Part 15
More about PRINT and INPUT

Subjects covered...

	CLS
	PRINT items
	Expressions (numeric or string type)
	TAB numeric expression
	AT numeric expression
	PRINT separators , ; '
	INPUT items
	Variables (numeric or string type)
	LINE string variable
	Scrolling
	SCREEN$

You have already seen PRINT used quite a lot, so you will have a rough
idea of how it is used. Expressions whose values are printed are
called PRINT items. They may be separated by commas, semicolons or
apostrophes, which are called PRINT separators. A PRINT item can also
be nothing at all, which is a way of explaining what happens when you
use PRINT on its own.

There are two more kinds of PRINT items, which are used to tell the +3
not what, but where to print. For example, the instruction...

	10 PRINT AT 11,16;"*"

...prints an asterisk '*' in the centre of the screen. This is
because...

	AT line,column

...moves the PRINT position (the place where the next item is to be
printed) to the line and column specified. Lines are numbered from 0
(at the top) to 21; columns are numbered from 0 (on the left) to 31.

SCREEN$ is the reverse function to PRINT AT, and will (within limits)
'read' the character which is located at a particular position on the
screen. It uses line and column numbers in the same way as PRINT AT,
but enclosed in brackets. For example, the instruction...

	20 PRINT AT 0,0; SCREEN$ (11,16)

...will read the asterisk printed in the centre of the screen, then
print it at location 0,0 (the top left-hand corner).

Characters from tokens are read normally (as single characters), and
spaces are read as spaces. However, attempting to read user-defined
characters, graphics characters, or lines drawn by PLOT, DRAW and
CIRCLE, result in a null (empty) string being returned. The same
applies if OVER has been used to create a composite character. (The
keywords PLOT, DRAW, CIRCLE and OVER are described in parts 16 and 17
of this chapter.)


               __ You cannot normally PRINT or PLOT
              |   on the bottom two lines
              |
              |  2 2 1 1 1 1 1 1 1 1 1 1                        
             _|_ 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0    
            '___`___________________________________________    
     P    0 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  0  C
     i    8 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  1  o
     x   16 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  2  l
     e   24 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  3  u
     l   32 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  4  m
         40 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  5  n
     x   48 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  6  s
         56 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  7   
     c   64 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  8  |
     o   72 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|  9  |
     o   80 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 10  v  
     r   88 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 11  
     d   96 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 12
     i  104 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 13
     n  112 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 14
     a  120 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 15
     t  128 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 16
     e  136 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 17
     s  144 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 18
        152 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 19
     |  160 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 20
     |  168 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 21
     v  176 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 22
        184 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 23
        192 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 24
        200 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 25
        208 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 26
        216 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 27
        224 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 28
        232 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 29
        240 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 30 
        248 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 31 
                                          1 1 1 1 1 1 1 1 1   
                    1 2 3 4 4 5 6 7 8 8 9 0 1 2 2 3 4 5 6 6
                0 8 6 4 2 0 8 6 4 2 0 8 6 4 2 0 8 6 4 2 0 8
                                                          
                Pixel y coordinates -->


The function...

	TAB column

...prints enough spaces to move the PRINT position to the column
specified. It stays on the same line, or, if this would involve
backspacing, moves to the next line. Note that the +3 reduces the
column number 'modulo 32' (i.e. it divides by 32 and takes the
remainder) - so 'TAB 33' means the same as 'TAB 1'.

As an example...

	PRINT TAB 30;1; TAB 12;"Contents"; AT 3,1;"Chapter"; TAB
		24;"Page"

...is how you might want to print out the heading on the contents page
(page 1) of a book.

Try running this...

	10 FOR n=0 TO 20
	20 PRINT TAB 8*n;n;
	30 NEXT n

This shows what is meant by the TAB numbers being reduced modulo 32.

For a more elegant example, change the 8 in line 20 to a 6.

Note the following points...

(i) TABs and print items are best terminated with semicolons, as we
have done above. You can use commas (or nothing, at the end of the
statement), but this means that after having carefully set up the
PRINT position, you immediately move it on again - not terribly
useful!

(ii) You cannot print on the bottom two lines (22 and 23) on the
screen because they are reserved for commands, INPUT data, reports,
error messages and so on. References to 'the bottom line' usually mean
line 21.

(iii) You can use AT to locate the PRINT position even where these is
already something printed - the new print item will simply overwrite
the old.

Another statement connected with PRINT is CLS. This clears the whole
screen.

When printing reaches the bottom of the screen, it starts to scroll
upwards rather like a typewriter. You can see this if you go into the
small screen using the edit menu option 'Screen' (described in chapter
6), and then type...

	CLS: FOR n=1 TO 30: PRINT n: NEXT n

When it has printed a screen full, the +3 will stop with the message
'scroll?' at the bottom of the screen. You can now inspect the first
22 numbers at your leisure. When you have finished with them, press Y
(for yes) and the +3 will give you the next screen full of numbers.
Actually, any key will make the +3 carry on except N (for no), the
BREAK key or the space bar. These will make the +3 stop running the
program with the report 'D BREAK - CONT repeats'.

The INPUT statement can do much more than we have told you so far. You
have already seen INPUT statements like...

	INPUT "How old are you?",age

...in which the +3 prints the caption 'How old are you?' at the bottom
of the screen, and then you have to type in your age. In fact though,
an INPUT statement can be made up of items and separators in exactly
the same way as a PRINT statement, so 'How old are you?' and 'age' are
both INPUT items. INPUT items are generally the same as PRINT items,
however, there are some very important differences.

