Re: [問題] 練習題

看板C_Sharp (C#)作者GreatShot (我要拿Ph.D.!!!)時間18年前 (2007/05/03 10:52)推噓9(9推 0噓 8→)

留言17則, 6人參與討論串4/8 (看更多)

: 另外，如果要算的次方數很大的話 : (應該到五次或是六次就應該呼叫了..) : 建議呼叫 Math.Pow 會快上很多.... : 裡面用的算法比你自己寫的連乘法好很多.. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 這點跟我認知很不一樣我沒看過Math.Pow()的source所以不敢說它演算法好不好我只能作實驗來觀察 source如下 public partial class Form1 : Form { [DllImport("kernel32.dll")] extern static short QueryPerformanceCounter(ref long x); [DllImport("kernel32.dll")] extern static short QueryPerformanceFrequency(ref long x); public Form1() { InitializeComponent(); } private void button1_Click(object sender, EventArgs e) { long ctr1 = 0, ctr2 = 0, freq = 0; double sum = 0; double a = 0; Random myrand = new Random(); if (QueryPerformanceCounter(ref ctr1)!=0) // Begin timing. { for (int i=0; i<1000000000; i++) // Code being timed. { a = myrand.NextDouble() * 10; //Generate Random number //3 options sum = 6 * (a * a * a) + (5 * (a * a)) + (3 * a) - 1; //sum = 6*Math.Pow(a,3) + 5*Math.Pow(a,2)+(3*a) - 1; //sum = a * (a * (a * (+6) + 5) + 3) - 1; } QueryPerformanceCounter(ref ctr2); // Finish timing. this.richTextBox1.AppendText("Start Value: " + ctr1 + "\n"); this.richTextBox1.AppendText("End Value: " + ctr2+"\n"); QueryPerformanceFrequency(ref freq); this.richTextBox1.AppendText("QueryPerformanceCounter minimum resolution: 1/" + freq + " seconds."+"\n"); this.richTextBox1.AppendText("100 Increment time: " + (ctr2 - ctr1) * 1.0 / freq + " seconds."+"\n"); } else this.richTextBox1.AppendText("High-resolution counter not supported."+"\n"); } } Result1: sum = 6 * (a * a * a) + (5 * (a * a)) + (3 * a) - 1; Start Value: 757143142739 End Value: 757167788963 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 1.72132379953318 seconds. Result2: sum = 6*Math.Pow(a,3) + 5*Math.Pow(a,2)+(3*a) - 1; Start Value: 758355648048 End Value: 758755628202 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 27.9351254139842 seconds. Result3: sum = a * (a * (a * (+6) + 5) + 3) - 1; Start Value: 759758768779 End Value: 759783404913 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 1.720619101031 seconds. 再來換成冪次更高的 Result1: sum = 9 * (a * a * a * a * a * a) + 8 * (a * a * a * a * a) + 7 * (a * a * a * a) + 6 * (a * a * a) + (5 * (a * a)) + (3 * a) - 1; Start Value: 766926901793 End Value: 766951596182 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 1.72468770472225 seconds. Result2: sum = 9 * Math.Pow(a, 6) + 8 * Math.Pow(a, 5) + 7 * Math.Pow(a, 4) + 6 * Math.Pow(a, 3) + 5 * Math.Pow(a, 2) + (3 * a) - 1; Start Value: 769278944190 End Value: 770282636993 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 70.0991887935478 seconds. Result3: sum = a * (a * (a * ( a * (a * (a * (+9) +8) +7 ) +6) + 5) + 3) - 1; Start Value: 774103750407 End Value: 774128401956 QueryPerformanceCounter minimum resolution: 1/14318180 seconds. 100 Increment time: 1.72169570434231 seconds. 這實驗其實很粗糙但是可以看的出有非常明顯的差異由此看來第一種寫法跟第三種寫法幾乎是沒有差別的也許是.net compiler會自己最佳化但是第二種(使用Math.Pow()) 很明顯的在執行時間上有段落差冪次越大差距就越大所以我的認知是要求效率的時候用連乘法會比Math.Pow()來的好不過也許是我實驗的方式有誤有錯的話麻煩請各位賢達指正謝謝 :) -- ※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 220.133.110.47