2D arrays and Coordinates

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Question

I'm trying to just print space in my "canvas" ( with coordinates (2,2) for example) by editing my 80x20 grid made by █ blocks in the console window.

  • Please suggest me better ways of creating the grid in the first place ( I've just learned for-each loops)

  • Why do I get those 3 characters after running the program ?

  • Why isn't the space on the (2,2) block but obviously on the first row somewhere in the mid ?

enter image description here

Code :

#include <iostream>
int main()
{
 uint8_t block {219}; // █
 uint8_t space {32};  // ' '
 uint8_t screen[80][20] {};
 for (auto &row : screen)   // make the "canvas"
    for (auto &col : row)
        col = block;
 for (int row = 1; row <= 80; ++row)
 {
    for (int col = 1; col <= 20; ++col)
    {
        if (col == 2 && row == 2)
            screen[row][col] = space;
    }
 }
std::cout << *screen;
return 0;
}

Answer

Some issues:

  • C++ uses 0-based indexing. You want for (int row = 0; row < 80; ++row).
  • You are looping through the entire array just to add a space, why not use screen[2][2]=space?
  • You are printing the entire array to the screen, but the array does not include newlines, so you are relying on the console to wrap, is this a safe assumptions?
  • Your string does not include the required null-termination, so extra characters are printed.
  • You have ordered your array as column-major. I suspect you'll want to use row-major instead.

Since you're using C++, I'd probably write your code like this:

#include <iostream>
#include <vector>
class Screen {
 public:
  typedef uint8_t schar;
  std::vector<schar> data; //Store data in a flat array: increases speed by improving caching
  int width;
  int height;
  Screen(int width, int height, schar initchar){
    this->width = width;     //'this' refers to the current instantiation of this object
    this->height = height;
    data.resize(width*height,initchar); //Resize vector and set its values
  }
  schar& operator()(int x, int y){
    return data[y*width+x];
  }
  void setAll(schar initchar){
    std::fill(data.begin(),data.end(),initchar);
  }
  void print() const {
    std::cout<<"\033[2J"; //ANSI command: clears the screen, moves cursor to upper left
    for(int y=0;y<height;y++){
      for(int x=0;x<width;x++)
        std::cout<<data[y*width+x];
      std::cout<<"\n";     //Much faster than std::endl
    }
    std::cout<<std::flush; //Needed to guarantee screen displays
  }
};
int main(){
  const int WIDTH  = 80;
  const int HEIGHT = 20;
  uint8_t block {219}; // █
  uint8_t space {32};  // ' '
  Screen screen(WIDTH,HEIGHT,block);
  screen(2,2) = space;
  screen.print();
  return 0;
}