Question

I recently started with C++; I’m an hobby programmer, and I know a bit of Python.

I programmed a little snake. I wanted to insert another snake guided by the computer.

I decided to put the possible direction that the snake can take in an enum:

enum directions{UP, DOWN, RIGHT, LEFT, IN, OUT, FW, RW,NONE};

void fill_map(std::map<directions,V4> &map_vec);

void fill_map(std::map<int, directions*> &map_dir);

void fill_map(std::map<directions,directions> &map);

and map the enum for the needed function:

void fill_map(std::map<directions,V4> &map_vec){
    map_vec[UP] = V4(0,1,0,0);
    map_vec[DOWN] = V4(0,-1,0,0);
    //others

  }

 void fill_map(std::map<directions, directions> &map){
    map[UP]= DOWN;
    map[DOWN]= UP;
    //others
  }


void fill_map_axis(std::map<int, directions*> &map_dir){
    directions array_x[2] = {RIGHT,LEFT};
    map_dir[0] = array_x;

    directions array_y[2] = {UP,DOWN};//store the array
    map_dir[1] = array_y;

    directions array_z[2] = {FW,RW};//store the array
    map_dir[2] = array_z;

    directions array_w[2] = {IN,OUT};//store the array
    map_dir[3] = array_w;

}

The fill_map functions are called in the snake constructor.
Basically what I wanted to do in the fill_map_axis is to map an integer corresponding to the index of the coordinate (0 coord x, 1 coord y etc) and map the two directions that move along those axis.
So I stored an array of two directions.

Now I call the function:

directions SnakeCPU::find_dir(V4 point){
    //point is the target point
    directions dir;
    int index = get_coord_index(point); //get the index where to move
    double diff = head_pos[index]-point[index]; //find the difference between the head and the target point

    directions* axis = dir_coords[index]; //call the map containing the directions stored in an array.

    if(diff<0.){

        dir = *axis; //use the first
    }
    else if(diff>0.) {
        axis++;
        dir = *axis; //use the second
    }
    else{
        dir = NONE;
    }

    return dir;
}

Although the map are initialized in the Snake constructor, it seems that the returned value from the pointer axis is a random memory block.

So my question: do you see a mistake in the code? did I used the pointer axis with sense?

I’m really not expert with pointer; in Python the map is instantiated with a dictionary like this:

dir_coords = {0:[LEFT,RIGHT], ...}

so I just need to call it:

axis = dir_coords[index]
dir = axis[0]
#or
dir = axis[1]

edit:

Snake constructor:

Snake::Snake()
{    
    fill_map(dir_vectors);
    fill_map(dir_coords);
    fill_map(opposite_dir);
    head_pos = V4(0.,0.,0.,0.);
    //other stuff...
}

Answer 1

Just a shot in the blue, here is how I would design this.

#include <map>

enum EDirection { NONE = 0, UP, DOWN, RIGHT, LEFT, IN, OUT, FW, RW };

typedef std::map<EDirection, V4>           DirectionMap;
typedef std::pair<EDirection, EDirection> DirectionPair;
typedef std::map<int, DirectionPair>            PairMap;

extern const DirectionMap map_vec {
  { UP,   (0, 1, 0, 0) },
  { DOWN, (0,-1, 0, 0) },
  // ...
};   // using C++11 initialization lists for convenience

extern const PairMap map_dir {
  { 0, { RIGHT, LEFT } },
  { 1, { UP, DOWN }    },
  // ...
};

Here I decided to make map_vec and map_dir global constants, because I gathered that that’s essentially what they are. To initialize those, I rely on the new C++11 initialization syntax. If that’s not an option, we can also fill the map in the traditional way:

PairMap map_dir;
map_dir.insert(std::make_pair(0, DirectionPair(RIGHT, LEFT)));
map_dir.insert(std::make_pair(1, DirectionPair(UP, DOWN)));
// ...

DirectionMap map_vec;
map_vec.insert(std::make_pair(UP,   V4(0, 1, 0, 0)));
map_vec.insert(std::make_pair(DOWN, V4(0,-1, 0, 0)));
// ...

(Yes, you can also write map_dir[0] = DirectionPair(RIGHT, LEFT)'. I don’t like the square brackets, though, they feel too violent for my taste.)