如何在 C++ 中读取和解析 CSV 文件? [英] How can I read and parse CSV files in C++?

问题描述

我需要在 C++ 中加载和使用 CSV 文件数据.在这一点上，它实际上可以只是一个逗号分隔的解析器(即不用担心转义换行符和逗号).主要需要一个逐行解析器，每次调用该方法时，它都会为下一行返回一个向量.

I need to load and use CSV file data in C++. At this point it can really just be a comma-delimited parser (ie don't worry about escaping new lines and commas). The main need is a line-by-line parser that will return a vector for the next line each time the method is called.

我发现这篇文章看起来很有前途:http://www.boost.org/doc/libs/1_35_0/libs/spirit/example/fundamental/list_parser.cpp

I found this article which looks quite promising: http://www.boost.org/doc/libs/1_35_0/libs/spirit/example/fundamental/list_parser.cpp

我从未使用过 Boost's Spirit，但我愿意尝试一下.但前提是没有我忽略的更直接的解决方案.

I've never used Boost's Spirit, but am willing to try it. But only if there isn't a more straightforward solution I'm overlooking.

推荐答案

如果你不关心转义逗号和换行符，
并且你不能在引号中嵌入逗号和换行符(如果你不能转义那么......)
那么它只有大约三行代码(OK 14 -> 但它只有 15 行才能读取整个文件).

If you don't care about escaping comma and newline,
AND you can't embed comma and newline in quotes (If you can't escape then...)
then its only about three lines of code (OK 14 ->But its only 15 to read the whole file).

std::vector<std::string> getNextLineAndSplitIntoTokens(std::istream& str)
{
    std::vector<std::string>   result;
    std::string                line;
    std::getline(str,line);

    std::stringstream          lineStream(line);
    std::string                cell;

    while(std::getline(lineStream,cell, ','))
    {
        result.push_back(cell);
    }
    // This checks for a trailing comma with no data after it.
    if (!lineStream && cell.empty())
    {
        // If there was a trailing comma then add an empty element.
        result.push_back("");
    }
    return result;
}

我只想创建一个代表一行的类.
然后流入该对象:

I would just create a class representing a row.
Then stream into that object:

#include <iterator>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <string>

class CSVRow
{
    public:
        std::string_view operator[](std::size_t index) const
        {
            return std::string_view(&m_line[m_data[index] + 1], m_data[index + 1] -  (m_data[index] + 1));
        }
        std::size_t size() const
        {
            return m_data.size() - 1;
        }
        void readNextRow(std::istream& str)
        {
            std::getline(str, m_line);

            m_data.clear();
            m_data.emplace_back(-1);
            std::string::size_type pos = 0;
            while((pos = m_line.find(',', pos)) != std::string::npos)
            {
                m_data.emplace_back(pos);
                ++pos;
            }
            // This checks for a trailing comma with no data after it.
            pos   = m_line.size();
            m_data.emplace_back(pos);
        }
    private:
        std::string         m_line;
        std::vector<int>    m_data;
};

std::istream& operator>>(std::istream& str, CSVRow& data)
{
    data.readNextRow(str);
    return str;
}   
int main()
{
    std::ifstream       file("plop.csv");

    CSVRow              row;
    while(file >> row)
    {
        std::cout << "4th Element(" << row[3] << ")
";
    }
}

但是通过一些工作，我们可以在技术上创建一个迭代器:

But with a little work we could technically create an iterator:

class CSVIterator
{   
    public:
        typedef std::input_iterator_tag     iterator_category;
        typedef CSVRow                      value_type;
        typedef std::size_t                 difference_type;
        typedef CSVRow*                     pointer;
        typedef CSVRow&                     reference;

        CSVIterator(std::istream& str)  :m_str(str.good()?&str:NULL) { ++(*this); }
        CSVIterator()                   :m_str(NULL) {}

        // Pre Increment
        CSVIterator& operator++()               {if (m_str) { if (!((*m_str) >> m_row)){m_str = NULL;}}return *this;}
        // Post increment
        CSVIterator operator++(int)             {CSVIterator    tmp(*this);++(*this);return tmp;}
        CSVRow const& operator*()   const       {return m_row;}
        CSVRow const* operator->()  const       {return &m_row;}

        bool operator==(CSVIterator const& rhs) {return ((this == &rhs) || ((this->m_str == NULL) && (rhs.m_str == NULL)));}
        bool operator!=(CSVIterator const& rhs) {return !((*this) == rhs);}
    private:
        std::istream*       m_str;
        CSVRow              m_row;
};


int main()
{
    std::ifstream       file("plop.csv");

    for(CSVIterator loop(file); loop != CSVIterator(); ++loop)
    {
        std::cout << "4th Element(" << (*loop)[3] << ")
";
    }
}

现在我们在 2020 年，让我们添加一个 CSVRange 对象:

Now that we are in 2020 lets add a CSVRange object:

class CSVRange
{
    std::istream&   stream;
    public:
        CSVRange(std::istream& str)
            : stream(str)
        {}
        CSVIterator begin() const {return CSVIterator{stream};}
        CSVIterator end()   const {return CSVIterator{};}
};

int main()
{
    std::ifstream       file("plop.csv");

    for(auto& row: CSVRange(file))
    {
        std::cout << "4th Element(" << row[3] << ")
";
    }
}

这篇关于如何在 C++ 中读取和解析 CSV 文件?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！