C: How to wrap a float to the interval [-pi, pi)

Question

I\'m looking for some nice C code that will accomplish effectively:

while (deltaPhase >= M_PI) deltaPhase -= M_TWOPI;
while (deltaPhase < -M_PI) deltaP         


        

Answer 1

Instead of working in radians, use angles scaled by 1/(2π) and use modf, floor etc. Convert back to radians to use library functions.

This also has the effect that rotating ten thousand and a half revolutions is the same as rotating half then ten thousand revolutions, which is not guaranteed if your angles are in radians, as you have an exact representation in the floating point value rather than summing approximate representations:

#include 
#include 

float wrap_rads ( float r )
{
    while ( r > M_PI ) {
        r -= 2 * M_PI;
    }

    while ( r <= -M_PI ) {
        r += 2 * M_PI;
    }

    return r;
}
float wrap_grads ( float r )
{
    float i;
    r = modff ( r, &i );

    if ( r > 0.5 ) r -= 1;
    if ( r <= -0.5 ) r += 1;

    return r;
}

int main ()
{
    for (int rotations = 1; rotations < 100000; rotations *= 10 ) {
    {
        float pi = ( float ) M_PI;
        float two_pi = 2 * pi;

        float a = pi;
        a += rotations * two_pi;

        std::cout << rotations << " and a half rotations in radians " << a << " => " << wrap_rads ( a ) / two_pi << '\n' ;
    }
    {
        float pi = ( float ) 0.5;
        float two_pi = 2 * pi;

        float a = pi;
        a += rotations * two_pi;

        std::cout << rotations << " and a half rotations in grads " << a << " => " << wrap_grads ( a ) / two_pi << '\n' ;
    }
    std::cout << '\n';
}}