Why does the program output -nan (ind)?

Question

C++ / C#

Wrote a function to rotate a straight line.

// Line - y = _k * x + _b
void Line::rotation(double angle) {
    double temp = _k;
    _k = (_k + tan(angle))/(1 - _k*tan(angle));
    _b *= _k/temp;
}

double Line::rotationAngle(const Line &l) { // counterclock-wise from l to this
    return atan((_k - l._k)/(1. + _k*l._k));
}

When entering the following data

Line Line1(0, 0), Line2(2, 1);

    cout << Line1;
    cout << Line2;

    Line1.rotation(Line2.rotationAngle(Line1));

    cout << Line1;

For some reason it gives the following

y = 0x + 0
y = 2x + 1
y = 2x + -nan(ind)

The program calculated the coefficient of the rotated straight line correctly (_k = 2), but for some reason then it cannot divide 2 by 2:
_b *= _k/temp;
Although if temp is replaced by 2, then everything is ok. Will issue: It seems that there are no roots and divisions by zero. What is the reason? y = 2x + 0

Answer 1

It seems to be no roots and divisions by zero.

If the tangent is 0.5 with a factor of 2, then division by zero appears.