Breaking out of a loop with a keypress

Question

I am writing a Win32 GUI app which has a loop which I would like to restart only when a keypress is made. The main loop is already running, and I can't simply restart it, so I have to insert a 'hang' point which the user can manually break out of. The code below represents what I have put at the end of my main loop. It is supposed to pause the program by putting it into an endless sub-loop which can only be broken when the letter 'q' is pressed.

for (;;)
 {
 char temp;
 temp = _getch();

 if (temp == 'q')
  {
  break;
  }
 }

This successfully makes the program hang, but pressing 'q' does nothing to end the loop. I understand that using cin.ignore() or cin.get() would be preferable, but for some reason when I add iostream to the header list, I get a bunch of errors, so I am currently trying to do it using _getch() with the conio.h header.

Any help much appreciated.

Just curious, but what errors are you getting from the headers? — MGZero, Aug 10 '11 at 15:07
are you sure your program reaches this loop? have you verified it? — Karoly Horvath, Aug 10 '11 at 15:07
Doing an infinite loop is not the correct strategy for "pausing" a program unless you want test the speed of your fan. — Felice Pollano, Aug 10 '11 at 15:07
@MGZero, these are a couple of the errors I get when I add iostream:1>c:\program files\microsoft visual studio 10.0\vc\include\cmath(19): error C2061: syntax error : identifier 'acosf' 1>c:\program files\microsoft visual studio 10.0\vc\include\cmath(19): error C2059: syntax error : ';' — CaptainProg, Aug 10 '11 at 15:07
Also, you might want to type cast that 113 to a char, or just say 'q'. — MGZero, Aug 10 '11 at 15:08
@yi_H yes, the program definitely reaches the loop, as it didn't hang before I added it — CaptainProg, Aug 10 '11 at 15:09
@Nobody and MGZero: Thanks, I've changed to make the code more readable now — CaptainProg, Aug 10 '11 at 15:10
Probably a dumb question, and not meant to be offensive, but you do press 'q' followed by enter right? :-) — love_me_some_linux, Aug 10 '11 at 15:11
@love_me_some_linux No, pressing enter doesn't work either. However I wonder is it possible that between pressing 'q' and 'enter' since the loop restarts perhaps it throws out the 'q'? — CaptainProg, Aug 10 '11 at 15:13

score 2 · Accepted Answer · edited Dec 30 '14 at 02:00

2

You can just call MessageBox.

edited Dec 30 '14 at 02:00

oɔɯǝɹ

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answered Aug 10 '11 at 15:13

Cheers and hth. - Alf

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Genius! Thanks, will try that instead – CaptainProg Aug 10 '11 at 15:14
Just checkin' (noob here) - will a messagebox pause my main program? – CaptainProg Aug 10 '11 at 15:14
`MessageBox` is modal, so it will block whichever thread called it until the user chooses an option or closes the window. – Justin ᚅᚔᚈᚄᚒᚔ Aug 10 '11 at 15:21
Thanks. In this situation, this has worked perfectly. The program hangs and until I press Enter, the program is paused. – CaptainProg Aug 10 '11 at 15:26

Justin ᚅᚔᚈᚄᚒᚔ · Answer 2 · 2011-08-10T15:20:13.033

It's hard to tell from your question, but it sounds like you're trying to prompt the user after each iteration of a loop if they wish to continue. I assume that the infinite for loop is at the end of your main processing loop.

You don't need the infinite loop: _getch() will halt execution anyway until a key is pressed. In truth, the fact that you've wrapped your conditional in a for loop is the reason that break isn't behaving like you want -- you're breaking out of the infinite loop, but not your main processing loop.

Example:

while(1)
{
   // Do some processing
   for (;;)
   {
      char temp;
      temp = _getch();

      if (temp == 'q')
         break;   // This will break out of the for and continue the while
   }
}

vs.

while(1)
{
   // Do some processing
   if ('q' == _getch())
      break;             // This will break out of the while
}

You'll also notice that since _getch() returns a char, you can just test the return code of the function (unless you need to store the input for later use).

score 0 · Answer 3 · answered Aug 10 '11 at 15:12

0

You have to create a message loop and wait until a keydown message arrives. Try to check this example.

In a windows GUI application there is no input to read from the console.

answered Aug 10 '11 at 15:12

Felice Pollano

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score 0 · Answer 4 · answered Aug 10 '11 at 15:27

You mentioned this is a Win32 GUI application. At the time of entering the loop, have you created any Windows that might have the input focus? if yes you should try trapping WM_KEYDOWN/WM_KEYUP messages. If it is a console application, look at Console Functions specially ReadConsole.

Sam

Breaking out of a loop with a keypress

4 Answers4