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C++统计精确时间
QueryPerformanceFrequency用法 精确获取时间: QueryPerformanceFrequency() - 基本介绍 类型:Win32API 原型:BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency); 作用:返回硬件支持的高精度计数器的频率。 返回值:非零,硬件支持高精度计数器;零,硬件不支持,读取失败。 QueryPerformanceFrequency() - 技术特点 供WIN9X使用的高精度定时器:QueryPerformanceFrequency()和QueryPerformanceCounter(),要求计算机从硬件上支持高精度定时器。需包含windows.h头文件。 函数的原形是: BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency); BOOL QueryPerformanceCounter (LARGE_INTEGER *lpCount); 数据类型LARGEINTEGER既可以是一个作为8字节长的整数,也可以是作为两个4字节长的整数的联合结构,其具体用法根据编译器是否支持64位而定。该类型的定义如下: typeef union _ LARGE_INTEGER { struct { DWORD LowPart; LONG HighPart; }; LONGLONG QuadPart; } LARGE_INTEGER; 在定时前应该先调用QueryPerformanceFrequency()函数获得机器内部计时器的时钟频率。接着在需要严格计时的事件发生前和发生之后分别调用QueryPerformanceCounter(),利用两次获得的计数之差和时钟频率,就可以计算出事件经历的精确时间。 测试Sleep的精确时间: #include <stdio.h> #include <windows.h> void main() { LARGE_INTEGER nFreq; LARGE_INTEGER nBeginTime; LARGE_INTEGER nEndTime; double time; QueryPerformanceFrequency(&nFreq); QueryPerformanceCounter(&nBeginTime); Sleep(1000); QueryPerformanceCounter(&nEndTime); time=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart; printf("%f\n",time); Sleep(1000); system("Pause"); } 再给出我写的一个Fibonacci的例子: #include <iostream> #include <cmath> #include <ctime> #include <windows.h> #include<iomanip> using namespace std; unsigned long Fib1(unsigned long n) { return (n == 1 || n == 2) ? 1 : Fib1(n - 1) + Fib1(n - 2); } unsigned long Fib2(unsigned long n) { if (n == 1 || n == 2) { return 1; } unsigned long m1 = 1, m2 = 1; for (unsigned long i = 3; i <= n; i++) { m2 = m1 + m2; m1 = m2 - m1; } return m2; } int main() { LARGE_INTEGER nFreq; LARGE_INTEGER nBeginTime; LARGE_INTEGER nEndTime; double time1 = 0, time2 = 0; unsigned long result1 = 0; unsigned long result2 = 0; unsigned long fib_num = 45; cout.precision(10); cout.setf(cout.showpoint); //设置为始终输出小数点后的数字,就是说 a = 3,它也输出 3.00000 这 QueryPerformanceFrequency(&nFreq); QueryPerformanceCounter(&nBeginTime); result1 = Fib1(fib_num); QueryPerformanceCounter(&nEndTime); time1=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart; cout<<"Fib1: "<< fixed<<setprecision(8)<<setiosflags(ios::showpoint)<<time1<<endl; QueryPerformanceFrequency(&nFreq); QueryPerformanceCounter(&nBeginTime); result2 = Fib2(fib_num); QueryPerformanceCounter(&nEndTime); time2=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart; cout<<"Fib2: "<< fixed<<setprecision(20)<<setiosflags(ios::showpoint)<<time2<<endl; cout<<"Fib2 cost "<<time2*(unsigned long)100/time1<<"% time of Fib1."<<endl; //cout << "Hello world!" << endl; return 0; }
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