#004 07 января 1987

Outclass - PRINTING IN AND CLEARING THE "INPUT ZONE"

OUTCLASS articles will appear
occasionally in OUTLET to cover
different aspects of BASIC and
machine code programming. OUTLET
readers are encouraged to write
in to the letters pages of
OUTLET with any queries arising
from these articles or any other
programming problems. We can't
guarantee to have all the
answers but then other readers
may be able to help.



You are also welcome to send in
articles yourself for inclusion
in this OUTCLASS section if you
feel you have expertise in some
area of programming. Programming
principles, hints, tips,
information specific to the
Spectrum, structure, creativity,
in short ANYTHING that might be
helpful or interesting to
programming enthusiasts or those
learning to program.

Those of you well acquainted
with the Spectrum will find much
that seems absurdly simple in
this section - things you take
for granted but remember there
are plenty of beginners who will
find such information helpful.



So much for the introduction.
Let's get on with this month's
subject which I have seen raised
again and again over the years
in the letters pages of other
magazines:-

  PRINTING IN AND CLEARING THE  
  "INPUT  ZONE" - Brian Cavers  
  ---------------------------- 

As you probably know, the
Spectrum screen is divided into
two main areas or "windows". If
you set the border to a
different colour to the main
screen with say, BORDER 0 or
BORDER 4, you can see quite
clearly that when you type



anything on the keyboard it
appears on a line below the
central screen. If you look
closely you will see it is
actually two lines below the
central screen. The first line
is blank. If you type to the end
of the line the cursor will drop
to the start of the next line
below and the border colour will
stretch up one line to cover the
bottom line of the central
screen.

You now have three lines in the
input area and this can expand
up the screen. Normally,
however, there are just two
lines in the input area. In
BASIC, these may be referred to
as lines 23 and 24. I am sure

you have already discovered that
PRINT AT 23,0;"ANYTHING" will
not work! You can only normally
print at lines 0 to 21.

So how do you print something in
the input area (other than when
you are entering something from
the keyboard)? Why should you
want to anyway? Well, you will
find it handy for prompts such
as "PRESS ANY KEY..." or the
score in a game. There are many
uses but mainly it just expands
the screen. The whole screen is
24 lines high (numbered 0 to 23)
so why not use it all?


The way to print into the input
zone is with the hash sign: #. I
will give a couple of examples
then a simple explanation. You
need not bother understanding
the explanation if you don't
want. You only need to know how
to do it!

10 PRINT #0;AT 0,0;"TESTING ONE
TWO THREE"

20 PAUSE 0

Notice that the # sign is
followed by the number 0. You
can also use the number 1. It
makes no practical difference.
Following a PRINT statement, the



# sign simply signifies a stream
number. A "stream" is like a
flow of information or data
leading somewhere. #0 and #1 go
to the input area. #2 goes to
the central screen and #3 goes
to a printer. Actually PRINT and
LPRINT, both without any # sign,
MEAN PRINT #2 and PRINT #3
respectively. The Spectrum
assumes that if you don't
mention the stream number for
PRINT then you mean #2 and
similarly #3 for LPRINT.

You can try PRINT #2 and you
will find it prints to the
central screen. This instruction
might occasionally be useful if
you want to print to both the
screen and to a printer.



You could have say:-           
10 FOR N = 2 TO 3 : PRINT # N;
"text...": NEXT N.

If you do not yet know what a
FOR...NEXT instruction is: the
above will first PRINT with N
valued at 2 and then loop back
and PRINT again with N valued at
3.

It's important to remember to
put colour statements AFTER the
# sign when printing to the
input area, e.g:.

10 PRINT #1; PAPER 6; INK 2;
BRIGHT 1; FLASH 1; AT 1,8; "STOP
THE TAPE!"      -       NOT:   
10 PRINT INK 7; #1; AT 1,7;
"PRESS M FOR THE MAIN MENU..."



I always put the print position
in when printing to the input
area, i.e. I use the AT
statement otherwise you often
get the input area scrolling up
when you don't want it to scroll
up. The lines in the input area
are numbered 0 and 1. The AT
statement, like the colour
statements, should come AFTER
the # number.

Finally, remember if you are
experimenting with the above you
will need something after the
instruction to keep the program
running even if it is just a
PAUSE 0 (which holds the program
until a key is pressed)
otherwise the program will stop
running and the input area will



be immediately cleared before
you can read what you put there!

Talking of clearing the input
area brings me nicely to the way
you can do that in your
programs. The simplest way is
just:  INPUT;

There are other variations, such
as INPUT "" but INPUT; is the
shortest and uses the least
memory.

This instruction will erase
anything in the input area.
Normally you would use this if
you wished to remove a message
that was no longer needed or to
print something new in the input
area which does not completely



cover the old message. Also, if
more text is printed on the main
screen, the text in the input
area may be scrolled up on to
the central screen making a mess
so you would need to erase the
input area first with INPUT;
print to the main screen and
then re-print any text you need
in the input area.

Well that's it for now. Tell us
if you find the presentation of
this section TOO simplified! All
suggestions welcome!

In future OUTCLASSes we shall be
looking at how to save memory
and when NOT to and making a
simple start with machine code.
See you then!