OnError

OnError (Statement)

Format

onerror label
onerror subroutine_name ( )

Description

Sets up an error trap. From then on, when a runtime error or a warning happens, BASIC-256 runs the trap instead of stopping the program.

• onerror label runs the code at the label like a Gosub. That code must end with a Return.
• onerror subroutine_name() calls the Subroutine. The brackets are needed: without them the name is taken as a label. The subroutine may not have parameters.

When the trap has finished, the program carries on in the statement that had the error. The operation that failed gives an empty value: 0 in a calculation, an empty string in a string. Inside the trap, LastError, LastErrorLine, LastErrorMessage and LastErrorExtra tell you what went wrong.

Error traps are kept in a stack, so the last trap defined that has not been removed with OffError is the active one. An error inside a Try / Catch / End Try goes to its catch, not to the trap.

If the trap itself causes an error, the trap is called again for that error, and again: the program does not stop by itself. Keep the trap simple.

Example

# trap errors with a label
onerror nicetrap
print 1
print 10 / 0
open 99, "bogus.file"
print "Runs Anyway"

# turn trapping off, and stop at the next error (the default)
offerror
print 2
print 10 / 0
print "never get here"
end

nicetrap:
print "trap - line " + lasterrorline + " - error " + lasterror + " - " + lasterrormessage
return

will display

1
trap - line 4 - error 50 - Division by zero
0.0
trap - line 5 - error 5 - Invalid File Number
Runs Anyway
2
ERROR on line 11: Division by zero.

The same with a subroutine:

# trap errors with a subroutine
onerror trap()
print "z = " + z
x = 10 / 0
print "x = " + x
end

subroutine trap()
   print "error " + lasterror + " on line " + lasterrorline + ": " + lasterrormessage
end subroutine

will display

error 13 on line 3: Variable z has not been assigned a value
z =
error 50 on line 4: Division by zero
x = 0.0

See additional example of usage on ThrowError page.

See Also

Error Codes, Lasterror, Lasterrorextra, Lasterrorline, Lasterrormessage, Offerror, Onerror, ThrowError, Try / Catch / EndTry

History

0.9.6z	New To Version