如何从 c++ 中的 csv 文件中只读取一列？

Question

这里我有一个 excel 文件，其第一列有 ID，即：ID 12 32 45 12.. 还有其他列，但我只想读取第一列中存在的数据，即 ID。

这是我抛出异常的代码。 我不知道为什么？

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <vector>
#include <algorithm> 
#include<string>
#include<cstdlib>
//std::find
#include<cstring>
using namespace std;
int main()
{
   

    ifstream fin("1.csv");
    string line;
    int rowCount = 0;
    int rowIdx = 0; //keep track of inserted rows

    //count the total nb of lines in your file
    while (getline(fin, line)) {
        rowCount++;
    }

    //this will be your table. A row is represented by data[row_number].
    //If you want to access the name of the column #47, you would
    //cout << data[0][46]. 0 being the first row(assuming headers)
    //and 46 is the 47 column.
    //But first you have to input the data. See below.
    std::vector<std::vector<std::string>> data;

    fin.clear(); //remove failbit (ie: continue using fin.)
    fin.seekg(fin.beg); //rewind stream to start

    while (getline(fin, line)) //for every line in input file
    {
        stringstream ss(line);  //copy line to stringstream
       string value;
        
        while (getline(ss, value, ',')) {       //for every value in that stream (ie: every cell on that row)
            data[rowIdx].push_back(value);//add that value at the end of the current row in our table
        }
        rowIdx++;   //increment row number before reading in next line
    }
fin.close();


//Now you can choose to access the data however you like.
//If you want to printout only column 47...

int colNum;
string colName = "ID";

//1.Find the index of column name "computer science" on the first row, using iterator
//note: if "it == data[0].end()", it means that that column name was not found 
vector<string>::iterator it = find(data[0].begin(), data[0].end(), colName);

//calulate its index (ie: column number integer)  
colNum = std::distance(data[0].begin(), it);

//2. Print the column with the header "computer science"
for (int row = 0; row < rowCount; row++)
{
    cout << data[row][colNum] << "\t";  //print every value in column 47 only
}
cout << endl;

return 0;
}

请帮我解决这个问题。 我只想显示包含 ID 的第一列。

Answer 1

这是上述代码的简化版本。 它摆脱了 2D 向量，只读取第一列。

std::vector<std::string> data;
ifstream fin("1.csv");
string line;
while (getline(fin, line)) //for every line in input file
{
    stringstream ss(line);  //copy line to stringstream
    string value;
    if (getline(ss, value, ',')) {
        data.push_back(value);
    }
}

编辑

如何显示数据

for (size_t i = 0; i < data.size(); ++i)
    cout << data[i] << '\n';

Answer 2

问题：

std::vector<std::vector<std::string>> data;

分配vector s 的vector 。 这两个维度当前都设置为 0。

data[rowIdx].push_back(value);

推入并可能调整内部vector的大小，但外部向量的大小保持为rowIdx的值无效。 天真的解决方案是使用rowCount来确定外部vector的大小，但事实证明这是一种浪费。 您可以组装整行，然后将行push_back到data中，但即使这样也是一种浪费，因为只需要一列。

vector 的优点之一是您不必知道行数。 您制作一个行向量，将列推入其中，然后将行推入数据。 当您到达文件末尾时，您就完成了阅读。 无需两次读取文件，但您可能会在数据上次自我调整大小时浪费一些存储空间。

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <vector>
// reduced includes to minimum needed for this example
// removed using namespace std; to reduce odds of a naming collision
int main() {

    std::ifstream fin("1.csv"); // Relative path, so watch out for problems 
                                // with working directory
    std::string line;
    std::vector<std::string> data; // only need one column? Only need one 
                                   // dimension
    if (fin.is_open())
    {
        while (std::getline(fin, line)) //for every line in input file
        {
            std::stringstream ss(line);
            std::string value;
    
            if (std::getline(ss, value, ',')) 
            {
                 // only take the first column If you need, say, the third 
                 // column read and discard the first two columns 
                 // and store the third
                data.push_back(value); // vector sizes itself with push_back, 
                                       // so there is no need to count the rows
            }
        }
    }
    else
    {
        std::cerr << "Cannot open file\n";
        return -1;
    }
    // use the column of data here
}