uint8_t values issues with vectors

Question

I am trying to create a 10 by 10 grid with uint8_t values. using some coordinates I am trying to place 255 values at those indexes. When I substitute 255 at those coordinates it works fine but as soon as I try to print out the values at those indexes it doesn't seem to be the values I wanted.

float grid_width = 10;
float grid_height = 10;
float grid_resolution = 0.05;
float rows = grid_width/grid_resolution;
float cols = grid_height/grid_resolution;

std::vector<std::vector<float>> landmarks = {{2,2},{8,8},{2,8},{8,2},{3,2},{3.5,2.1},{4.0,3.5},{4.5,2.0},{5.0,3.0}};
std::vector<std::vector<uint8_t>> grid(rows, std::vector<uint8_t>(cols, 0));

for(auto i: landmarks)
{
    for(auto m = i[0]-(grid_resolution*2) ; m <= i[0] +(grid_resolution*2) ; m=m+grid_resolution )
    {
        for(auto n = i[1]-(grid_resolution*2); n <= i[1]+(grid_resolution*2); n=n+grid_resolution)
        {
            int m_co = m/grid_resolution;
            int n_co = n/grid_resolution;
            grid[m_co][n_co] = 255;
            cout<<"m_co: "<< m_co <<" n_co:" << n_co <<endl;
            cout<<"grid[m_co][n_co]: "<< unsigned(grid[m_co][n_co])  <<endl; //check the printout over here for index values of 18,22 you will see 255 but when you try it outside the for loops those values are rest to 0
            
        }
        
    }
}

// for(auto m: grid)
// {
//     for(auto n: m)
//     {
//         cout<<unsigned(n)<<" ";
//     }
//     cout<<endl;
// }
cout<<unsigned(grid[18][18])<<endl; //expected 255 got 255
cout<<unsigned(grid[18][22])<<endl; //expected 255 got 0
cout<<unsigned(grid[22][22])<<endl; //expected 255 got 0
cout<<unsigned(grid[22][18])<<endl; //expected 255 got 0
cout<<unsigned(grid[17][17])<<endl; //expected 255 got 0
cout<<unsigned(grid[20][18])<<endl; //expected 255 got 255

So I am taking indexes using landmarks vector and trying to replace 0 from grid vector to 255 on the indexes of landmarks, but I also want to replace 0 on the indexes around that index. like if 2,2 is my coordinate I want to replace 1,1 and 1,2 and 1,3 and 2,1 and 2,2, and 2,3 and 3,1 and 3,2 and 3,3 to 255 but I do not get 1,3 and 2,3 and 3,3 255 i get 0 this is just an example I am trying to achive

Answer 1

I was able to find the solution. the old logic was too complex and not working with different resolutions. So I made a simpler one that does need to take care of rounding up issues. first took each point and tried to go replace values index by index.

for(auto i: landmarks)
{
    for(int m = (i[0]/grid_resolution)-2; m <= (i[0]/grid_resolution)+2; m++)
    {
        for(int n = (i[1]/grid_resolution)-2; n <= (i[1]/grid_resolution)+2; n++)
        {
            grid[m][n] = 255;
        }
    }
}

Answer 2

In the loop you are printing n/grid_resolution. When it prints 22, it is actually 21.99999809265, but because the default precision of iostream is 6, it is rounded to 22. On the other hand, when you assign it to int, it is rounded down to 21, so you actually store 255 in grid[18][21].

Answer 3

The problem is that m/grid_resolution has type float, while m_co has type int:

int main()
{
    float grid_width = 10;
    float grid_height = 10;
    float grid_resolution = 0.1;
    float rows = grid_width/grid_resolution;
    float cols = grid_height/grid_resolution;
    cout.precision(5);
    
    std::vector<std::vector<float>> landmarks = {{2,2},{8,8},{2,8},{8,2},{3,2},{3.5,2.1},{4.0,3.5},{4.5,2.0},{5.0,3.0}};
    std::vector<std::vector<uint8_t>> grid(rows, std::vector<uint8_t>(cols, 0));
    
    for(auto i: landmarks)
    {
        for(auto m = i[0]-(grid_resolution*2) ; m <= i[0] +(grid_resolution*2) ; m=m+grid_resolution )
        {
            for(auto n = i[1]-(grid_resolution*2); n <= i[1]+(grid_resolution*2); n=n+grid_resolution)
            {
                int m_co = m/grid_resolution;
                int n_co = n/grid_resolution;
                grid[m_co][n_co] = 255;
                // Uncomment the following two lines for showing that are different
                //if (m_co != m/grid_resolution || n_co != n/grid_resolution)
                //    cout << "different" << endl;
                cout << "grid["<<m_co<<"]["<<n_co<<"]: "<< unsigned(grid[m_co][n_co]) << endl;
            }
            
        }
    }
    
    // for(auto m: grid)
    // {
    //     for(auto n: m)
    //     {
    //         cout<<unsigned(n)<<" ";
    //     }
    //     cout<<endl;
    // }
    cout<<"grid[18][18]" << unsigned(grid[18][18])<<endl;
    cout<<"grid[18][22]" << unsigned(grid[18][22])<<endl;
    cout<<"grid[22][22]" << unsigned(grid[22][22])<<endl;
    cout<<"grid[22][18]" << unsigned(grid[22][18])<<endl;
    cout<<"grid[17][17]" << unsigned(grid[17][17])<<endl;
    cout<<"grid[20][18]" << unsigned(grid[20][18])<<endl;
    return 0;
}

If you run that code, the right values of m_co and n_co will be printed.

Best regards.