Commodore PET - SUBTRACTION; Method 1: Subtraction via Numeric Strings

Using CHR$ and ASCII values makes it possible to program or do com-
parison checking for cursor controls such
as RETURN and INSERT /DELETE. Sup-
pose a program must check incoming characters from the keyboard to
see if the
programmer depressed the
RETURN key; this may be needed when using an IN-
PUT or GET statement. Vou could check for a RETURN which has an ASCII code of
13,
as follows:
1(1 GET ;";$
2(1 IF X$=CHR$(13) GOTO 340
30 GOTO 10
This test would be impossible if vou tried to put a RETURN between quota-
tion marks:
21<:1 IF ;:";$='" F:ETUF.:N l''OI)TO 340
t
Impossible
This is impossible because when vou depress the RETURN key following a set of
quotes,
it automatically moves the cursor to the next line:
As vou can
see, use of the CHR$ function and ASCII numbers is extremelv
helpful because
it allows vou to program characters and controls which could not
otherwise
be programmed.
ARITHMETIC
With the PET, vou can do addition, subtraction, multiplication, and
division, using
up to 9-digit numbers in either calculator mode or program
mode.
A 9-digit length limit can cause problems. For example. integer numbers
range from 0 to 999999999; in dollars and cents this gives vou a range of $0.00 to
$9,999.999.99
(or $999.999,999 if cents are not used). Fractional numbers mav
range from 0.000000001 to 99999999.9. Although these limits pose no problems
in manv applications, numbers
will frequent1v exceed these limits in business and
scientific applications.
Two programming methods can overcome the PET's numeric length
limitations. The first method calculates the numbers
as numeric strings. The
second method calculates
with multiple integer math, where a large number
is separated into smaller segments, and each segment is handled separately.
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