ON…GO SUB

Syntax

ON condition GO TO line1 *[,linei]* or

ON condition GO SUB line1 *[,linei]*

Location QL ROM

The QL supports a structure which enables the program to jump to specific lines depending upon the value of a variable. condition must be an integer expression which returns a value of one or more. After the command GO TO or GO SUB must appear a list of line numbers to jump to depending on the value of the condition. The value returned is then used to determine which of these line numbers will be jumped to, by counting the different options. If the result of the condition is zero, or more than the number of options, then the error ‘Out of Range’ will be returned. If ON…GO SUB is used, then when a RETurn is executed from within the sub-routine, the program will continue from the statement following ON..GO SUB.

Example

A procedure to print out the locations in an adventure might look like this:

100 no_of_locations=3
110 start=0
120 PRINT_LOC 2
130 DEFine PROCedure PRINT_LOC (xa)
140   IF xa=0 OR xa>no_of_locations THEN PRINT 'Undefined Location':RETurn
150   ON xa+start GO SUB 170,180,190
160   PRINT 'What now?':RETurn
170   PRINT 'This is location 1':RETurn
180   PRINT 'This is location 2':RETurn
190   PRINT 'This is location 3':RETurn
200 END DEFine

For a simpler (and clearer) way of writing this PROCedure, please refer to the example given for SELect ON.

NOTE

ON can also be used with the SELect ON structure - please refer to SELect ON for further details.

CROSS-REFERENCE

These two commands can generally be replaced either by a calculated GO SUB or GO TO statement, or the SELect ON structure.