Part 5
Subroutines

Subjects covered...

	GO SUB, RETURN

Sometimes, different parts of the program will have rather similar
jobs to do, and you will find yourself typing in the same lines two or
more times; however, this is not necessary. Instead, you need only
type in the lines once (in what's called a subroutine) and then call
the subroutine into action whenever you need it in the program.

To do this, you use the statements GO SUB (go to subroutine) and
RETURN. This takes the form...

	GO SUB xxx

...where 'xxx' is the line number of the first line in the subroutine.
It is just like 'GO TO xxx' except that the +3 remembers where the GO
SUB statement was, so that it can come back again after carrying out
the subroutine.

(In case you're interested, the +3 does this by remembering at which
point in the program the GO SUB command was issued (in other words
where it should continue from afterwards) and storing this return
address on top of a pile called the GO SUB stack.)

When the command...

	RETURN

...is met (at the end of the subroutine itself), the +3 takes the top
return address off the GO SUB stack, and continues from the next
statement.

As an example, let's look at the number guessing program again. Retype
it as follows...

	 10 REM "A rearranged guessing game"
	 20 INPUT "Enter a secret number",a: CLS
	 30 INPUT "Guess the number",b
	 40 IF b=a THEN PRINT "Correct": STOP
	 50 IF b<a THEN GO SUB 100
	 60 IF b>a THEN GO SUB 100
	 70 GO TO 30
	100 PRINT "Try again"
	110 RETURN

The GO TO 30 statement in line 70 (and the STOP statement in line 60
of the next program) are very important because otherwise the programs
will run on into their subroutines and cause an error ('7 RETURN
without GO SUB') when the RETURN statement is reached.

The following program uses a subroutine (from line 100 to 150) which
prints a 'times table' corresponding to the value of parameter 'n'.
The command 'GO SUB 100' may be issued from any point in the program
to call the subroutine. When the RETURN command in line 150 of the
subroutine is reached, control returns to the main program, which
continues running from the statement after the GO SUB call. Like GO
TO, GO SUB may be typed in as GOSUB.

	 10 REM times tables for 2, 5, 10 and 11
	 20 LET n=2: GO SUB 100
	 30 LET n=5: GO SUB 100
	 40 LET n=10: GO SUB 100
	 50 LET n=11: GO SUB 100
	 60 STOP
	 70 REM end of main program, start of subroutine
	100 PRINT n;" times table"
	110 FOR t=1 TO 9
	120 PRINT t;" x ";n;" = ";t*b
	130 NEXT t
	140 PRINT
	150 RETURN

One subroutine can happily call another, or even itself (a subroutine
that calls itself is known as recursive).