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Contents / Structure within BASIC / Previous chapter / Next chapter / Index
There is often a need, in a computer program, to proceed in one of a number of directions. For example your program might present a "menu" of 8 options for the user to choose from. When the user has made the choice your program will need to branch off in the appropriate direction. There are a number of ways of doing this. Here is one in part of a program
100 MODE 7
110 PROCINTRO
120 REPEAT
130 PROCMENU
140 IF M=1 THEN PROCOscar7
150 IF M=2 THEN PROCOscar8
160 IF M=3 THEN PROCUOSAT
170 IF M=4 THEN PROCorbit
180 IF M=5 THEN PROCtransmit
190 IF M=6 THEN PROCshowfigs
200 IF M=7 THEN PROCMercator
210 IF M=8 THEN PROCLocator
220 IF M=9 THEN PROCgetdatetime
230 UNTIL M=-1
240 END
Lines 140 to 220 provide exits to a number of procedures all of which will automatically return to the main program. Which procedure is selected depends on the value of M as selected by the user during the procedure PROCMENU.
The above method is easy to understand and is recommended but there are other methods which have their place. The statement ON...GOTO also provides a number of exits.
100 ON M GOTO 1000,1200,1250,1600
would provide an exit to line 1000 of the BASIC program if M =
1. If M=2 then control will pass to line 1200 and so on.
An alternative format is
100 ON M GOSUB 1000,1200,1350
In this case control is passed to the subroutines indicated and then return to the next line.
Both these techniques are widely used but are less clear than the use of procedures as indicated at the beginning of this section.