Question

我编写了一个代码，通过两种方法将大量字母从小格式转换为大小格式。第一种方法，我使用逐位操作，第二种方法是使用switch语句。我的最终目标是使用此示例作为理解基准的参考。我应该在代码中考虑哪些参数来判断使用按位运算是否优于使用switch。到目前为止，我使用经过的时间作为更好性能的指标。在我的例子中，

Lower2UpperBitwise:
      elapsed time: 0.655201s

 Lower2UpperSwitch:
      elapsed time: 3.30221s

我正在使用cygwin在Windows中运行代码，这是我的代码

#include <iostream>
#include <random>
#include <string>
#include <chrono>


void Lower2UpperBitwise(std::string& str)
{
    int mask = 0xDF;
    long i(0); 

    while ( i != (long)str.size() )
        str[i++] &= mask;

}

void Lower2UpperSwitch(std::string& str)
{
    for ( long i(0); i < (long)str.size(); ++i ){
        switch(str[i]){
            case 'a': str[i] = 'A'; break;  
            case 'b': str[i] = 'B'; break;
            case 'c': str[i] = 'C'; break;  
            case 'd': str[i] = 'D'; break;
            case 'e': str[i] = 'E'; break;  
            case 'f': str[i] = 'F'; break;
            case 'g': str[i] = 'G'; break;  
            case 'h': str[i] = 'H'; break;
            case 'i': str[i] = 'I'; break;  
            case 'j': str[i] = 'J'; break;
            case 'k': str[i] = 'K'; break;  
            case 'l': str[i] = 'L'; break;
            case 'm': str[i] = 'M'; break;  
            case 'n': str[i] = 'N'; break;
            case 'o': str[i] = 'O'; break;  
            case 'p': str[i] = 'P'; break;
            case 'q': str[i] = 'Q'; break;  
            case 'r': str[i] = 'R'; break;
            case 's': str[i] = 'S'; break;  
            case 't': str[i] = 'T'; break;
            case 'u': str[i] = 'U'; break;
            case 'v': str[i] = 'V'; break;  
            case 'w': str[i] = 'W'; break;
            case 'x': str[i] = 'X'; break;  
            case 'y': str[i] = 'Y'; break;
            case 'z': str[i] = 'Z'; break;
            default: break;
        }
    }
}



int main()
{
    //@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    /* Setting up the random generator */
    std::random_device rd;
    std::mt19937 mt(rd());
    std::uniform_int_distribution<int> di(97,122);  // (97 : a)  (122 : z)


    // string size: 200,000,000
    const long SIZE( 200000000 );
    std::string str;

    //@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    /* Fill string with random small letters (a-z) */
    for (long i(0); i < SIZE; ++i) {
        int temp = di(mt);
        str.push_back( static_cast<char>(temp) );
    }
    /* =========================================== */


    std::string temp;
    temp = str;

    std::chrono::time_point<std::chrono::system_clock> start, end;
    std::chrono::duration<double> elapsed_seconds;

    // Using bitwise operation )##################################
    start = std::chrono::system_clock::now();  // <-- Start 
    Lower2UpperBitwise(temp);                  // <-- Run 
    end = std::chrono::system_clock::now();    // <-- End

    elapsed_seconds = end-start;
    std::cout << "\nLower2UpperBitwise:\n";
    std::cout << "      elapsed time: " << elapsed_seconds.count() << "s\n";


    // Using Switch operation )#####################################
    temp = str;

    start = std::chrono::system_clock::now(); // <-- Start 
    Lower2UpperSwitch(temp);                  // <-- Run 
    end = std::chrono::system_clock::now();   // <-- End

    elapsed_seconds = end-start;
    std::cout << "\n Lower2UpperSwitch:\n";
    std::cout << "      elapsed time: " << elapsed_seconds.count() << "s\n";


    return 0;
}

这是Makefile。

CXX      = g++
CXXFLAGS = -std=c++11 -Wall -O3 
TARGET   = test

.PHONY: all 
all: $(TARGET)

$(TARGET): main.o
    $(CXX) $(CXXFLAGS) main.cpp -o $(TARGET) 

.PHONY: clean   
clean:
    rm *.o $(TARGET)

我还应该考虑对此代码进行基准测试

0 个答案: