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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| C math question |
 Posted: Fri Jul 16, 2004 1:26 pm |
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Given this code
| Code: | Signed int T,dT,Num;
Num = -123;
T = Num/10;
dT = Num%10;
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Will this properly give integer and decimal parts of Num/10 for both positive and negative numbers? I have done this many times, but never had to worry about negatives. K&R says the result of % on negatives is machine dependent. How do people usually code this?
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The search for better is endless. Instead simply find very good and get the job done. |
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newguy
Joined: 24 Jun 2004
Posts: 1908
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| signed math |
| Posted: Fri Jul 16, 2004 2:28 pm |
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Ooh, is your question ever timely!
I just finished doing something similar, and I had some unexpected results until I figured out how the compiler was evaluating things.
I had to figure out the travel of a mechanical part, given a 10 bit adc reading from an electronic height sensor. The sensor's output is in the range 0 - 5V, and I had to take into account two possibilities: that 0V (0x0000) = 0 travel, and 5V (0x03ff) = full travel, and vice-versa: 0V (0x0000) = full travel, and 5V (0x03ff) = 0 travel. This was to take into account the height sensor being installed backwards, which is a possibility. Also, in the real world, the sensor's output not be the full 5V, which is why I coded things the way I did.
How I ended up doing it is as follows:
| Code: | signed int16 min; // 0 travel value
signed int16 diff; // full travel value - 0 travel value
signed int16 step;
signed int16 temp; // used for math later
int16 adc_reading; // no explanation necessary
int full_travel; // full travel of the part (inches)
int travel; // actual position of part - range 0 - full_travel
int1 ascending = FALSE;
if (diff < 0) { // if diff is negative, then 5V = 0 travel, 0V = full travel
ascending = FALSE;
}
else {
ascending = TRUE;
}
step = diff/full_travel; // this is the value corresponding to 1 inch of travel (one step)
temp = adc_reading - min;
travel = temp/step; // this ends up being a pretty good approximation of the part's position in inches, truncated to the nearest inch
// now to take care of rounding - if the remainder > step/2, round up travel
if (ascending && ((temp % step) > (step / 2))) { // remainder positive
travel++;
}
else if (!ascending && ((temp % step) < (step / 2))) { // remainder negative
travel++;
} |
Originally I simply checked to see if (temp % step) > (step/2), without the ascending flag. It turns out that if things are backwards (5V = 0 travel, 0V = full travel), then the remainder of temp % step and step are both negative. However, if the remainder is, say -1, that's 0xffff, and if step is -2, that's what, 0xfffe? Since 0xffff > 0xfffe, the if evaluates to true, and travel would be incremented, even though I (that's the important part) didn't think that it should be.
Hope this helps. |
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Guest
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| Posted: Sat Jul 17, 2004 1:21 am |
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To be sure about the modulo's result I use something like this:
signed int unsig;
unsig = Num;
if (unsig < 0) unsig = -unsig;
dT = unsig % 10;
This gives in you example (-123) -12 for T and 3 for dT. |
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Sherpa Doug
Guest
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| Posted: Sat Jul 17, 2004 1:43 pm |
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| Anonymous wrote: | To be sure about the modulo's result I use something like this:
signed int unsig;
unsig = Num;
if (unsig < 0) unsig = -unsig;
dT = unsig % 10;
This gives in you example (-123) -12 for T and 3 for dT. |
That is the simple answer I was hoping for. Rathter obvious in retrospect, as many good answers are. Hopefully I can try it Monday if my boss has no new crises for me. |
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