Commodore 128D - Page 92

a contingency. Neither line is executed UNLESS
there is an error. When an error occurs, the line
with the TRAP statement is enacted, and control
is directed to another part of the program. You
can use these statements to catch anticipated
errors in entering data, resume execution, or
return to text mode from a graphics mode, to
name just a few options. If you run the DO/LOOP
example (which doubled numbers) without an
UNTIL statement, you can get an OVERFLOW
error and the program crashes. You can prevent
that from happening by adding two lines, one at
the beginning of the program and one at the
end. For this example, you might add these two
lines:
5 TRAP 100
100 IF N>1 THEN END
Even though N has been much greater than one
for the entire program, the statement isn't con
sidered until there is an error. When the number
"overflows" (is greater than the computer can
accept), the TRAP statement goes into effect.
Since N is greater than one, the program is
directed to END (rather than crashing.)
Here is an example in which trapping is used to
prevent a zero from being input for division:
U
\ /
LJ
U
LJ
USING C128 MODE—Some BASIC Commands and Keyboard Operations Unique to
84 C128Mode