Question

Has anyone seen this weird value while handling sin / cos/ tan / acos.. math stuff?

===THE WEIRD VALUE===

-1.#IND00

=====================

void inverse_pos(double x, double y, double& theta_one, double& theta_two)
{
    // Assume that L1 = 350  and L2 = 250


    double B = sqrt(x*x + y*y);
    double angle_beta = atan2(y, x);
    double angle_alpha = acos((L2*L2 - B*B - L1*L1) / (-2*B*L1));
    theta_one = angle_beta + angle_alpha;
    theta_two = atan2((y-L1*sin(theta_one)), (x-L1*cos(theta_one)));
}

This is the code I was working on.

In a particular condition - like when x & y are 10 & 10, this code stores -1.#IND00 into theta_one & theta_two.

It doesn t look like either characters or numbers :(

Without a doubt, atan2 / acos / stuff are the problems.

But the problem is, try and catch doesn t work either cuz those double variables have successfully stored some values in them.

Moreover, the following calculations never complain about it and never break the program!

I m thinking of forcing to use this value somehow and make the entire program crash... So that I can catch this error..

Except for that idea, I have no idea how I should check whether these theta_one and theta_two variables have stored this crazy values.

Any good ideas?

Thank you in advance..

Answer 1

The "weird value" is NaN (not a number).

The problem is because (L2*L2 - B*B - L1*L1) / (-2*B*L1) = 6.08112… is outside of the range [-1, 1] where acos is well-defined for real numbers. So NaN is returned.

Are you sure the formula is correct?

If you want to catch an NaN, the NaN needs to be a signaling NaN. For gcc, compile with the -fsignaling-nans option.

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