Detecting if 'Control' or 'Shift' key was down when key pressed

[Icyculyr]

I have been looking into detecting key stuff, it works great, I am trying to figure out if control or shift was pressed, I read in some MSDN documentation (I use Visual C++), that lParam is 0 if ctrl or shift isn't down, or 1 if they are, but how do I tell which one?

I may be wrong, it had a table structure saying stuff like:

0-15 repeat count
16-18 reserved, do not use

19-23 = something

24 = if the right hand shift or control key is down on an extended 101-102 key keyboard.. returns 1 if it is, otherwise returns 0

My code for keys:

CASE WM_KEYDOWN:
    {
    if (LOWORD(lParam) == 1)
    {
        MessageBox(hWnd, _T("Ctrl Or Shift down"), _T("CAP"), 0);
    }
    int iKey = LOWORD(wParam);
    if (iKey == 37) //LEFT ARROW
    {
        MessageBox(hWnd, _T("LA"), _T("CAP"), 0);
    }
    else if (iKey == 39) //RIGHT ARROW
    {
        MessageBox(hWnd, _T("RA"), _T("CAP"), 0);
    }
    }

I get the message Ctrl Or Shift down, when I hold either of them down, but I don't if I don't hold them down...

How do I tell which one was pressed? Thanks

[Boris K K]

Your code does not make much sense to me but if you want to check the state of other keys, such as Ctrl, Alt, or Shift while processing WM_KEYUP/DOWN, you can use GetKeyState API function.

[kabilius]

Hi Icyculyr,

I have been reading information about keyboard messages recently, at one point, I was exposed to the topic you are investigating.

Like Boris K K suggested, you can use GetKeyState( virtual key) to discover if a shift key, such as SHIFT, CTRL, and ALT, is pressed.
For example,

nRet = GetKeyState ( VK_SHIFT ) ;

You can obtain a more detailed usage on MSDN GetKeyState () ;

However, according to the book I was reading, it mentioned a flaw of using GetKeyState. GetKeyState does not check keyboard status in real-time.

It refects the keyboard status up to and including the current message being processed.

If this is not what you want, you can also check VK_SHIFT at WM_KEYDOWN

Hope this helps.

kab

[Boris K K]

Actually it is not a flow. You are interested if Ctrl was down at the time the other key was pressd, not whether it is down or up at the moment you are processing the message.

[kabilius]

Hi Boris K K,

I was confused with GetKeyState after I was reading the book. After reading your post, I guess I am still confused

Here is my understanding of GetKeyState:
Let's say we are using GetKeyState to determine if a letter should be displayed as capitalized or not
Let's say the message queue contains ['a']['p']['p']['shift']['l']['e'], so we know the user types 'appLe'
If we hold 'shift' key while the message queue is being processed, we will get 'APPLE' instead of 'appLe'

I believe your statement

not whether it is down or up at the moment you are processing the message.

is targeting this case I just described.

The lesson that I got out of this is that "it is not a good practice to use GetKeyState to determine if a letter is capitalized or not."


Question:
Could you give me an example of when you will want to use GetKeyState?

kab

[golanshahar]

You can use ::GetAsyncKeyState().

Cheers

[Icyculyr]

Hi all, thanks for your input,

I have read the MSDN documentation on this a little while ago,

GetKeyState() checks if the key was pressed at the same time as the WM_KEYDOWN message came,


GetAsyncKeyState() checks if it is still down...

(As far as I know)

Thanks alot guys

[Boris K K]

...Let's say the message queue contains ['a']['p']['p']['shift']['l']['e'], so we know the user types 'appLe'
If we hold 'shift' key while the message queue is being processed, we will get 'APPLE' instead of 'appLe'

I believe your statement
is targeting this case I just described.

The lesson that I got out of this is that "it is not a good practice to use GetKeyState to determine if a letter is capitalized or not."

No, using GetAsyncKeyState (is Shift down right now) may result in incorrect capitalization. GetKeyState (was Shift down when 'l' was pressed; was Shift down when 'e' was pressed, etc.) should be OK.

[bauran]

void CMousepresentView::OnDraw(CDC* pDC)
{
int shiftValue=::GetKeyState(VK_SHIFT);
if(!shiftValue)
pDC->TextOut(0,50,"Shift not pressed");
else
pDC->TextOut(0,50,"Shift pressed");

int ctrlValue=::GetKeyState(VK_CONTROL);
if(!ctrlValue)
pDC->TextOut(0,100,"Ctrl not pressed");
else
pDC->TextOut(0,100,"Ctrl pressed");
}
In this program when I run the program output is
shift pressed

ctrl not pressed

Why "shift pressed" comes even I don't press shift.

[David19]

Like for Ctrl+A, Try

 if (GetKeyState(VK_CONTROL) != 1)
		{
				switch(wParam)
				{
				case 65:
					//statements
				}
		}

[hoxsiew]

If this is in your OnDraw routine, you'll need to invalidate your drawing area to get a repaint. Try handling WM_KEYDOWN, and call Invalidate() from it's handler.