code: def sparse_grid_from_unmasked_sparse_grid(unmasked_sparse_grid, sparse_to_unmasked_sparse):
      
      total_pix_pixels = sparse_to_unmasked_sparse.shape[0]
  
      pix_grid = np.zeros((total_pix_pixels, 2))
  
      for pixel_index in range(total_pix_pixels):
          pix_grid[pixel_index, 0] = unmasked_sparse_grid[sparse_to_unmasked_sparse[pixel_index], 0]
          pix_grid[pixel_index, 1] = unmasked_sparse_grid[sparse_to_unmasked_sparse[pixel_index], 1]
  
      return pix_grid
docs: Use the central arc-second coordinate of every unmasked pixelization grid's pixels and mapping between each
      pixelization pixel and unmasked pixelization pixel to compute the central arc-second coordinate of every masked
      pixelization grid pixel.
  
      Parameters
      -----------
      unmasked_sparse_grid : ndarray
          The (y,x) arc-second centre of every unmasked pixelization grid pixel.
      sparse_to_unmasked_sparse : ndarray
          The index mapping between every pixelization pixel and masked pixelization pixel.