C++ vector容器如何快速读取txt文档数据？

Question

事先不知道文档大小（MxN的矩阵），将数据读到vector<vector<double> >容器中，有什么快速的方法？
我测试过500x500的txt文档，读进去需要18.945s，时间太长了！

/*************************
*函数名:split
*函数功能：分割字符串
*参数说明：
*src:源字符串
*separator: 分割字符
*dest:存储结果
**************************/
void split(const string& src, const string& separator, vector<string>& dest)
{
    string str = src;
    string substring;
    string::size_type start = 0, index;

    do
    {
        index = str.find_first_of(separator,start);
        if (index != string::npos)
        {
            substring = str.substr(start,index-start);
            dest.push_back(substring);
            start = str.find_first_not_of(separator,index);
            if (start == string::npos) return;
        }
    }while(index != string::npos);

    //the last token
    substring = str.substr(start);
    dest.push_back(substring);
}

/**************************
*函数名:readFile
*函数功能:读取文件,VS平台下内存映射采用SDK的API
*参数说明:
*fileName:文件全名
*vecDoubleList:输出数据引用
**************************/
void readFile(const string &fileName,vector< vector<double> >&vecDoubleList)
{
    HANDLE hFile = CreateFileA(fileName.c_str(),GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL);
    if(hFile == INVALID_HANDLE_VALUE)
    {
        return ;
    }
    // Create a file-mapping object for the file.
    HANDLE hFileMapping = CreateFileMappingA(hFile,NULL,PAGE_WRITECOPY,0, 0, NULL);
    char *data = (char *)MapViewOfFile(hFileMapping, FILE_MAP_COPY, 0, 0, 0);

    vector<string>vec;
    vector< vector<string> >veclist;
    split(data,"\n",vec);

    for(int i = 0;i != vec.size();i++)
    {
        vector<string>tmp;
        split(vec[i]," ",tmp);
        veclist.push_back(tmp);
    }

    /*********************
     vec.size()修改为veclist.size(), 否则会出现vector越界。同时，移除了除以10000.主要因为
     本函数是一个读取文件通用函数，不是针对地面反射率数据，因此移除了除以10000，并把除以10000
     放到主函数外面。
    **********************/
    for(int i = 0; i != veclist.size();i++)
    {
        vector<double>tmp;
        for(int j = 0;j != veclist[0].size();j++)
        {
            tmp.push_back(atof(veclist[i][j].c_str()));//参数除以10000转换
        }
        vecDoubleList.push_back(tmp);
    }

    UnmapViewOfFile(data);
    CloseHandle(hFileMapping);
    CloseHandle(hFile);
}

Answer 1

What does your code look like? Can you post it? Normally 100M/s is no problem

#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>
#include <cstdlib>

int main() {
std::ifstream is("test_file.txt");
int line_size = 500 * 3;
char line[line_size];

is.seekg(0, is.end);
int file_size = is.tellg();
is.seekg(0, is.beg);

int line_number = 1;
std::vector< std::vector<double> > result;
while(is.good()) {
    is.getline(line, file_size);
    if(strlen(line) == 0) {
        continue;
    }
    std::stringstream ss;
    ss << line;
    char number_str[100];
    std::vector<double> line_doubles;
    while(ss.good()) {
        ss.getline(number_str, line_size, ' ');
        if(strlen(number_str) == 0) {
            continue;
        }
        line_doubles.push_back(atof(number_str));
        //std::cout << "get number:" << atof(number_str) << std::endl;
    }
    //std::cout << "get line_doubles size:" << line_doubles.size() << std::endl;    
    result.push_back(line_doubles);
    //std::cout << "get line" << "(" << line_number++ <<"):" << line << std::endl;
    }
    //std::cout << "get result size:" << result.size() << std::endl;
    is.close();
}

Tested it, it’s about 34ms

Answer 2

Optimization is not enabled?