Appendix A - Line Numbers and the Dreaded GOTO
Most seasoned programmers will throw up their hands at the mention of
line numbers and GOTO. They are not considered polite conversation and
ladies should be asked to leave the room before being discussed. There
are an equal number of programmers who swear by them, rather than at
them, insisting that with proper use they are valid. The truth is
probably somewhere between the two. Don't e-mail me on this as it is a
point over which many kilobytes have been exchanged without convincing anyone to change sides. At some point you
will come across them in other people's programs so it would be remiss
not to mention them.
Line Numbers
Traditional BASICs, of which this is one (and that's something to be
proud of), always had line numbers. Though their use has fallen
somewhat by the wayside, in BBC BASIC they are still useful. If you make a
mistake on a line, when the program is run Basic will present you with
an error message specifying the line on which it found the mistake.
BBC BASIC allows lines in the range 0 - 65535. It is traditional to start
with 10 and go up in increments of 10.
Lines must be in numeric order
and no duplicates are allowed. If you need to insert another line, make
sure it falls between the two numbers before and after or BBC BASIC will
complain. If you need to insert lots of lines, BBC BASIC allows you to enter
the lines without numbers and will renumber them afterwards if you ask
it nicely. To demonstrate this, enter the following revolutionary piece
of code:
| REM Line number demo |
|
PRINT "Hello, world" |
|
WAIT
100 |
|
PRINT "That's all folks!" |
|
END
|
Now, from the toolbar select Renumber. A dialogue appears: accept the
default settings which start at 10 and go up in 10s and press OK. The
listing is updated. For a bigger program, this is a lot easier than
renumbering by hand. Run the program to prove it works. It is also
possible to remove line numbers. To do this, invoke the renumber box
again and tick 'Remove unused line numbers'. 'Unused ' will be
qualified in the next section on GOTO, but it is possible to get a
program to leap around within itself during execution. The target of
these jumps can be a line number, so if a line jumps to a line number,
this number is classed as used and will not be removed.
GOTO
GOTO is a command that is used in conjunction with a line number. When
the program executes, a GOTO statement will cause an immediate jump to
the line number specified and execution will continue from there. For
example, in our well worn circle program, we can make it loop endlessly
by inserting line 50.
| 10 REM Area of a circle |
|
20 INPUT "Enter the radius " Radius |
|
30 Area=PI*Radius^2 |
|
40 PRINT "Area of the circle is "
;Area |
|
50 GOTO 20 |
|
60 END
|
GOTO can be used as a quick and dirty way to skip a block of code when
testing:
| 400 REM Stuff |
|
410 GOTO 500 |
|
420 REM Help screens you've already tested |
|
430 REM ... |
|
440 REM ... |
|
450 REM ... |
|
460 REM ... |
|
470 REM ... |
|
480 REM ... |
|
490 REM ... |
|
500 REM Interesting stuff starts
here |
GOTOs can be made conditional:
| 10 REM Area
of a circle |
|
20 INPUT "Enter
the radius " Radius |
|
30 Area=PI*Radius^2 |
|
40 PRINT "Area of the circle is "
;Area |
|
50 INPUT "Another go (Y/N)"; Reply$ |
|
60 IF Reply$="Y"
OR
Reply$="y" THEN
GOTO 20 |
|
70 END |
And in single line IFs (don't ever jump out of or into a multi-line
block with GOTO) you can miss the THEN or GOTO out completely:
| 50 IF
TestResult>50 THEN 1000 |
Or:
| 50 IF
TestResult>50 GOTO 1000 |
But never:
Overindulgence can lead to what is commonly known as spaghetti code,
which is why people hate it so much. Don't bother to type this in but
see if you can follow it:
| 10 GOTO 60 |
|
20 PRINT "Help!" |
|
30 PRINT " lines 20 and 50 "; |
|
40 GOTO 80 |
|
50 PRINT "I'm stranded!" |
|
60 PRINT "Why are"; |
|
70 GOTO 30 |
|
80 PRINT "never executed?" |
|
90 END |
| Tip: Not every line needs a number |
You
don't have to have a line number on each line, only the ones that
are a target for a GOTO or similar instruction. The following is
perfectly legal:
| REM
Area of a circle |
|
20 INPUT "Enter
the radius " Radius |
|
Area=PI*Radius^2 |
|
PRINT "Area of the circle is "
;Area |
|
GOTO 20 |
|
END |
This techinque can be used when converting old programs until you have
the structure just right and can eliminate the GOTOs completely. When
you select 'Remove used line numbers' from the renumber dialog, this
would be the result.
|
The very, very early (we're talking late 60s or early 70s) BASIC programmers
had to use GOTO because there wasn't a choice. BBC BASIC has many options to
help with conditional execution and repeating blocks of code. It's
largely not needed, but, as previously stated, you will see code with
it in. If you do use GOTO:
a) Keep jumps short, preferably on one page so you can see the
destination.
b) Don't jump in and out of blocks of code like PROCs, loops or IFs
(can't emphasize this enough), you completely mess BBC BASIC's internal
stacks up when you do this.
c) Never admit to it in public, you'll cause a riot.
© Peter Nairn 2006
