码:
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));
}
}