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Getting a program typed in is often only the<br />
start of your problems. Ed to the rescue.<br />
Typing in a program is a useful<br />
exercise. Apart from the patience<br />
required, techniques<br />
learned and the end program to<br />
be used, probably the most<br />
educational part of it is tracking<br />
down the bugs introduced by<br />
yourself or occasionally by our<br />
publication system.<br />
In debugging you gain a<br />
much deeper insight and<br />
understanding on how the program<br />
actually works than by<br />
merely typing it in, but tracking<br />
down these errors is an art in<br />
itself and needs some skill. So<br />
here are some tips to help you in<br />
your efforts when faced with<br />
that cryptic error report!<br />
1 NEXT without FOR<br />
Look back through the program,<br />
either the loop has not been set<br />
up — no related FOR 'letter' =<br />
No1 TO No2 line, or the letter<br />
has been re-used as an ordinary<br />
variable within the loop with a<br />
LET 'letter' = No.<br />
2 Variable not found<br />
This is one of the most common<br />
errors. Again, the problem may<br />
not lie in the line where the error<br />
was detected and reported. If<br />
there is only one variable, which<br />
may be one or more letters or a<br />
string 10 THEN LET X= 10<br />
4 Out of memory<br />
As well as for programs which<br />
are too big, it may happen if the<br />
previous program set RAMtop,<br />
Before despairing, enter CLEAR<br />
USR"a"-1 on the Spectrum: on<br />
the ZX81 SAVE the program,<br />
turn the machine off and on,<br />
then reload the program.<br />
7 RETURN without GOSUB<br />
Somehow the computer has<br />
reached a RETURN command<br />
other than via a GOSUB instruction,<br />
Check a GOTO hasn't been<br />
entered in place of a GOSUB.<br />
Check for a missing GOSUB.<br />
B Integer out of range<br />
An integer {whole number)<br />
either as a number of variable is<br />
too big or small and you are attempting<br />
to do something like<br />
PRINT AT 0,33 not allowed!<br />
Check any variables involved as<br />
per report 2 and trace it back<br />
through the program looking for<br />
adjustments to it by + - * / :<br />
Add limiting code if needed —<br />
see report 3<br />
E Out of DATA<br />
A Spectrum problem. Check the<br />
number of DATA items match<br />
the number of READs; usually<br />
one (or more) has been missed<br />
out. Attempting to reread a<br />
DATA list without first using a<br />
RESTORE command will cause<br />
this and it can happen on an auto<br />
start program (saved with a LINE<br />
number). Good programming<br />
usually RESTORES to the correct<br />
line number before using<br />
READ.<br />
I FOR without NEXT<br />
See report 1 but this time the<br />
NEXT is missing!<br />
Note that the letters I have used<br />
for examples could by ANY letters<br />
not just A$,X,Y etc and depend<br />
on the particular choice of<br />
the programmer.<br />
This is by no means a comprehensive<br />
list but I have tried to<br />
cover many of the most common<br />
error reports. Personally, I<br />
get almost as much satisfaction<br />
from debugging as I do from programming<br />
I do assure you,<br />
however, that there is absolutely<br />
no truth in the rumour that we<br />
deliberately inject bugs into our<br />
listings in order to introduce you<br />
to the dubious delights of<br />
debugging!<br />
123<br />
'
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