循环加快——C++

来源：互联网发布：万睿科技有限公司知乎编辑：程序博客网时间：2024/05/07 04:46

#ifndef LOCALVARIABLES_H_
#define LOCALVARIABLES_H_

#include <iostream>
#include <ctime>

const int N = 10000000;

class A
{
public:
int getValue()
{
return a;
}
private:
int a;
};

class TestTime
{
public:
void test();
clock_t test1();
clock_t test2();
clock_t test3();
private:
clock_t duration1;
clock_t duration2;
clock_t duration3;
};
#endif

#include "localVariables.h"

void TestTime::test()
{
clock_t d1 = test1();
clock_t d2 = test2();
clock_t d3 = test3();

std::cout<<"duration1 = "<<d1<<"ms // 变量定义在for循环里面"<<std::endl
  <<"duration2 = "<<d2<<"ms // 变量定义在for循环外面，且采用达夫编程技巧"<<std::endl
  <<"duration3 = "<<d3<<"ms // 变量定义在for循环外面"<<std::endl;
}
clock_t TestTime::test1()
{
clock_t begin1,end1;
begin1 = clock();
for ( int i=0; i<N; ++i )
{
  A a;  //编译器会自动优化，定义在外面或者里面效果一样
  int b = a.getValue();
}
end1 = clock();
duration1 = end1 - begin1;
return duration1;
}

clock_t TestTime::test2()
{
A a;
int b;
int N2 = N/10;
clock_t begin2,end2;
begin2 = clock();
//达夫编程技巧降低循环判断次数
for ( int i=0; i<N2; ++i )
{
  b = a.getValue();//1
  b = a.getValue();
  b = a.getValue();//3
  b = a.getValue();
  b = a.getValue();//5
  b = a.getValue();
  b = a.getValue();//7
  b = a.getValue();//8
  b = a.getValue();//9
  b = a.getValue();//10
}
end2 = clock();
duration2 = end2 - begin2;
return duration2;
}

clock_t TestTime::test3()
{
A a;
clock_t begin3,end3;
begin3 = clock();
for ( int i=0; i<N; ++i )
{
int b = a.getValue();
}
end3 = clock();
duration3 = end3 - begin3;
return duration3;
}

#include "localVariables.h"
//using namespace std; //尽量少用，降低编译时间

void main()
{
TestTime tt;
tt.test();

system("pause");
}

输出：

duration1 = 249ms // 变量定义在for循环里面
duration2 = 234ms // 变量定义在for循环外面，且采用达夫编程技巧
duration3 = 250ms // 变量定义在for循环外面
请按任意键继续. . .

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