Drawing
on
the
Screen
A
SIMPLE
LOOP
Computers can
do
things very qUickly but
it
would not be possible
to
exploit this great •
power
If
every action had
to
be written
as
an
Instruction. A building foreman has a similar
problem.
If
he wants a workman
to
lay a hundred paving stones that
is
roughly what
he
says.
He does not give a hundred separate Instructions.
A traditional
way
of
achieving looping or repetition
In
BASIC
is
to
use
a GO
TO
(or
GOTO,
they
are
the same) statement
as
follows:
NEW"II
10
PAPER 6 : CLS
..
"
20
BORDER
1,2
..
'
30
INK
RND(5)
..
',
40
LINE
50,60
TO
RNDC10OJ,
RND(100)
..
,
50
GOTO
0
..
",
RUN
.11
You
may prefer not
to
type
in
this program because SuperBASIC allows a better way
of
dOing
repetition. Note certain things about each
line.
~~
~~~p~a~rt_'
-
~~=t
=-re-pe_at_ed'
'I
I"
----------'-1
~~
L~han~e
part_--:
repeated.J
50
~ntrols
pr6gr~~~=
You
can
re-write
the above program
by
omitting
the
GOTO
statement and, instead, putting
REPeat and END REPeat around the part
to
be repeated.
NEW"II
10
PAPER 6 :
CLS
..
"
20
BORDER
1
,2
..
"
30
REPEAT
star
..
"
40
INK
RND(5)
..
"
50
LINE
50,60
TO
RND(100),
RND(100)
..
"
60
END
REPEAT
star
..
"
RUN
"It
•
We
have give the repeat
structure
a name,
star.
The structure consists of the two lines:
REPeat star
END REPeat star
and what lies between them
is
called the content of the structure The use of upper
case letters Indicates that REP
IS
a valid abbreviation of REPeal.
This program should produce coloured lines Indefinitely
to
make a star as shown
in
the •
figure
below.
16
The
STAR
program
You
can stop
it
by pressing the break
keys:
Hold down I CTRL I and then press
rl
-~S"'P"'1>."C"E~----'
12/84
•
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