How to Parse a CSV File

c++11 files intermediate

Related: How to read a file

A CSV file is a file with comma separated values. CSV files are useful because they can represent tabular data very simply. To parse a CSV file in C++ we’ll use the streams library from the Standard Library.

To open a file for reading in C++ we use std::ifstream (input file stream). We’ll read the file line by line, parsing each line with a std::istringstream (input string stream). The input file we’ll parse as an example is:

name,distance,radius
Mercury,0.4,4879.4
Venus,0.7,6051.8
Earth,1.0,6371.0
Mars,1.5,3389.5
Jupiter,5.2,69911
Saturn,9.5,58232
Uranus,19.2,25362
Neptune,30.1,24622

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

int main() {
  std::string filename{"csv_file.txt"};
  std::ifstream input{filename};

  if (!input.is_open()) {
    std::cerr << "Couldn't read file: " << filename << "\n";
    return 1; 
  }

  std::vector<std::vector<std::string>> csvRows;

  for (std::string line; std::getline(input, line);) {
    std::istringstream ss(std::move(line));
    std::vector<std::string> row;
    if (!csvRows.empty()) {
       // We expect each row to be as big as the first row
      row.reserve(csvRows.front().size());
    }
    // std::getline can split on other characters, here we use ','
    for (std::string value; std::getline(ss, value, ',');) {
      row.push_back(std::move(value));
    }
    csvRows.push_back(std::move(row));
  }

  // Print out our table
  for (const std::vector<std::string>& row : csvRows) {
    for (const std::string& value : row) {
      std::cout << std::setw(10) << value;
    }
    std::cout << "\n";
  }
}

Of course, you may want to interpret your data as types other than just strings. For that we’ll need the “string to” family of functions such as string to int.