如何提高C++中for循环的效率

    clock_t startTime,endTime;
    startTime = clock();
     vector<double> row(100);
     double sum=0;
     for(unsigned int i=0;i<1000;i++)
     {
         for(unsigned int j=0;j<row.size();j++)
         {
             sum+=row[j];
         }
     }     
  endTime = clock();
  cout << "The run time is: " <<(double)(endTime - startTime) / CLOCKS_PER_SEC << "s" << endl;

    vector<double> row(100);
     double sum=0;
     unsigned int n=row.size();
     for(unsigned int i=0;i<1000;i++)
     {
         for(unsigned int j=0;j<n;j++)
         {
             sum+=row[j];
         }
     }     

     vector<double> row(100)
     double sum=0;
     for(unsigned int i=0;i<1000;i++)
     {
         for(unsigned int j=row.size();j>0;j--)
         {
             sum+=row[j];
         }
     }     

 for(unsigned int i=0;i<1000;i++)
     {
         for(unsigned int j=0， n=row.size();;j<n;j++)
         {
             sum+=row[j];
         }
     }

for(auto iter =row.cbegin(); iter != row.cend(); iter++) 
  {
         sum+=row[*iter];
   }

//前
for(int i=0;i<row.size();i++)
{
   try{}
   catch(Exception e){}
   }
 //后
 try{
 for(int i=0;i<row.size();i++){
 }
 }
 catch(Exception e){}

//前
int a=1,b=2;
for(int i=0;i<row.size();i++)
{
  i=i*a*b;
  }
 //后
 int a=1,b=2;
 int c=a*b;
for(int i=0;i<row.size();i++)
{
  i=i*c;
  }