C++ - Simple - nested while loops never terminate?

Question

I'm just revising some file i/o for C++. Writing a program that prints its own source code to the terminal without comments

Here are the loops in question WORKING with if statements

while (!inputstream.eof())
{
 if(in_comment == false)
    {
      inputstream.get(temp);
      if(temp == '/')
        {
          inputstream.get(temp1); 
          if (temp1 == '*')
            {
              in_comment = true;

            }
          else
            {
              inputstream.putback(temp1);
            }
        }

      cout << temp;
    }

  if(in_comment == true)
    {
      inputstream.get(temp);
      if(temp == '*')
        {
          inputstream.get(temp); 
          if (temp == '/')
            {
              in_comment = false;
            }
        }
    }
}

And here they are NOT working with while loops

while (!inputstream.eof())
{
 while(in_comment == false)
    {
      inputstream.get(temp);
      if(temp == '/')
        {
          inputstream.get(temp1); 
          if (temp1 == '*')
            {
              in_comment = true;
             break;
            }
          else
            {
              inputstream.putback(temp1);
            }
        }

      cout << temp;
    }

  while(in_comment == true)
    {
      inputstream.get(temp);
      if(temp == '*')
        {
          inputstream.get(temp); 
          if (temp == '/')
            {
              in_comment = false;
            }
        }
    }
}

I would have expected the eof marker to cause the program to break out of the outer while loop but it doesn't. Why is this?

Thanks

Compile with all warnings and debug info (`g++ -Wall -Wextra -g`). Then **use the debugger** (`gdb`) — Basile Starynkevitch, Dec 27 '14 at 14:21
Were you expecting the outer while to be broken while the inner while was still running? If so, that's your problem. It only checks the condition `!inputstream.eof()` at the beginning of each loop: If the outer loop never has a chance to the beginning, it won't be able to check. — Numeri, Dec 27 '14 at 14:21
Basile Starynkevitch also has a very good suggestion—learn to use a debugger well! Stepping through your code line by line with a variable watch is very, very helpful. (If you use an IDE, it almost certainly has a debugger in it.) — Numeri, Dec 27 '14 at 14:24
Also have a look on [Why is iostream::eof inside a loop condition considered wrong?](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) please. — πάντα ῥεῖ, Dec 27 '14 at 15:16

score 3 · Answer 1 · answered Dec 27 '14 at 14:26

3

Your inner loops do not break on eof so you get an endless loop - simple as that. The outer loop only gets a chance to break when the inner loops are left. The working example has no inner loops, so the outer loop can end.

answered Dec 27 '14 at 14:26

Ole Dittmann

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C++ - Simple - nested while loops never terminate?

1 Answers1