Commodore VIC-20 - Using the GET Statement

USING THE GET STATEMENT
Most simple programs use the INPUT statement to get data from
the person operating the computer. When dealing with more
complex needs, such as protection from typing errors, the GET
statement offers more flexibility and gives the program more
intelligence. This section shows you how to use the GET statement
to add some special screen editing features to your programs.
The ViC has a keyboard buffer that holds up to 10 characters.
This means if the computer is busy doing some operation and is not
reading the keyboard, you can still type in up to 10 characters, and
the VIC will use them as soon as it finishes what it was doing.
This can be demonstrated with a simple program. Type in the
program shown below. When you tell it to RUN, type the word
HELLO on the keyboard. Since the VIC is busy in a loop, nothing
appears on the screen—until the program stops after 15 seconds,
and the word HELLO that you typed appears on the screen.
10 Tl$ = "000000"
20 IF TIS < "000015" THEN 20
The VIC's input buffer is also called a queue, which is a good
image
to use to better understand how it works. Imagine standing in
line wailing to buy a ticket to get into a movie. The first person in line
is the first to get a ticket and leave the line, and the last person in line
is the last to get a ticket. (In accounting, this is called the "first in, first
out" method, or FIFO, as opposed to the "last in, first out", or LIFO
method.)
The GET statement in the VIC acts as the ticket taker. First it
looks to see if there are any characters "in line" (if a key or keys
have been typed). If there are, the first character typed gets placed
in a "variable" and out of the queue. If no characters are waiting in
the buffer, then an empty value is returned.
One other point should be mentioned when talking about the
queue. Any characters typed on the VfC's keyboard afterthe queue
is fulf are lost, since the queue was full. So imagine that the ticket
line is long enough to hold 10 people, and there is a cliff at the end of
the line. Anyone trying to get into the line afterthe line is full simply
falls off the cliff, never to be seen again.
Since the GET statement will keep going even when no character
was typed, it is often necessary to put the GET statement into a
loop, having it wait until the operator hits a key (actually, until a
character has been received}. Here is the recommended form:
(Type NEW to erase the previous program.)
10 GET AS : IF AS - "" THEN 10
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