sum of image pixels

Question

I tried to sum pixels values on black and white area, through dividing the image into a grid of blocks and getting the sum of pixels for each block.

When I printed the values of each, the values were all the same = 255. My question is: Why does this happen? Some blocks have only black pixels and some have black and white?

double* divide (Mat I)
         {
             //double* pointer;
         static double*  sums= new double [9];
        //pointer= sums[0];
          //double sums2[10];

          Mat block;
              //Mat block2;

    int numberblocks=9; 
    int bh;
    int bw;
    //int bh2;
    //int bw2;

    bh=I.cols/numberblocks;
    bw=I.rows/numberblocks; 
    //bh2=u.cols/numberblocks;
    //bw2=u.rows/numberblocks; 
    //
    double blockarea=bh*bw;
    //double num=0;
    //double blockarea2=bh2*bw2;

    //int r=0;
    //int c=0;
    //int err=0;

    for(int a=0;a<9;a++)
    {
       for (int r = 0; r < I.rows; r += bw)
       {
             for (int c = 0; c < I.cols; c += bh)
                 {

                 block  = I(cv::Range(r, min(r + bw, I.rows)),cv::Range(c, min(c + bh, I.cols)));


                  }
       }
       double sum=0;

         for(int i=0;i<block.rows;i++)
         {
             for(int j=0;j<block.cols;j++)
                 {
                sums[a] = sum + block.at<uchar>(i,j); //sums[k-1] + block.at<uchar>(i,j);
             }
         }
         cout<<"sum of"<<a<<"is"<<sums[a]<<endl;


    }


return sums;

}    

Answer 1

I think this will do

     for(int i=0;i<block.rows;i++)
     {
         for(int j=0;j<block.cols;j++)
             {

              sum = sum + block.at<uchar>(i,j); //sums[k-1] + block.at<uchar>(i,j);
         }
     }
     sums[a]=sum;

Debug it to be sure. Without using sum it would be:

     for(int i=0;i<block.rows;i++)
     {
         for(int j=0;j<block.cols;j++)
             {

               sums[a] =  sums[a] + block.at<uchar>(i,j); //sums[k-1] + block.at<uchar>(i,j);
         }
     }         

Answer 2

The row

sums[a] = sum + block.at(i,j);

looks strange. sums[a] has always the value of the last element of block. If this is by chance always 255, you have your explanation.