Strange keyboard??

**ripper079** · 08-20-2002

I´m trying to write a program that can handle different keypresses. I´ve 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;
}

Shouldn´t each key return a different value???

**fletch** · 08-20-2002

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.

**fletch** · 08-20-2002

Whoops. Cast the char into an unsigned integer.

printf(" %d", unsigned int (ch));

**ronin** · 08-20-2002

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);

**moonwalker** · 08-20-2002

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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