First steps this month will concentrate on a problem which I know is
common to many of our readers. How to get VULTURES III running!
Whilst much of what we will say will be specific to Vultures many of
the principles will apply to all listings which are typed in from
magazines. The first hard lesson to learn - like it or not - is that 98%
(or so) of the time the reason that a program won't run is because YOU
have made typing mistakes! Don't get upset, I make typing errors so does
everyone I have come across - we all have to learn the hard way.
Although the listing of Vultures was not very clear (blame the Atari 822
printer which has now died completely) the program does run as listed.
Lesson number one is, when any program has machine language in it,
SAVE IT BEFORE YOU RUN IT. Any mistakes in entering data can lock up
your machine making the only means of recovery to switch off and start
again. Machine language is usually included in a Basic program as a
series of DATA statements so when you see blocks of DATA - beware! It is
probably a good idea to get into the habit of saving ALL Programs before
you run them and you don't have to wait until you have completed a
listing, you can save it at any time making it possible to put in a long
listing over several evenings. The biggest headache is to try and find
out where you are likely to have made your mistake - not easy! Common
errors in typing DATA statements include typing an 0 (letter) instead of
a 0 (figure), using a full stop instead of a comma or missing out commas
thus ending up with numbers greater than 255. Most of the problems in
Vultures stemmed from these type of errors in the big block of DATA
between lines 2000 and 2080. If you have still not got it running try
making the following changes which will check the DATA.
LINE
2000 delete :NEXT I
Add
line 2085 TOT=TOT+A:N.I:IF TOT<>39259 THEN ? "ERROR IN
DATA": TOT=0:STOP
If you get the error message, there is no getting away from it - you
must check every one of those DATA statements!
If you get ERROR 8 messages when reading in DATA then you have
characters or punctuation in the statements. if you are using a FOR..
NEXT loop to read in the data then you can pinpoint the area of the
error by asking the computer to print the value of the FOR ... NEXT
variable and the value of the last statement read. For example if you
get an ERROR 8 message in line 2000 of Vultures, the loop variable is I
and the variable for the DATA is A. Ask the computer to PRINT I,A. The
two figures you get are first the number of times the loop has executed
before coming to the error and secondly the value of the last CORRECT
data statement read. You should be able to count through the DATA
statements to the I'th number which should be wrong. The number before
should be your last correctly read item of data.
This idea of checking variables is very useful for tracing bugs. You
can always stop a program at any time by using the BREAK Key and then
ask the computer to PRINT the value of a variable or the content of
strings. If you can figure out whether a variable is behaving correctly
you should be able to narrow down the likely area of error.
If you still can't get Vultures going and you feel it is because you
may have read the bad (awful?) printing wrongly, send me a stamped
addressed envelope and I will send you off a full original listing.
Would anyone like to write this column for a future issue? It would
be interesting to get alternative views - perhaps a humorous story,
incorporating a few hints and tips of how you survived that nerve-wracking
first few months with your new Atari computer - Let me know........
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