Subject:
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RE: Incorrect result
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Newsgroups:
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lugnet.robotics.rcx.legos
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Date:
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Wed, 30 Jan 2002 19:30:03 GMT
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Reply-To:
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<rhempel@bmts.#nomorespam#com>
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Viewed:
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2191 times
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<snipped code>
> I have tried replacing the '>> 7' with '/ 128', substituting a->magnitude
> and a->direction with int am and ad (and equating), and removing the (long)
> cast while changing rx to int. No matter what I do, 256 * 128 / 128 = -256
> (???). Tell me there is a fix and what it is.
Robert, this is a classic problem with ranges of singed numbers in C
rx = (long)(a->magnitude * TA_costable[a->direction]) >> 7;
I don't have an RCX in frot of me, but I think you need to cast a->magnitude
to long before the multiplication, otherwise you're just casting the int result
of the multiplication to long, which gets you no benefit.
256*128 is 32768, which is -1 in signed ints.
Try the following:
rx = ((long)a->magnitude * TA_costable[a->direction]) >> 7;
This should help your situation...and you could try this out
interactively if you programmed in pbForth :-)
Regards, Ralph Hempel
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Message has 1 Reply:
 | | Re: Incorrect result
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| (...) Yes, this is what happens when one is used to just making everything long and not considering overflows. Even worse was that I was doing the math on the computer's calculator, but not looking at the intermediate result, just typing in the (...) (23 years ago, 30-Jan-02, to lugnet.robotics.rcx.legos)
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Message is in Reply To:
| | Incorrect result
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| This problem, unlike the other, is definitely not a result of my eyes/brain being tired or something. I thoroughly displayed every value before and after evaluation and the result is ***WRONG***. Here is the code and the values: typedef struct { int (...) (23 years ago, 30-Jan-02, to lugnet.robotics.rcx.legos)
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