[ ON selector ] GOTO lineNumber [ , lineNumber ] ...

 

 

 

Program flow control.  Transfer program execution to the start of a (typically other) program line.

 

The original GOTO (without a space) and the newer GO TO (with one or more spaces) are suppose to be indentical / synomyous.  However, a SYNTAX ERROR will be generated if the ON preposition is used with the (newer) GO TO version.  Fail!  Or as I like to say: if it ain't broke, don't fix it.

 

GOTO can be used for unconditional program flow re-direction.  However it is typically used in conjunction with a decision making process.  The most typical case being the IF/THEN/ELSE construct.  Since some BASIC versions lack ELSE, excpect them to have more instances of pure GOTOs.

 

Although the main (trailing) argument to GOTO, the lineNumber, is simply a number, CBM BASIC does not allow the number to be contained in a variable or calculated from an expression.  Why, why, why?  It seems to make a RENUMBER routine more complex, but the original version(s) of CBM BASIC did not have RENUMBER so the answer must be something else...  You might need to ask Bill Gates!

 

Anyway, a kludgy method around the "no variables, no expressions" restriction is the use of the ON preposition.  When this is present, the selector is evaluated.  If the result is less than 0 (or greater than 255), then an ILLEGAL QUANTITY ERROR is generated.  Otherwise the selector's value (call it N) is used to select the N^th lineNumber in the list.  The default N is 1 (i.e., when no ON preposition exists).  Not all N possibilities need to be entered.  If N is greater than the number of lineNumbers following GOTO, then the GOTO is completely ignored and the statment following it (whether on the same line or the next line) will be executed instead.  If N is less/equal to the count of lineNumbers listed, then N^th one is used.  Not all N values need to have a corresponding lineNumber, but for any N that is actually choosen, its corresponding line number must exist in the program or an UNDEF'D STATEMENT ERROR occurs.  If the N^th lineNumber is "empty" (no number) and is selected, then instead a SYNTAX ERROR message will be generated (all other cases work as described above).  If the N^th lineNumber is "garbage" then everything will run fine when N is > 0 and N < "garbage" entry, but when N = 0 or N >= to the "garbage" entry, you will also get SYNTAX ERROR.

 

Sorry if that seems complex!  Now you know why I call it a kludge.  Some examples are below.  Also it should be noted that ON/GOTO is very similar (and more compact) than the SELECT/CASE of VisualBASIC or SWITCH/CASE of C.  Anyway, it is cleaner and faster than a long chain of IF/THEN/ELSE statements, which would otherwise be needed.

 

If GOTO (or GO TO) is executed in direct mode, the BASIC interpreter will change its internal state from from "direct mode" to "run mode".  While the concept may seem obvious, the details are not well-documented!  See this topic for details.  When this happens, BASIC updates a secret variable (sets it to 128).  It is secret because BASIC provides no way to read the value (except for machine-specific PEEK).

 

Before the examples, I would like to mention (in case it isn't obvious) that GOTO is weird in a few more ways.  For instance, the selector won't allow an Integer like most BASIC statements, only an unsigned byte.  Second the line number(s) is(are) not a normal integer either, but a restricted unsigned integer.  (Normal unsigned integers in BASIC are 0 ~ 65535, but then again, unsigned integers are not normal in BASIC anyway).  Ah, well that has more to due with the rules of line numbers in general than GOTO in particular.  But overall, you have to admit this simple programming concept is riddled with exceptions!

 

Example:

10 GOTO 10 : REM infinite loop, do not RUN!
NEW

READY.
10 INPUT "N"; N
20 ON N GOTO 100, , 300, 400
30 PRINT "INVALID" : GOTO 10
100 PRINT "ONE" : GOTO 10
300 PRINT "THREE" : GOTO 10
RUN
N? 3 (user input)
THREE
N? 5 (user input)
INVALID
N? 0 (user input)
INVALID
N? -1 (user input)
?ILLGAL QUANTITY ERROR IN 20

READY.
RUN
N? 1 (user input)
ONE
N? 2 (user input)
?SYNTAX ERROR IN 20

READY.
RUN
N? 3 (user input)
THREE
N? 4 (user input)
?UNDEF'D STATEMENT ERROR IN 20

READY.

 

I almost forgot!  If the newer GO TO syntax is used, only the 1^st lineNumber will ever be used.  This is because there it is no way to specify a selector (always defaults to one).

 

CHAR, COLLISION, CONT, GOSUB, LOCATE, MOVSPR, RECORD, RESTORE, RUN, TRAP

 

END, STOP

 

DO, EL, FOR, GO, IF, ON, TO