remainder function

[Ether]

Does the C++ compiler used for FRC support the REMAINDER operator "x REM y" per IEC 60559

as specified on Page 235 Section 7.12.10.2 of ISO/IEC 9899:TC3 ?



If someone would be willing to test that with y=360

and x values of 179, 180, 181, 359, 721, -1, -179, -180, -359, -361, -721

and post results it would be much appreciated.

[sur]

It seems to be defined if you use the C compiler, but not the C++ one. You could use fmod, which has slightly different behavior.

You can test it out here: http://codepad.org/86MlmNXU

[Ether]

fmod(x,y) is quite different from remainder(x,y)

The 3 columns below are x, remainder(x,360), and fmod(360):

 180.000     -180.000      180.000
 181.000     -179.000      181.000
-181.000      179.000     -181.000
 359.000       -1.000      359.000
-359.000        1.000     -359.000
 719.000       -1.000      359.000
-719.000        1.000     -359.000

[nightpool]

What's the issue with using the modulus operator instead of the built-in remainder function?

[Ether]

Originally Posted by nightpool

What's the issue with using the modulus operator instead of the built-in remainder function?

Are you saying there is a remainder function in the C++ compiler used for FRC?

[sur]

It looks like you need to define it using extern "C" for WindRiver to compile it correctly:

#include <cmath>
using namespace std;

extern "C" double remainder(double, double);

[Ether]

Originally Posted by sur

It looks like you need to define it using extern "C" for WindRiver to compile it correctly:

#include <cmath>
using namespace std;

extern "C" double remainder(double, double);

Would you mind running that and see if it produces the same output as this:

double myRemainder(double x, double y){
return x-y*floor(0.5+x/y);
}

[Racer26]

What about:

double EthersRemainder( double x, double y ){

int wholenumber;

wholenumber = (int)(x/y);

double result;

result = (x - (wholenumber * y));
if(result > 0.5*y) {
result -= y;
}

return result;

}

[Ether]

Originally Posted by 1075guy

What about:
EthersRemainder

That's not my remainder.

Did you compile and run it ? It doesn't work correctly.

I'm sure that there's a way, with enough conditional logic, to torture fmod() or % into into a remainder* function, but what I am really asking in this thread is

a) does the C++ compiler provided with FRC have the remainder* function, and

b) does it behave like this:


*This* is my remainder:

double myRemainder(double x, double y){
  return x-y*floor(0.5+x/y);
}

*per IEC 60559 as specified on Page 235 Section 7.12.10.2 of ISO/IEC 9899:TC3

[Racer26]

If you have a function that works, then use it?

I don't understand your qualm?

I have not ever encountered a programming scenario where modulus division didn't solve my desire for a remainder, so I'm a bit lost.

If there doesn't exist an operator that does what you want, then you need to either a) fabricate a solution from the operators you have access to, or b) find a library that does that for you.

[Ether]

Originally Posted by 1075guy

If you have a function that works, then use it?

I do use it. But I don't want to be telling other people to use my custom function if there's already a perfectly good remainder function provided with FRC C++. I don't have access to a working C++ installation, otherwise I would test it myself. I am seeking help from knowledgeable C++ users on this forum.

I don't understand your qualm?

Honestly, that's because I don't have a qualm. I am perfectly calm.

I have not ever encountered a programming scenario where modulus division didn't solve my desire for a remainder, so I'm a bit lost.

I have encounted at least one such scenario, and that's why I posted this thread.

For example, the IEC60559 remainder function can take any angle and normalize it to the range -180..+180 with a single C instruction:

normalizedAngle = remainder(angle,360);

I'd like to know if the remainder function is supported in FRC C++ so I can recommend it to students who are trying to do this type of conversion.

Neither the fmod() function nor the % operator can replicate the functionality of the remainder() function without using a bunch of conditional logic

If there are any Java programmers following this thread, Java appears to have the IEC 60559 remainder function; in Java it is called IEEERemainder. See attachments. If someone would be willing to test that (compare it to the implementation I posted ealier in this thread) I'd be grateful.



qualm
/kwä(l)m/
Noun

1) An uneasy feeling of doubt, worry, or fear; a misgiving.
2) A momentary faint or sick feeling.

Synonyms
nausea - queasiness

[Racer26]

Originally Posted by Ether

Are you saying there is a remainder function in the C++ compiler used for FRC?

There's definitely a modulus division operator, which amounts to the same thing.

719 % 360 = 359

[nightpool]

Originally Posted by 1075guy

There's definitely a modulus division operator, which amounts to the same thing.

719 % 360 = 359

Right, but it rounds the same as fmod() (see http://www.gnu.org/software/libc/man...Functions.html). I don't have enough experience to say when this would be more or less useful in practice, but if you DO need it extern "C" seems to be the way to go about it.

[Ether]

Originally Posted by 1075guy

There's definitely a modulus division operator, which amounts to the same thing.

719 % 360 = 359

The modulus division operator is *not* the same thing as the remainder function.

remainder(719,360) is -1, not 359.

Look at the definition of the remainder function which I attached to the original post in this thread.

[Racer26]

OK, so do this instead:

output = (719 % 360 > 180) ? (-360 + (719 % 360)) : (719 % 360);