#include <stdio.h>
#include <stdlib.h>
                      // exit(int) is in stdlib

void main()
{ FILE * f = fopen("mystery.dat", "r");
  if (f == NULL)
  { printf("file not found\n");
    exit(1); }

                      // Mistake warning:
                      //    fseek returning the new position is not standard
                      // after all (I thought it was). To find the length, you
                      // have to fseek the end then get the answer from ftell.
  fseek(f, 0, SEEK_END);
  int filesize = ftell(f);
  fseek(f, 0, SEEK_SET);

  const int columns = 800;
  const int rowsize = columns * sizeof(short int);
  int rows = filesize/rowsize;

  // This is how you create a true two dimensional array dynamically.
  // rows is allowed to be a variable, but columns must be a constant.
  short int (* a)[columns];
  a = new short int[rows][columns];

  int n = fread(a, rowsize, rows, f);
  printf("%d rows read\n", rows-1);

  for (int row = 1; row < rows; row += 10)
  { for (int col = 0; col < columns; col += 10)
    { 
      int watercount = 0;
      for (int i = 0; i < 10; i += 1)
        for (int j = 0; j < 10; j += 1)
          if (a[row+i][col+j] == -500)
            watercount += 1;

      if (watercount == 100)
        putchar('#');
      else if (watercount > 50)
        putchar('O');
      else
        putchar(' '); }

    putchar('\n'); } }