Commodore Plus 4 - Page 197

Using the Built-in Error-Trapping Routine 18S
Using the Built-in Error-Trapping Routine
The TRAP command lets you prevent a program from being aborted because of
any BASIC error condition except an UNDEFINED STATEMENT error.
TRAP catches the error and branches to the line number named as the TRAP
parameter. At this line number, you can write any sort of error-handling and/or
error-reporting routine. You should use RESUME to resume execution after
completing your error routine. Using a GOTO to leave a trap routine causes
BASIC to think it is still in the TRAP routine (unless it receives another TRAP
command). Because errors inside TRAP routines cannot be trapped, it is best
never to leave a trap routine through a GOTO. RESUME reexecutes the line in
which the error occurred. RESUME NEXT resumes executing at the statement
following the error. RESUME linenumber resumes execution at the specified line
number. Error trapping can be turned off by using a TRAP command with no
line number. Error trapping is also turned off by a CLR command.
TRAP does not trap disk drive errors (read from the disk error channel) or
other errors not generated by BASIC. For a list of all the BASIC errors. See
Appendix A.
Example: In this program, a DIVISION BY ZERO error can occur if 0 is input as a value
for X. The error is trapped, and the line number (EL), error number (ER), and
error message (ERR$(ER)) are printed before execution resumes with the
INPUT statement. The program prints out 10 valid results. An input resulting in
an error is not counted because execution resumes at 30.
10 TRAP60
20 FOR 1=1 TO 10
30 INPUT Y,X
40 PRINT Y/X
50 NEXT:END
60 PRINT 'TINE'';EL;/'ERROR/';ER;ERR$(ER)
70 PRINT X:RESUME 30
Line-by-Line Explanation
10 Turn on error trapping with trap routine starting at line 60.
20 Count 10 valid results.
30 Get values for X and Y.
40 Print Y divided by X.
50 Go on to next result, and quit when done.
60 Print out line number, error number, and error message,
70 Print out the value of X and resume program execution at 30.