Posted By
SVS
on 2006-06-26
05:05:44
 |  The Plus4 programmer Summer quiz!
Hello, this is the quiz:
10 Q=(255-6)/7
20 PRINT "FORMULA RESULT=";6+Q*7
30 IF 255<>6+Q*7 THEN PRINT "IS NOT";: ELSE PRINT "IS";
40 PRINT " EQUAL TO 255"
What is the result? ...and why?
Are you sure to know it?
S\_/S (hehehe...)
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Posted By
Csabo
on 2006-06-26
09:01:08
| Re: The Plus4 programmer Summer quiz!
More of the same weird BASIC rounding error? It doesn't look like it's new, I recall us discussing this error before here on the forum. ?10=(10/3*3) also prints false, even though ?10/3*3 prints 10.
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Posted By
SVS
on 2006-06-26
09:31:08
| Re: The Plus4 programmer Summer quiz!
Csabo, this time there is something more... even odder,
for this reason I've written "are you sure to know it?"
I wait for other answers before to publish my discovery 
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Posted By
JamesC
on 2006-06-26
22:35:38
| Re: The Plus4 programmer Summer quiz!
I think someone is full of themselves from the World Cup or something..... kicking poor Q around, makes him feel like a spotted ball instead of a proper variable!
It is simple rounding error in the BASIC PRINT evaluation routine. The fraction of the number over 255 is too small to worry about, so the computer forgets it when PRINTING
the expression 6+Q*7. However the actual variable Q holds the value 35.5714826, which becomes 249.0000002 when multiplied by 7. Add the 6, and you have 255.0000002, which is definately not 255 at the point where the computer evaluates the <>.
But then again, I used the Microsoft calculator gadget in Vista Beta 2 to get those 7 places after the decimal, so there could be a problem in Microsoft code, as well.
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Posted By
SVS
on 2006-06-27
03:01:55
| Re: The Plus4 programmer Summer quiz!
Are you all sure? ))
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Posted By
Csabo
on 2006-06-27
09:42:18
| Re: The Plus4 programmer Summer quiz!
I'm sure. The -6 part does nothing, 249/7*7 is enough to cause this issue, so we can remove that from the equasion. I briefly tried to see if 255 was in some way special, but it's not.
? INT ( 10 / 3 * 3 ) prints 9, and ? ( 249 / 7 * 7 ) prints 248. It's the same round error we've already known about. Of course I could be wrong. If you have anything new you'd like to share, just go ahead and let us know.
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Posted By
SVS
on 2006-06-28
02:46:15
| Re: The Plus4 programmer Summer quiz!
The "nice" solution is:
10 Q=(255-6)/7
20 PRINT "FORMULA RESULT=";6+Q*7
30 IF 6+Q*7<>255 THEN PRINT "IS NOT";: ELSE PRINT "IS";
40 PRINT " EQUAL TO 255"
I've only inverted the arguments of equation in line 30. Even if it seems incredible, in this way the result is correct.
Any idea about the reason?
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Posted By
siz
on 2006-06-28
03:22:09
| Re: The Plus4 programmer Summer quiz!
I've tried this version before, but I could not find the reason why it works one way and not the other way. :O
Probably a bug in IF?
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Posted By
Csabo
on 2006-06-29
12:01:53
| Re: The Plus4 programmer Summer quiz!
Nice find! It's really weird. The -6 is still not needed, but indeed, if the "constant" number is on the right side of the equals sign, it works.
? 10 = 10/3*3, 10/3*3 = 10
The first one is false, the second is true. The bug is in the expression evaluation.
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Posted By
SVS
on 2006-06-30
09:51:43
| Re: The Plus4 programmer Summer quiz!
To be complete, I faced the problem during BalSys code updating.
With a massive operations and different numbers, even the second case *sometime* produces a wrong result. The first case always produces wrong results.
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Posted By
Rachy
on 2006-07-03
05:26:43
| Re: The Plus4 programmer Summer quiz!
SVS: and do you know the reason of this? Probably operator precedence problem?
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Posted By
SVS
on 2006-07-03
08:11:53
| Re: The Plus4 programmer Summer quiz!
It's probable, maybe system "pipe" the data during expression evaluating. This piping could insert a further rounding.
Another interesting thing is that internal values uses 1 cipher more than the output values, then it is impossible to view (print) the complete value of any variable/calculus.
Anyway I did not debug that code, then I'm not sure.
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