Question

我是一名新生计算机科学专业，主修我的编程原理1课程。其中一小部分涉及询问用户是否要覆盖已存在的文件。我正在使用Code :: Blocks，请记住，我只是在我的第一个编程课程中。这是我为此写的代码：

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
   char outputFileName[81];
   ifstream tempInputFile; // to check if file already exists
   char overwrite;
   ofstream outputFile;

   system( "cls" );
   cout << endl << endl;

   cout << "Please enter the file for data to be written to: ";
   cin >> outputFileName;
   cout << endl << endl;
   tempInputFile.open( outputFileName );
   while ( tempInputFile )
   {
      cout << "This file already exists. Would you like to overwrite?  (Y/N):";
      cin >> overwrite;
      cout << endl << endl;
      if ( overwrite == 'Y' || overwrite == 'y' )
         tempInputFile.close();
           // I also tried a block if statement here with .clear() before the .close()
      else
      {
         tempInputFile.close();
         cout << "Please enter the file for data to be written to: ";
         cin >> outputFileName;
         cout << endl << endl;
         tempInputFile.open( outputFileName );
      } // end else
   } // end while

   tempInputFile.close();
   outputFile.open( outputFileName );

   cout << "The file is ready to be written to..." << endl << endl;

   cout << endl << endl;
   system( "pause" );
   return 0;
} // end main()

问题是如果输入现有文件，则用户回答＆＃34; Y＆＃34;要覆盖该文件，程序将再次询问是否要覆盖该文件。回答＆＃34; Y＆＃34;第二次将继续，但我不明白为什么它第二次问。我最后编写了一些正确处理它的代码（使用do ... while循环），但我想知道为什么其他代码没有做我认为应该做的事情。有人可以向我解释一下吗？这是我写的代码，它正在做我想要的：

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
   char outputFileName[81];
   ifstream tempInputFile;
   char overwrite;
   ofstream outputFile;

   system( "cls" );
   cout << endl << endl;

   cout << "Please enter the file for data to be written to: ";
   cin >> outputFileName;
   cout << endl << endl;

   tempInputFile.open( outputFileName );
   if ( tempInputFile )
      do
      {
         cout << "This file already exists. Would you like to overwrite?  (Y/N):";
         cin >> overwrite;
         cout << endl << endl;
         if ( overwrite == 'N' || overwrite == 'n' )
         {
            tempInputFile.close();
            cout << "Please enter the file for data to be written to: ";
            cin >> outputFileName;
            cout << endl << endl;
            tempInputFile.open( outputFileName );
            if ( !tempInputFile )
               overwrite = 'Y';
         } // end if
      } while ( overwrite == 'N' || overwrite == 'n' );

   cout << "The file is ready to be written to..." << endl << endl;

   tempInputFile.close();
   outputFile.open( outputFileName );

   cout << endl << endl;
   system( "pause" );
   return 0;
} // end main()

Answer 1

让我们专注于您的代码的这一部分：

tempInputFile.open( outputFileName );
while ( tempInputFile )
{
  cout << "This file already exists. Would you like to overwrite?  (Y/N):";
  cin >> overwrite;
  cout << endl << endl;
  if ( overwrite == 'Y' || overwrite == 'y' )
     tempInputFile.close();       
  else
  {
      // not important
  } // end else
} // end while

成功while后第一次调用open时，您询问用户是否要覆盖该文件，如果答案是Y，则关闭该文件while循环再次检查条件。

问题是while ( tempInputFile )表达式检查是否有errors。由于关闭文件通常不会导致任何问题，因此条件为true并且您再次输入while的正文。然后你尝试再次关闭文件，这里是文件已经关闭失败的地方。因此，当第3次调用while ( tempInputFile )时，它将以false结束。

Answer 2

在循环的第一部分放置一个break语句。虽然你做tempinputfile.close()我的猜测是，就while循环而言，它仍然是一个真正的值。

所以编辑它是这样的：

cin >> overwrite;
cout << endl << endl;
if ( overwrite == 'Y' || overwrite == 'y' )
{
tempInputFile.close();
break;
}

那应该有用！中断将立即退出while循环并允许您覆盖。不在else语句下放置一个中断，因为在这种情况下，您希望循环再次执行。

干杯！

C ++编程：询问用户是否要覆盖已存在的文件

2 个答案: