My Function Doesn't Work

[zb0S216C]

I'm working with vectors (not the STL vector), and I'm trying to get the angle between two vectors. I've created a function that returns the result of a dot operation (·) (Which is required). Strangely enough, when I call std::acos( ), I get the IND output (I believe that it's a result of diving by zero). Here's my function:

float GetDotProduct( VECTOR &A, VECTOR &B )
{
    return( ( A.X * B.X ) + ( A.Y * B.Y ) );
}

VECTOR is a simple structure. X and Y are members of VECTOR that're of type float.

Here're the vectors I've used:

VECTOR VectorA;
VectorA.X = 3.0f;
VectorA.Y = 1.5f;

VECTOR VectorB;
VectorB.X = 1.5f;
VectorB.Y = 3.0f;

Here's the function call to std::acos( ):

std::cout << std::acos( GetDotProduct( VectorA, VectorB ) ) << std::endl;

Any ideas?

Additional Information:
- I've tried normalizing the vectors but it gives me the same result.

_Wazzak

[zwA4jE8b]

std::acos() only takes values between 1 and -1

or so it seems.

[zb0S216C]

Thanks for the reply, CreativeMFS. That explains why I get the IND result. One more thing, though: When I normalize my vector, my vector doesn't equal the value of 1. All of the websites say that the vector should equal to one after normalization. Any thoughts on this?

_Wazzak

[webJose]

To normalize, you basically divide each dimension (or coordinate or whatever you call it) by the vector's magnitude. If that doesn't yield a normalized vector, remember that PC's have rounding issues. Personally, I would work with doubles by default.

[zb0S216C]

Thanks for that, WebJose. I'll look it up :)

Thanks again.

_Wazzak

[rambo1177]

this is only the dot product... the angle is something like this
cos(∂)=A.B/|A||B|...
A.B is dot product....
|A||B| is the product of their magnitudes.....

[zb0S216C]

Rambo, your equation doesn't work. I broken down your equation (so C++ can understand it) and I was left with this:

Angle( GetDotProduct( VectorA, VectorB ) / ( GetVectorMagnitude( VectorA ) * GetVectorMagnitude( VectorB ) ) );

The output was (without normalization): 0.97308
The output was (with normalization): 0.993353

Rambo, if possible, can you convert your computation into C++ code? That way, I'll have better understanding.

_Wazzak