ON

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BASIC keyword
Keyword: ON
Abbreviation:
Type: Command
Token code: 145/$91
Handling routine
in BASIC ROM:
43339–43370
$A94B–A96A
List of all BASIC keywords


Remark: This article describes the BASIC-keyword ON in BASIC V2 at the Commodore 64.

Typ: Command
General Programming-Syntax: ON <index> GOSUB|GOTO <line>,<line>,...

In Commodore BASIC V2, the command ON is part of a structure which jumps to a specific line in the given list of BASIC <line> numbers, either as an unconditional GOTO or as a subroutine call through GOSUB. If <index> (which must be either a floating point or integer expression) equals 1, the first <line> on the list is taken, if <index> equates to 2, the second line number is taken.

If the numerical <index> expression evaluates to a non-integer result, ON will round it down to nearest lower integer, as if the INT function had been used on the result; e.g. an <index> of 2.2 resolves to 2, and so the second line number on the list is taken.

If the <index> expression, after eventual rounding, exceeds the count of line numbers given, ON does not GOTO or GOSUB anywhere, and program execution continues with the subsequent command.

Attempting to use a string expression as the <index> causes the ?TYPE MISMATCH ERROR in line error message to appear. A negative <index> yields an ?OVERFLOW ERROR IN line. As with "normal" GOTO and GOSUB statements, referring to a non-existent line number causes an ?UNDEF'D STATEMENT ERROR in line as soon as ON tries to call it. Other calculation failures may raise other error messages, e.g. ?DIVISION BY ZERO ERROR or ?OVERFLOW ERROR.

Important: Two-letter keywords like ON "blocks" the use of "on" as the name of a BASIC variable; you cannot create e.g. a string variable named ON$; any reference to it in the BASIC code will cause a ?SYNTAX  ERROR IN line.


Starting with BASIC 3.5 the keywort ON can be also used in disk oriented BASIC commands to specify the device number (in addition to or instead a comma which separates parameters).

Examples[edit]

ON ... GOTO example[edit]

10   INPUT "Write an integer number, please "; A
20   PRINT: ON A GOTO 1000,2000,3000
999  END
1000 PRINT "1. jump"
1001 A=A+1: GOTO 20
2000 PRINT "2. jump"
2001 A=A+1: GOTO 20
3000 PRINT "3. jump"
3001 A=A+1: GOTO 20

ON ... GOSUB example[edit]

10   INPUT "Write a integer number, please"; A
20   ON A GOSUB 1000,2000,3000: PRINT
999  END
1000 PRINT "1. Subroutine"
1001 A=A+1: RETURN
2000 PRINT "2. Subroutine"
2001 A=A+1: RETURN
3000 PRINT "3. Subroutine"
3001 A=A+1: RETURN

ON ... GOSUB with calculated input[edit]

20 ON A/4 GOSUB  1000,2000,3000

The number variables can be used directly for easy calculations.

An equivalent to IF ... GOTO[edit]

10 IF A > 0 GOTO 99

corresponds to

10 ON -(A > 0) GOTO 99 

but with the difference that the IF version ignores everything after the GOTO until the line ends. In contrast, the ON version continues right after a given colon (":") in case the GOTO is no taken.

Usage in disk oriented commands[edit]

DIRECTORY ON U9
BASIC V2.0 (second release) Commands

ABS | AND | ASC | ATN | CHR$ | CLOSE | CLR | CMD | CONT | COS | DATA | DEF | DIM | END | EXP | FN | FOR | FRE | GET | GET# | GOSUB | GOTO | IF | INPUT | INPUT# | INT | LEFT$ | LEN | LET | LIST | LOAD | LOG | MID$ | NEW | NEXT | NOT | ON | OPEN | OR | PEEK | π  |POKE | POS | PRINT | PRINT# | READ | REM | RESTORE | RETURN | RIGHT$ | RND | RUN | SAVE | SGN | SIN | SPC | SQR | STATUS/ST | STEP | STOP | STR$ | SYS | TAB | TAN | THEN | TIME/TI | TIME$/TI$ | TO | USR | VAL | VERIFY | WAIT