Sinclair QL User Manual

Procedures and Functions
Program
Output
Program
100 REMark LOCAL price
110 LET price =70
120 item 3
130item4
140 PRINT I price I
150 DEFine PROCedure item(num)
160 LOCal price
170 IF num <= 3 THEN LET pri ce = 300 + 10*num
180 IF num >= 4 THEN LET pri ce = 12*num
190 PRINT I price I
200 END DEF i ne
100 REMark Global price
110 LET price = 70
120 item 3
130item4
140 PRINT I price I
150 DEFine PROCedure item(num)
160 IF num <= 3 THEN LET pri ce = 300 + 10*num
170 IF num >= 4 THEN LET pri ce = 12*num
180 PRINT I price I
190 END DEFine
330 48 48
The price of the lager has been altered by the procedure. We say that the vanable, price,
is global because It can be used anywhere in the program.
Make the procedure vanable,
price, LOCAL to the procedure. This means that EXAMPLE
SuperBASIC will treat it as a special vanable accessible only within the procedure. The
vanable,
price, In the main program will be a different thing even though it has the same
name.
•
•
330 48 70
ThiS time everything works properly. Line 70 causes the procedure variable, price to
be Internally marked as 'belonging' only to the procedure, item. The other vanable, price
is not affected. You can see that local variables are usetul things.
Output
•
Local variables are so useful that we automatically make procedure formal parameters EXAMPLE
local. Though we have not mentioned it before parameters such as num In the above
programs cannot interfere
with main program variables. To prove this we drop the LOCAL
statement from the above program and use
num for the price of lager Because num
in the procedure is local everything works.
100 REMark LOCAL parameter Program
110 LET num = 70
120 item 3
130 item 4
140 PRINT I num I
150 DEFine PROCedure item(num)
160 IF num <= 3 THEN LET price = 300 + 10*num
170 IF num >= 4 THEN LET pri ce = 12*num
180 PRINT I price I
190 END DEFine
330 48 70
Output
•
So far we have only used procedure parameters tor passing values to the procedure.
But suppose the main program wants the cost ot an item
to be passed back so that
it can compute the total bill. We can do this easily by providing another parameter in
the procedure call. This must be a variable because it has to receive a value from the
procedure.
We therefore call it a vanable parameter and it must be matched by a
corresponding variable parameter
in the procedure definition.
VARIABLE
PARAMETERS
12/84
87

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)