写入文本文件时避免附加第一行

Question

我有一个程序，它接受用户输入并将其写入文本文件。但是，当我尝试附加到我的文本文件时，它会重写我的“标题”行。我怎么能避免这种情况？

// StudyTime.cpp : 
/* Track study time
accept user input
store in .txt file
*/

#include "stdafx.h"
#include <string>
#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

int main()
{
    string date = "";
    double hours = 0;
    const string filename = "StudyTime.txt";

    ofstream myfile (filename, ios_base::app); //create filename and allow appending

    myfile << "Date" << setw(15) << "Study Time" << endl; //Header line

    //prompt user for Date
    cout << "Date: "; 
    cin >> date;

    //prompt user for hours
    cout << "Hours: ";
    cin >> hours;

    if (date.size() == 3) {
        myfile << date << setw(8) << hours << endl; //write date and hours to txt file
    }

    else if (date.size() == 4) {
        myfile << date << setw(7) << hours << endl; //write date and hours to txt file
    }

    else
        myfile << date << setw(6) << hours << endl; //write date and hours to txt file

    myfile.close(); //close file

    return 0;
}

Answer 1

你不止一次地运行它。

每次运行时，都会使用ios_base :: app

打开

这意味着结果将附加到之前的文件内容中。

每次都要重写文件，请使用ios_base :: trunc

ofstream myfile (filename, ios_base::trunc);

另外，您也可以只写一行而不是if / else if / else：

myfile << date << setw(11 - date.size()) << hours << endl; //write date and hours to txt file

如果你想追加，你需要决定是否写标题，检查文件是否存在：Fastest way to check if a file exist using standard C++/C++11/C?

例如：

 ifstream f(filename);
 bool exists = f.good();
 f.close();

Answer 2

在添加标题之前，您可以检查文件以查看它是否已有标题。下面的代码应该有效。我添加了一个函数hasHeader，用于检查文件中是否已存在标头，如果是，则返回true。在这种情况下，不会附加标题。

#include "stdafx.h"
#include <string>
#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

bool fileHasHeader(string file)
{
  char line[100];
  ifstream myfile;
  myfile.open(file, ifstream::in);
  myfile.getline(line, 100);
  bool hasHeader = false;
  if (strcmp(line, "Date     Study Time") == 0) {
    hasHeader = true;
  }
  myfile.close();

  return hasHeader;
}

int main()
{
  string date = "";
  double hours = 0;
  const string filename = "StudyTime.txt";

  ofstream myfile (filename, ios_base::app); //create filename and allow appending

  if (!fileHasHeader(filename)) {
    myfile << "Date" << setw(15) << "Study Time" << endl; //Header line
  }

  //prompt user for Date
  cout << "Date: "; 
  cin >> date;

  //prompt user for hours
  cout << "Hours: ";
  cin >> hours;

  if (date.size() == 3) {
    myfile << date << setw(8) << hours << endl; //write date and hours to txt file
  }

  else if (date.size() == 4) {
    myfile << date << setw(7) << hours << endl; //write date and hours to txt file
  }

  else
    myfile << date << setw(6) << hours << endl; //write date and hours to txt file

  myfile.close(); //close file

  return 0;
}