Commodore VIC-20 - The Keyboard as Joystick

166 The VIC 20 User Guide
2. If no key has been pressed, the numeric variable is assigned a value
of O. The program has no way of determining whether no key was
pressed
or a "0" was typed.
With a string variable, you can
GET almost any key, including the
cursor control keys and
RETURN. (The STOP and RESTORE keys can't be
read, and the various shifts and the
CTRL key operate as usual.) If no key is
pressed, GET assigns an empty string to the variable and your program
detects this.
If you want to wait until something is typed in, use a line like the
following:
113 GET AI : IF A$ .. 1111 THEN 113
Notice that there is no space between the quotes, resulting in an empty
string.
If the variable you GET is equal to this empty string, then no key was
pressed.
Echoing Keystrokes
As mentioned earlier, characters entered with GET do not appear on
the screen. Sometimes, however, you need to see what you type. This can be
accomplished
by adding a PRINT statement to the program.
113 GET AI : IF AS = 1111 THEN 113
213 PRINT AS;
The simple program above will echo to the screen exactly what is typed
at the keyboard. Pressing the STOP key will stop the program.
The Keyboard as Joystick
This section is presented in a "case study"form. Rather than start with
the program listing,
we will first show you some of the steps taken in
designing it. Following these steps will help you to understand
both the
program listings and the programming process itself.
Our make-believe joystick will act
just like the ones made by Commo-
dore. You can make it point in various directions (left, right, up, down, and
diagonally) by pressing different keys.
It will also have a Fire button,
although it can't be pushed while a direction key
is being held down.