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Using GetKeyboardState

Posted on 1998-11-14
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Last Modified: 2013-11-20
Hi,

How can I use the GetKeyboardState function?  I mean, I know how to get the array filled with information, but I don't know how to interpret it.  Can someone give me an example of how I can, for example, get whether or not any of the arrow keys are pushed down?

Thank you very much for you help.
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Question by:a121496
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jkr earned 100 total points
ID: 1324697
Well, you could use it in this way:

BYTE aKeyState[ 256];

if ( GetKeyboardState( aKeyStaze))
{
 if ( 0x80000000 & aKeyState [ V_UP])
 {
   // 'cursor up' is pressed...
 }
}

Or you alternatively could use

if ( 0x80000000 & GetKeyState( VK_UP))
{
   // 'cursor up' is pressed...
}

(0x80000000 represents the bit mask for the highest bit in the highest byte. If this is set to '1', the key is pressed)
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by:jkr
ID: 1324698
Ooops sorry, a typo ;-)

if ( GetKeyboardState( aKeyStaze))

should of course read

if ( GetKeyboardState( aKeyState))

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by:a121496
ID: 1324699
Thank you, but I have one more question now, how can I find out which elements in the array correspond to things like the letters on the keyboard ("asdf...") etc?  What numbers are associated with them?  If you answer this as well (I'm sure you can), I will give you an 'A'.
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Expert Comment

by:jkr
ID: 1324700
That's easy ;-)
the 'VK_ *' constants can be used as indices to the array, and the ASCII codes for the characters correspond directly to the virtual keycodes:
(from 'winuser.h')
/* VK_0 thru VK_9 are the same as ASCII '0' thru '9' (0x30 - 0x39) */
/* VK_A thru VK_Z are the same as ASCII 'A' thru 'Z' (0x41 - 0x5A) */

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Author Comment

by:a121496
ID: 1324701
Huh.  That does not seem to work.  I have included "stdafx.h" so winuser should be included too, but the compiler says that those constants are undefined

"O:\stuff\newgame\AsteroidDlg.cpp(256) : error C2065: 'VK_A' : undeclared identifier"

Ideas?
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Author Comment

by:a121496
ID: 1324702
I also tried using the character constants like so:
keys['a']

but the if statment I'm using isn't called when the a key is pressed.

Things I checked:
1) The obvious: Caps lock is off.

2) I know the code is right.  Here it is though:
  if (0x80 & keys['a'])  // 'a'
  { m_Players[1].AdjustDegrees (-10);
    if (m_Players[1].GetDegrees () < 0)
    { m_Players[1].AdjustDegrees (360); }
  }
  if (0x80 & keys['d'])  // 'd'
  { m_Players[1].AdjustDegrees (10);
    if (m_Players[1].GetDegrees () > 359)
    { m_Players[1].AdjustDegrees (-360); }
  }

  if (0x80 & keys['w'])  // 'w'
  { m_Players[1].Accelerate (.025); }
  if (0x80 & keys[VK_SHIFT])
  { m_Players[1].Fire (1); }

The only 'if' that is called is the shift key one (the last one).  The others are ignored.

Forgot to mention also that 0x80000000 didn't work, I had to cut off a bunch of zeros.  I know that is not the problem because I am using this same code in another area with the VK_LEFT, UP, etc constants and it works there.
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Author Comment

by:a121496
ID: 1324703
I am using VC++ 5.0 compiler.  Perhaps you are using a different version in which those constants were defined?
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Author Comment

by:a121496
ID: 1324704
Ignore all that.  After experimenting, I found that I had to use CAPITAL letters in the [ ] like
keys['A']

Anyway, thanks for the help.
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