nbsrujan - 8 months ago 78

C++ Question

I am using 3D vector as key in C++ map containers.For which I have to implement comparison of two vectors. I used magnitude to compare vectors. But problem arises when two vectors are different but their magnitudes are same, which is resulting in overwriting of keys in C++ map container.

You can find small snipped of implementation.

`class Vector3f`

{

public:

float x, y, z;

double magnitude() const { return sqrt(x*x + y*y + z*z); }

}

std::map<Vector3f, std::vector<int>, Vector3fCompare> vector_index;

struct Vector3fCompare

{

bool operator() (const Vector3f& lhs, const Vector3f& rhs) const

{

return lhs.magnitude() < rhs.magnitude();

}

};

Is there any way to compare two vectors?

Answer

Rather than simply comparing their `magnitude`

you can do a comparision using `std::tuple`

which has the comparison operators defined lexicographically.

```
struct Vector3fCompare
{
bool operator() (const Vector3f& lhs, const Vector3f& rhs) const
{
return std::make_tuple(lhs.x, lhs.y, lhs.z) < std::make_tuple(rhs.x, rhs.y, rhs.z);
}
};
```

Also note that if you define `operator<`

in your class, then you do not need to make this struct, as the template arguments for `map`

are

```
template<
class Key,
class T,
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T> >
> class map;
```

So `std::less`

will be defined since you defined `operator<`