Sinclair QL User Manual

From Basic to SuperBAS1C
In this case w=1 is a condition or logical expression which is either true or false If it •
is true then a subroutine starting at line 300 would be executed. This subroutine may
deal with
kite flying but you cannot tell from the above line. A careful programmer would
write:
40
IF w=l THEN GOSUB 300 : REM fly_ki te
to make it more readable. But the SuperBASIC statement is readable as it stands. The
identifier
windy is interpreted as true or false though it is actually a floating point van able.
A value of 1 or any non-zero value is taken as true. Zero is taken as false. Thus the
single word,
windy, has the same effect as a condition of logical expression.
The other word,
fly_kite, IS a procedure. It does a job similar to, but rather better than,
GOSUB 300.
The following program Will convey the idea of logical variables and the simplest type
of named procedure.
100 INPUT wi ndy
110 IF windy THEN fly kite
120 IF NOT windy THEN-tidy shed
130 DEFine PROCedure fly kite
140 PRINT "See it in the air."
150 END DEFine
160 DEFine PROCedure tidy shed
170 PRINT "Sort out rubbish."
180 END DEFine
INPUT OUTPUT
a Sort out rubbish.
1 See it in the air
2 See
it
intheair
-2
See
it
in the air
You can see that only zero is taken as meaning false. You would not normally write
procedures
with only one action statement but the program illustrates the idea and syntax
in a very simple context. More is said about procedures later in this chapter.
LET STATEMENTS In SuperBASIC LET is optional but we use it in thiS manual so that there will be less
chance
of confusion caused by the two possible uses of =. The meanings of = in:
LET count = 3
and in
IF count = 3 THEN EXIT
are different and the LET helps to emphasise this. However, if there are two or a few
LET statements dOing some simple job such as setting initial values, an exception may
be made.
For example:
100 LET fi rst = a
110 LET second = a
120 LET t h i rd = a
may be re-written as:
100 LET first = 0: second = 0: third = a
without loss of clarity or style. It is also consistent With the general concept of allOWing
short forms of other constructions where they are used in simple ways.
The colon: is a valid statement terminator and may be used with other statements besides
LET.
12/84
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Sinclair QL Specifications

General

Processor	Motorola 68008
Clock Speed	7.5 MHz
RAM	128 KB (expandable to 640 KB)
ROM	48 KB
Operating System	Sinclair QDOS
Release Year	1984
Storage	Microdrive tape loop
Graphics	256x256 pixels, 8 colors
Ports	RS-232, ROM cartridge
Sound	Beeper (internal speaker)