Converting radians to degrees in a reference-circle manner

Question

Context: I'm creating a map-replanner for a robot in C, it should add obstacles within the environment to the map depending on which IR-sensors are getting high readings.

For this i need to check how the robot is positioned IRL vs in the internal map (pointing north). But for some reason, i sometimes get negative values in the conversion. Any ideas what i'm doing wrong?

There should never be any negative values. The allowed values should lie between 0-360. As of now, i get negative values sometimes.

#define     PI   3.14159265358979323846264338327950
#define     DEG(x)   (x*57.2957795130823208767981548141)

Code:

float currentRot  = now.th; //current rotation-reading in RAD from function.
if(currentRot > PI){        //normalize values       
   currentRot     = currentRot - 2 * PI;
}
if(currentRot < -PI){
   currentRot     = currentRot + 2 * PI;
}
currentRot        = fmod(currentRot,PI*2); //convert into RAD-modular
currentRot        = DEG(currentRot);       //convert to degrees

Any ideas of what i'm doing wrong?

Answer 1

π cannot be exactly represented as a float, so any mod-ding with fmod(currentRot,PI*2); or comparing if(currentRot > PI){ may fail to provide the expected results for edge cases.

Note that currentRot = currentRot + 2 * PI; is a (foat) + (double) and the conversion to float. The edges of this conversion are problem to avoid slight negative results. Best to avoid mixing float and double math for this sensitive conversion.

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Even with the good comment of @M Oehm, the inexactness PI and mixing float/double math can result in negative currentRot.

Instead, convert radians to degrees first and then mod.

float RadianToDegree(float r);
  float d = (float)(PI/180.0) * r;
  d = fmodf(d, 360.0f);
  if (d < 0.0f) d += 360.0f;
  return d;
}

The result will be [0.0 - 360.0] inclusive.

Mod-ding by 360 can be expected to incur to no round-off error.

Use fmodf() with float and fmod() with double to avoid unnecessary conversions.

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On a higher level note, the goal of "The allowed values should lie between 0-360" sacrifice precision. Consider a primary range of between -180 - +180.