Question

I have a vector 3 (x,y,z) 1D dynamic array full of vertex positions.

How would I return the y value at a given x and z coordinate?

Edit:: Sorry about the lack of detail.

I am using c++, compiling in Visual Studio 2008.
The vector is a vector class storing 3 float values defining an x, y and z variable, it is used for position.

It’s like Vector3 *array;

array = new Vector3[100];

Lots of positional values are added to the array.

When you access a member of the array for a specific value it’s like

array[0].y

But I want to find a the y value that corresponds to a specific x and z

like

GetY(float x, float z)

…

return y;

Answer 1

I suppose you have something like

struct Vec3D{
  float x, y, z;
};

Vec3D vec3d_arr[20];

Then, to get what you want, you’ll need to iterate over the array.

float GetYforXZ(Vec3D* arr, unsigned int length, float x_val, float z_val){
  for(unsigned i=0; i < length; ++i){
    if(arr[i].x == x_val && arr[i].z == z_val)
      return arr[i].y;
  }
}

int main(){
  Vec3D arr[20];

  // init arr

  float y = GetYforXZ(arr,20,15.4f,23.3f);
}

---

Edit: On your comment:

#include <map>
#include <math>
using namespace std;

struct Vec3D{
  float x, y, z;
};

const float float_eps = 1e-5;

struct float_wrapper{
    float _value;

    float_wrapper()
        : _value(0.0f) {}

    float_wrapper& operator=(float f){
        _value = f;
        return *this;
    }

    operator float() const{
        return _value;
    }
};

bool operator==(float_wrapper const& lhs, float_wrapper const& rhs){
    float tmp = fabs(lhs._value - rhs._value);
    return tmp < float_eps && tmp >= 0;
}

bool operator<(float_wrapper const& lhs, float_wrapper const& rhs){
    return lhs._value < rhs._value;
}

typedef map< float_wrapper,float_wrapper > zy_map;
typedef map< float_wrapper,zy_map > xzy_map;

void init_vertex_mapping(xzy_map& a_map, Vec3D* arr, size_t length){
  for(size_t i=0; i < length; ++i){
    Vec3D& vertex = arr[i];
    zy_map& zy = a_map[vertex.x];
    zy[vertex.z] = vertex.y;
  }
}

int main(){
  xzy_map vertex_map;
  Vec3D vertex_array[100] = { {0,0,0},{0,0,0},{0,0,0},{-3.14f,42.0f,-13.37f},{0,0,0} };

  init_vertex_mapping(vertex_map, vertex_array, 100);

  float y = vertex_map[-3.14f][-13.37f];
}

Though a problem that I forgot is the inaccuracy of floats, so maybe you get problems with the map. Comment back if you do. 🙂

---

Edit:
Added a more safe version, employing a float_wrapper.