Strange keyboard??

This is a discussion on Strange keyboard?? within the C Programming forums, part of the General Programming Boards category; Im trying to write a program that can handle different keypresses. Ive downloaded an ASCII table so I know what ...

  1. #1
    Veni Vidi Vice
    Join Date
    Aug 2001
    Posts
    343

    Strange keyboard??

    Im trying to write a program that can handle different keypresses. Ive downloaded an ASCII table so I know what value each key has http://www.asciitable.com.

    If I understand it right there is two different versions of ASCII tabel, one "usual" (0-127) and one extended (0-255). Im using the getch() function and it works well on "alfabetic"(visible) character but when I retreive a value from leftarrow I get a negative value (-32). Is it possible (apperntly it is) to get a negative value?? But what is more strange is that I get the same value for left/right/up/down arrow (-32).

    Simple code
    Code:
    int main()
    {
    	char ch;
    		ch = getch();
    		printf(" %d", ch);
    return 0;
    }
    Shouldnt each key return a different value???

  2. #2
    Registered User fletch's Avatar
    Join Date
    Jul 2002
    Posts
    176
    A char is unsigned. So a -32 is really 224. Some keys, namely the arrow keys and function keys, return two values. The first number is an escape code (for F1-F10 it's char(0) and for F11, F12, and the arrow keys it's char(224)) and the second number is the actual key value.
    Code:
    int main()
    {
    	char ch;
    
    	do
    	{
    		ch = getch();
    		printf(" %d", ch);
    	} while (ch != 27);	// loop until press escape
    
    	return 0;
    }
    Try this code. When you press the left arrow key you should get 224 and 75.
    "Logic is the art of going wrong with confidence."
    Morris Kline

  3. #3
    Registered User fletch's Avatar
    Join Date
    Jul 2002
    Posts
    176
    Whoops. Cast the char into an unsigned integer.

    printf(" %d", unsigned int (ch));
    "Logic is the art of going wrong with confidence."
    Morris Kline

  4. #4
    Casual Visitor
    Join Date
    Oct 2001
    Posts
    350
    I just toyed with this same issue not long ago. I never got an absolute confirmation that this would cause random errors, but it traps function and arrow keys.

    Code:
    unsigned key1, key2;
    
    do
    {
         key1 = getch();
         if(key1 == 0)
         {
               key2 = getch();
               switch(key2)
               {
                    case 72  /*up arrow */
                     and so on
               };
          }
          else
         {
          
               switch(key1)
              {
                 do whatever here.
                 printf("%c", key1);
             };
          }
    }while(sentinel == true);
    I haven't used a compiler in ages, so please be gentle as I try to reacclimate myself. :P

  5. #5
    Registered User moonwalker's Avatar
    Join Date
    Jul 2002
    Posts
    282

    here...

    Code:
    #include <stdio.h>
    #include <conio.h>
    
    main()
    {
    	char c;
    	
    	printf("KEY\tCODE\n");
    
    	while ((c = getch()) != 27) //as long as 'Esc' is not pressed
    	{
    		printf("%3c\t%4d\n", c, (int) c);
    	}
    }

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