如何使用逗号分隔值读写/写入文本文件

Question

如果我的文件是逗号分隔值,我如何从文件中读取数据

1, 2, 3, 4, 5\n
6, 7, 8, 9, 10\n
\n

在读完文件后,我想将数据写回到上面相同格式的其他文件中.

我可以使用获得总行数

string line;
while(!file.eof()){
      getline(file,line);
      numlines++;
    }
    numline--; // remove the last empty line

但我怎么知道一行/一行的总位数？

我还有一些存储数据的向量.所以,我想读取第一行,然后计算该行中元素的总数,这里是5(1,2,3,4,5)并将它们存储在数组/向量中,然后读取下一行并将它们存储在向量中再等等,直到我到达EOF.

然后,我想将数据写入文件,我想这将完成将数据写入文件的工作,

numOfCols=1;
for(int i = 0; i < vector.size(); i++)
{
    file << vector.at(i);
    if((numOfCols<5) file << ",";//print comma (,)
    if((i+1)%5==0)
    {
                  file << endl;//print newline after 5th value
                  numOfCols=1;//start from column 1 again, for the next line
    }
    numOfCols++;
}
file << endl;// last new line

所以,我的主要问题是如何用逗号分隔值从文件中读取数据？

谢谢

Answer 1

第1步:不要这样做:

while(!file.eof())
{
    getline(file,line);
    numlines++;
}
numline--; 

在您尝试阅读之前,EOF是不正确的.标准模式是:

while(getline(file,line))
{
    ++numline;
}

另请注意,std::getline()可以选择采用第三个参数.这是要打破的角色.默认情况下,这是行终止符,但您可以指定逗号.

while(getline(file,line))
{
    std::stringstream   linestream(line);
    std::string         value;

    while(getline(linestream,value,','))
    {
        std::cout << "Value(" << value << ")\n";
    }
    std::cout << "Line Finished" << std::endl;

}

如果将所有值存储在单个矢量中,则使用固定宽度将其打印出来.然后我会做这样的事情.

struct LineWriter
{
        LineWriter(std::ostream& str,int size)
                :m_str(str)
                ,m_size(size)
                ,m_current(0)
        {}

        // The std::copy() does assignement to an iterator.
        // This looks like this  (*result) = <value>;
        // So overide the operator * and the operator = to
        LineWriter& operator*() {return *this;}   
        void operator=(int val)
        {
                ++m_current;
                m_str << val << (((m_current % m_size) == 0)?"\n":",");
        }

        // std::copy() increments the iterator. But this is not usfull here
        // so just implement too empty methods to handle the increment.
        void operator++()       {}
        void operator++(int)    {}

        // Local data.
        std::ostream&           m_str;
        int const               m_size;
        int                     m_current;
};

void printCommaSepFixedSizeLinesFromVector(std::vector const& data,int linesize)
{
    std::copy(data.begin(),data.end(),LineWriter(std::cout,linesize));
}

Answer 2

这里我发布了CSV读取代码以及编写代码.我检查了它的工作正常.

#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
using namespace std;

void readCSV(istream &input, vector< vector<string> > &output)
{
   string csvLine;
    // read every line from the stream
    while( getline(input, csvLine) )
    {
            istringstream csvStream(csvLine);
           vector<string> csvColumn;
            string csvElement;
            // read every element from the line that is seperated by commas
            // and put it into the vector or strings
            while( getline(csvStream, csvElement, ',') )
            {
                    csvColumn.push_back(csvElement);
            }
            output.push_back(csvColumn);
    }
}

int main()
{
    ofstream myfile;
    string a;
    fstream file("b.csv", ios::in);
    myfile.open ("ab.csv");
    if(!file.is_open())
    {
           cout << "File not found!\n";
            return 1;
    }
    // typedef to save typing for the following object
    typedef vector< vector<string> > csvVector;
    csvVector csvData;

    readCSV(file, csvData);
    // print out read data to prove reading worked
    for(csvVector::iterator i = csvData.begin(); i != csvData.end(); ++i)
    {
            for(vector<string>::iterator j = i->begin(); j != i->end(); ++j)
            {
          a=*j;
                  cout << a << " ";
          myfile <<a<<",";

 }
 myfile <<"\n";
 cout << "\n";
 }
 myfile.close();
 system("pause");

}