First, an obvious extra INPUT item is the variable whose value you
require to be typed in - 'age' in our example above. The rule is that
if an INPUT item begins with a letter, then it must be a variable
whose value is to be input.

This would seem to mean that you can't print out the values of
variables as part of a caption. However, you can get round this by
putting brackets around the variable. Any expression that starts with
a letter must be enclosed in brackets if it is to be printed as part
of a caption.

Any kind of PRINT item that is not affected by these rules is also an
INPUT item. Here is an example to illustrate what's going on...

	LET my age = INT ( RND * 100): INPUT ("I am ";my age;".");
		" How old are you?", your age

'my age' is contained in brackets, so its value gets printed out.
'your age' is not contained in brackets, so you have to type its value
in.

Everything that an INPUT statement writes goes to the bottom part of
the screen, which acts somewhat independently of the top part. In
particular, lines are numbered relative to the top line of the bottom
half, even if this has moved up the actual TV screen (which it does if
you type lots of INPUT data). Whatever the small screen does during
INPUT, however, it will always revert to being two lines in size when
the program stops, and you start editing.

To see how AT works in INPUT statements, try this...

	10 INPUT "This is line 1.", a$; AT 0,0;"This is line 0.",a$;
		AT 2,0;"This is line 2."; AT 1,0;"This is still line
		1.",a$

Run the program (just press ENTER each time it stops). When 'This is
line 2' is printed, the lower part of the screen moves up to make room
for it; but the numbering moves up as well, so that the lines of text
keep their same numbers.

Now try this...

	10 FOR n=0 TO 19: PRINT AT n, 0;n;: NEXT n
	20 INPUT AT 0, 0;a$; AT 1, 0;a$; AT 2, 0;a$; AT 3, 0;a$; AT 4,
		0;a$; AT 5, 0;a$;

As the lower part of the screen goes up and up, the upper part remains
undisturbed until the lower part threatens to write on the same line
as the PRINT position. Then the upper part starts scrolling up to
avoid this.

Another refinement to the INPUT statement that we haven't seen yet is
called LINE input and is a different way of inputting string
variables. If you use LINE before the name of a string variable to be
input, as in...

	INPUT LINE a$

...then the +3 will not give you the string quotes that it normally
does for a string variable (though it will pretend to itself that they
are there). So if you type in...

	bugs

...as the INPUT data, 'a$' will be given the value 'bugs'. Because the
string quotes do not appear with the string, you cannot delete them
and type in a different sort of string expression for the INPUT data.
Remember that you cannot use LINE for numeric variables.

There's an interesting side effect to INPUT. Whilst typing into an
INPUT request, the old Spectrum single-key entry system enjoys a brief
moment of freedom before being locked away again when you press ENTER.
Run this program if you're interested...

	10 INPUT numbers
	20 PRINT numbers
	30 GO TO 10

Input a few numbers, and they'll be printed faithfully onto the
screen. Now press EXTEND MODE followed by the M key. The word PI
appears, and if you press ENTER, then '3.1415927' will appear as if
by magic. However, if you type PI as two letters without the aid of
EXTEND MODE then the +3 will stop with the report '2 Variable not
found, 10:1'.

There's no simple explanation for this behaviour, and it's best just
to be aware that it can happen if you press some combinations of keys
during INPUT. If for some reason you're keen to experiment, chapter 7
(Using 48 BASIC) will tell you which keys produce which effects.

The control characters 'CHR$ 22' and 'CHR$ 23' have effects rather
like AT and TAB. Whenever the +3 is instructed to print one of them,
the character must be followed by two more characters that do not have
their usual effect, but that are treated instead as numbers (their
codes) to specify the line and column (for AT) or the tab position
(for TAB). You will almost always find it easier to use AT and TAB in
the usual way rather than use control characters, however, they might
be useful in some circumstances. The AT control character is 'CHR$
22'. The first character after it specified the line number and the
second specifies the column number, so that...

	PRINT CHR$ 22+ CHR$ 1+ CHR$ c;

...has exactly the same effect as...

	PRINT AT 1, c;

This is so that even if 'CHR$ 1' or 'CHR$ c' would normally have a
different meaning (for instance if 'c=13'); the 'CHR$ 22' before them
overrides that.

The TAB control character is 'CHR$ 23' and the two characters after it
combine to give a number between 0 and 65535, specifying the number
you would have in a TAB item. The statement...

	PRINT CHR$ 23+ CHR$ a+CHR$ b;

...has the same effect as...

	PRINT TAB a+256*b;

You can use POKE to stop the computer asking if you wish to 'scroll?'
by typing...

	POKE 23692,255

...every so often. After this it will scroll u p255 times before
stopping with 'scroll?'. As an example, try...

	10 FOR n=0 TO 1000
	20 PRINT n: POKE 23692,255
	30 NEXT n

...and watch everything whizz off the screen!


Exercise...

1. Try this program on some children, to test their multiplication
tables...

	 10 LET m$=""
	 20 LET a= INT (RND*12)+1: LET b= INT (RND*12)+1
	 30 INPUT (m$) ' ' "what is ";(a);" x ";(b);"?";c
	100 IF c=a*b THEN LET m$="Right.": GO TO 20
	110 LET m$="Wrong. Try again.": GO TO 30

If they are perceptive, they might manage to work out that they do not
have to do the calculation themselves. For instance, if the +3 asks
them to type the answer to 2 x 3, then all they have to do is type in
'2*3' literally.