如何在c++中从csv文件中只读取一列？

edqdpe6u 于 2022-12-05 发布在其他

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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;
}

请帮助我解决这个问题。我只想显示包含ID的第一列。

c++

来源：https://stackoverflow.com/questions/74679310/how-to-read-only-one-column-from-a-csv-file-in-c

2条答案

按热度按时间

jogvjijk1#

下面是上面代码的简化版本，它去掉了二维向量，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';

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ipakzgxi2#

问题在于：

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

配置vector的vector。两个维度目前都设定为0。

data[rowIdx].push_back(value);

推入内部的vector并可能调整其大小，但外部向量的大小仍为0。rowIdx的值无效。最简单的解决方案是使用rowCount来调整外部vector的大小，但结果证明这是一种浪费。您可以组装整个行，然后将行push_back为data，但即使这样也是浪费，因为只需要一个柱。
向量的好处之一是你不必知道行数。你创建一个行向量，将列推入向量，然后将行推入数据。当你到达文件的末尾时，你就完成了阅读。不需要读取文件两次，但是你可能会在数据的最后一次自我调整大小上浪费一点存储空间。

#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
}

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如何在c++中从csv文件中只读取一列？

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