Q(uick)BASIC Statement: ERROR

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ERROR

An error-trapping statement that simulates the occurrence of a BASIC error or allows the user to define error codes

Worth knowing

Useful and cross-version information about the programming environments of QBasic and QuickBasic.

Syntax

ERROR expression%

Description/Parameter(s)

expression%

The error code of a Basic or user-defined error; a value in the range 1 through 255. To define your own error, use a value that isn't listed in the Basic ⮜ Run-Time Error Codes ⮞ table.

Example

The ⮜ ERRDEV function programming example ⮞ illustrates the use of ERDEV, ERDEV$, ERL, ERR, ERROR, ON ERROR, and RESUME.

Syntax

ERROR integerexpression

Description/Parameter(s)

The integerexpression represents the error code. It must be greater than 0 and less than or equal to 255. If the integerexpression is an error code already used by BASIC, then the ERROR statement simulates the occurrence of that error and prints the corresponding error message.

To define your own error code, use a value that is greater than any used by the standard BASIC error codes. (Start at 255 and work down to maintain compatibility with future Microsoft BASIC error codes.)

If an ERROR statement specifies a code for which no error message has been defined, the message "Unprintable error" is printed. Executing an ERROR statement for which there is no error-handling routine causes an error message to be printed and execution to halt.

Example

This example uses an ERROR statement in an error-handling routine, Handler, to trap a user input error.
Tip: You must supply a text file when you run this example. Use a text file you have already created, create a file with a text editor, or specify the README.DOC text file.

ON ERROR GOTO Handler
OpenFile:
    INPUT "Name of file to update"; FileSpec$
    IF FileSpec$ = "" THEN END
    OPEN FileSpec$ FOR INPUT AS #1
    PRINT "The first five lines of "; FILESPEC$;" are:" : PRINT
    FOR I = 1 TO 5
        LINE INPUT #1, Temp$
        PRINT Temp$
    NEXT
    PRINT : INPUT "Is this the correct file"; R$
    'Define error 200.
    IF LEFT$(R$,1) <> "y" THEN ERROR 200
    END

Handler:        'Error-handling routine.
    Number = ERR
    'If program generates run-time error for "file not found,"
    'a special message is printed and the user is prompted to
    'enter a new file specification or end the program.
    IF Number = 53 THEN
        CLOSE #1
        PRINT "File not in this directory"
        PRINT "Enter new file spec ([d:]xxx...xxx) or"
        PRINT "press <RETURN> to end program"
        RESUME OpenFile
    ELSEIF Number = 200 THEN    'User entered "n"
        CLOSE #1
        RESUME OpenFile
    ELSE
        ERROR Number       'Error other than 53 or 200.
        ON ERROR GOTO 0    'Print message, disable error
    END IF                 'handling, and stop program.

Syntax

ERROR integerexpression%

Description/Parameter(s)

Usage Notes

If integerexpression% is an error code already used by BASIC, the ERROR statement simulates the occurrence of that error.
To define your own error code, use a value that is greater than any used by the standard BASIC error codes. (Start by using error code 255 and work down to avoid compromising compatibility with future Microsoft BASIC error codes.) In general, the error codes used by BASIC are between 1 and 100 (although not all these are used).
If an ERROR statement is executed when no error-handling routine is enabled, BASIC generates an error message and stops program execution. If the ERROR statement specified an error code that is not used by BASIC, the message "Unprintable error" is generated.

Example

The program uses the ON ERROR statement to enable error trapping. The program attempts to write a large file to the disk. If an error occurs, control is passed to a handling routine, and the ERR function is used to determine an appropriate message to display. The ERROR, ERR, and RESUME statements are used to exit the handling routines, and the END, STOP, and SYSTEM statements demonstrate ways to exit a program.

DECLARE SUB ErrorMessage (Message$)
DECLARE SUB WriteBigFile (Filenum%)
ON ERROR GOTO ErrHandler
CLS
PRINT "This program will attempt to write a large file to a disk drive"
PRINT "you select. The file will be erased when the program ends."
PRINT
DO
    INPUT "Which drive"; DR$
    DR$ = UCASE$(DR$)
LOOP UNTIL LEN(DR$) >= 1 AND LEN(DR$) <= 2 AND DR$ >= "A" AND DR$ <= "Z"
IF LEN(DR$) > 1 THEN
    IF RIGHT$(DR$, 1) <> ":" THEN
  DR$ = LEFT$(DR$, 1) + ":"
    END IF
ELSE
    DR$ = DR$ + ":"
END IF
'Put together a complete file specification.
FileSpec$ = DR$ + "BIGFILE.XXX"
'Get the next available file number.
Filenum% = FREEFILE
'Try to open the file.
OPEN FileSpec$ FOR OUTPUT AS Filenum%
WriteBigFile Filenum%
CLOSE Filenum%
CLS
PRINT "Everything was OK. No errors occurred."
PRINT "Deleting the file that was created."
KILL FileSpec$
'Same as END, returns to operating system.
SYSTEM
 
ErrHandler:
 
SELECT CASE ERR
    CASE 52  'Bad file name or number.
  END
    CASE 53  'File not found.
  RESUME NEXT
    CASE 57  'Device I/O error.
  ErrorMessage "You should probably format the diskette."
  END
    CASE 64  'Bad File Name.
  ErrorMessage "The drive name you specified was not correct."
  END
    CASE 68  'Device unavailable.
  ErrorMessage "The drive you named is unavailable."
  END
    CASE 71  'Drive not ready.
  ErrorMessage "The drive was not ready. Check the drive!"
  END
    CASE ELSE
  ErrorMessage "An unexpected FATAL error has occurred."
  STOP
END SELECT

SUB ErrorMessage (Message$)
    ON LOCAL ERROR GOTO MessageError
    CLS
    PRINT Message$
    PRINT "Cannot continue."
    PRINT
    PRINT "Press any key to exit."
    DO
    LOOP WHILE INKEY$ = ""
    EXIT SUB
MessageError:
    RESUME NEXT
END SUB

SUB WriteBigFile (Filenum%)
    ON LOCAL ERROR GOTO LocalHandler
    TEXT$ = STRING$(1024, "A")
    FOR I% = 1 TO 400
  PRINT #Filenum%, TEXT$
    NEXT I%
    EXIT SUB
LocalHandler:
    SELECT CASE ERR
  CASE 61             'Disk full.
      ErrorMessage ("There is no room remaining on the disk.")
      KILL "BIGFILE.XXX"
      END
  CASE ELSE
      ERROR ERR
    END SELECT
END SUB