Chapter 17 - Printing with frills
You will recall that a
PRINT
statement has a list of items, each one an expression (or possibly nothing at
all), & that they are separated by commas or semicolons. There are two more kinds of
PRINT
item which
are used to tell the computer not what
, but where to print. For example,
PRINT
AT 11,16; "*" prints a star in
the middle of the screen.
AT
line, column
moves the
PRINT
position (the place where the next item is to be printed) to the line & column specified.
Lines are numbered from 0 (at the top) to 21, & columns from 0 (on the left) to 31.
TAB
column
moves the
PRINT
position to the column specified. It stays on the same line, or, if this would invoke
back-spacing, moves on to the next one. Note that the computer reduces the column number modulo 32 (it
divides by 32 & takes the remainder); so
TAB
33 means the same as
TAB
1.
For example
PRINT TAB
30,1;
TAB
12;"CONTENTS";
AT
3,1;"CHAPTER";
TAB
24;"PAGE"
(This is how you might print out the heading of a Contents page, with 1 as the page numbers.)
Some small points:
(i) These new 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 immidiately move it on again - not usually terribly useful.
(ii) Although
AT
&
TAB
are not functions, you have to type the function key (shifted newline) to get them.
(iii) You cannot print on the bottom two lines (22 & 23) of the screen because they are reserved for
commands,
INPUT
data, reports and so on. References to the 'bottom line' usually mean line 21.
(iv) You can use
AT
to put the
PRINT
position even where there is already something printed; the old
stuff will be overwritten.
There are two more statements connected with
PRINT
, namely
CLS
&
SCROLL
.
CLS
Clears the Screen (but nothing else).
SCROLL
moves the whole display up one line (losing the top line) & moves the
PRINT
position to the
beginning of the bottom line.
To see how this works, run this program:
10
SCROLL
20
INPUT
A$
30
PRINT
A$
40
GOTO
10
Summary
PRINT
items:
AT
,
TAB
Statements:
CLS
,
SCROLL
Exercises
1. Try running this:
10
FOR
I=0
TO
20
20
PRINT
TAB
8*I;I;
30
NEXT
I
This shows what is meant by the
TAB
number's being reduced modulo 32. For a more elegant example,
change the 8 in line 20 to a 6.
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