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ON ... GOSUB

Syntax

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ON <expression> GOSUB <label0>, <label1>, <label2>, ..., <labelN>
ON <expression> GO SUB <label0>, <label1>, <label2>, ..., <labelN>

Transfers control to one of a series of specified line numbers or labels based on the value of an expression. This statement expects a RETURN statement in the subroutine to return control to the line following the ON ... GOSUB statement.

This statement uses a jump table for faster execution.

Parameters

  • <expression>: An integer expression that determines which label to jump to. It can be any numeric expression.
  • <label0>, <label1>, <label2>, ..., <labelN>: A comma-separated list of line numbers or labels. These are the destinations for control transfer based on the value of <expression>.

The value of <expression> is converted to a uByte (Unsigned Byte), so, for example, a value of 256 will be converted to 0. If the <expression> value is greater than the number of labels, this instruction will be ignored and the execution will continue normally.

Example with ON ... GOSUB

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 5 REM Random value between [0..3] both included
10 LET X = INT(RND * 4): LET ok = 0
20 ON X GOSUB 50, 100, 150: IF ok THEN END
30 PRINT "Invalid choice: "; X
40 GOTO 10
50 PRINT "You chose option 0": LET ok = 1
60 RETURN
100 PRINT "You chose option 1": LET ok = 1
110 RETURN
150 PRINT "You chose option 2": LET ok = 1
160 RETURN

In this example: * If X is 0, the execution will jump to line 50. * If X is 1, it will jump to line 100. * If X is 2, it will jump to line 150.

In each block, after executing the corresponding print statement, the program sets the ok variable to 1, indicating a valid choice. Then, the RETURN statement returns control to the line following the ON ... GOSUB statement.

Remarks

  • The <expression> is evaluated, and if it results in a value outside the range 0 to N (where N is the number of labels), no action is taken.
  • ON ... GOSUB is a structured programming alternative to multiple IF...THEN...GOSUB statements.

Compatibility

  • This statement is not compatible with Sinclair BASIC.

See also