3d array c/c++ initialization/conversion

Question

float sampleGrid1[5][5][5] =
{
    {
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0}

    },

    {
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 1.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0}

    },

    {
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 1.0, 0.0, 0.0},
        {0.0, 1.0, 1.0, 1.0, 0.0},
        {0.0, 0.0, 1.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0}

    },

    {
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 1.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0}

    },

    {
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0},
        {0.0, 0.0, 0.0, 0.0, 0.0}

    }
};

typedef struct
{
  int Nx;
  int Ny;
  int Nz;
  float*** M;
}OG;

Relevant function:

OG *newOG(){
    OG *newOG = (OG *)malloc(sizeof(OG));
    if (newOG == NULL)
    {
        throw std::exception("newOG : no memory is available");
    }

    return newOG;
}

int initiateOG(OG *MyOG)
{

    ifstream dump("OGdump3.txt");

    if (dump.is_open())
    {   
        while ( dump.good() )
        {   
            dump >> MyOG->Nx;
            dump >> MyOG->Ny;
            dump >> MyOG->Nz;

            MyOG->M = new float**[MyOG->Nx];
            for(int i = 0; i < MyOG->Nx; i++)
            {
                MyOG->M[i] = new float*[MyOG->Ny];

                for(int j = 0; j < MyOG->Ny; j++)
                {
                    MyOG->M[i][j] = new float[MyOG->Nz];
                }
            }
            for(int z=0;z < MyOG->Nz; z++){

                for(int y=0;y < MyOG->Ny; y++){

                    for(int x=0;x < MyOG->Nx; x++){

                        dump >> MyOG->M[x][y][z];
                    }

                }

            }


        }

    dump.close();
    }
    else return 0; 
    return 1;
}

I want to hard code some sample grids into the code, but don't know the best way to create them, do i have to use for loops?

i don't want to change my typedef struct OG, if possible

Modified:

OG *occupancyGrid;

 void initialize3dArray(int x, int y, int z,float*** array)
    {
        array = new float**[x];
        for(int i = 0; i < x; i++)
        {
            array[i] = new float*[y];

            for(int j = 0; j < y; j++)
            {
                array[i][j] = new float[z];
            }
        }

    }

   void sampleOG1()
    {
        occupancyGrid = newOG();
        occupancyGrid->Nx = 5;
        occupancyGrid->Ny = 5;
        occupancyGrid->Nz = 5;

        initialize3dArray(5, 5, 5,occupancyGrid->M);

        for(int z=0;z < occupancyGrid->Nz; z++){

            for(int y=0;y < occupancyGrid->Ny; y++){

                for(int x=0;x < occupancyGrid->Nx; x++){

                    occupancyGrid->M[x][y][z] = sampleGrid1[x][y][z];
                }

            }

        }

    }

initialize3dArray this function doesn't have compiling error， but still causing the program to crash

rectest.cpp: In function ‘OG* sampleOG1()’: rectest.cpp:59: error: ‘newOG’ was not declared in this scope rectest.cpp:63: error: ‘occupancyGrid’ was not declared in this scope — Oswald
– Oswald, Commented Apr 3, 2011 at 14:55
I'm happy to that there is a related function (relevance?). No problem with the initial snippet here. What is your exact question? — sehe
– sehe, Commented Apr 3, 2011 at 14:55
@Oswald: reporting your own compilation failures is hardly helpful — sehe
– sehe, Commented Apr 3, 2011 at 14:55

Sarfaraz Nawaz · Accepted Answer · 2011-04-03 15:37:16Z

3

Yes. that will not compile, because float[5][5][5] and float *** aren't same type. They're not even compatible type. One cannot convert to other automatically.

However, float[5][5][5] can convert to float (*)[5][5] automatically. So this is legal code:

    float (*m)[5][5];
    m = sampleGrid1;  //legal - allowed!

Demo : http://ideone.com/RwAwI

So define OG as,

struct OG
{
  int Nx;
  int Ny;
  int Nz;
  float (*M)[5][5];
};

If you OG as defined above, then you can write this:

OG* temp = newOG();
temp->Nx = 5;
temp->Ny = 5;
temp->Nz = 5;
occupancyGrid->M = sampleGrid1; //DONT use &

edited Apr 3, 2011 at 15:37

answered Apr 3, 2011 at 14:55

Sarfaraz Nawaz

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Comments

Lightness Races in Orbit · Accepted Answer · 2011-04-03 14:55:16Z

1

Isn't the error clear? float[5][5][5] is not as related to float*** as you think it is.

Use a std::vector<std::vector<std::vector<float> > > instead and avoid the whole mess.

answered Apr 3, 2011 at 14:55

Lightness Races in Orbit

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2 Comments

Mzq Over a year ago

i’m really new to c++，only had experience in java and c，could u give an example of using std::vector<std::vector<std::vector<float> > >

Lightness Races in Orbit Over a year ago

@Miranda: I think that's beyond the scope of this site. Read the chapter in your C++ book about vectors.

Erik · Accepted Answer · 2011-04-03 15:01:31Z

1

float x[2][2] is a 2D array of float - it's not an 1D array of pointers to float. The conversion from array to pointer only works for the first dimension of such arrays.

Given float x[2][2] you reserve space for 4 floats. A float ** variable on the other hand is a pointer to a pointer to a float - there's no pointers anywhere in float x[2][2]

The same of course holds true for a 3D array - your 3D array has no sneakily hidden pointers inside it, it cannot be treated as a float ***

edited Apr 3, 2011 at 15:01

answered Apr 3, 2011 at 14:55

Erik

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