51 — Grid: a board of lists

A list can hold lists. The outer list is the board, each inner list is one row, and every cell is a text mark. Reading and writing a cell uses two indexes — first the row, then the column.

#Steps

1. A 3x3 board is three rows, each a list of three marks. board[0] is the first row, board[1] the second. Reading a cell needs both indexes — row first, column second — and writing uses the same with =. All indexes start at 0, so board[0][0] is the top-left corner:

   board = [["-", "-", "-"], ["-", "-", "-"], ["-", "-", "-"]]
   show board[0]
   board[0][0] = "X"
   board[1][2] = "O"
   show board[0][0]
   show board[1][2]
   

   copyrun it →

   It prints the first row, then X and O. A - means an empty cell.

2. A for loop visits every row, so printing a whole board is three short lines:

   board = [["X", "O", "-"], ["-", "-", "-"], ["-", "-", "-"]]
   for row in board:
       show row
   

   copyrun it →

   The loop reads the outer list top to bottom; each row is one inner list.

3. Now a program that builds the board, prints it, places marks, reads them back, and counts them. The counting loop uses a second, inner loop so every cell of every row is visited. Save it as grid.nme:

   # grid.nme — a 3x3 board made of lists.
   # Run: nme r grid
   # A grid is a list of rows; each row is a list of cells.
   
   board = [["-", "-", "-"], ["-", "-", "-"], ["-", "-", "-"]]
   
   def show_board(board):
       for row in board:
           show row
   
   show "Empty board:"
   show_board(board)
   
   board[0][0] = "X"
   board[0][1] = "O"
   board[1][1] = "X"
   board[2][2] = "X"
   
   show "After four marks:"
   show_board(board)
   
   show f"Top-left corner: {board[0][0]}"
   show f"Center: {board[1][1]}"
   show f"Bottom-right corner: {board[2][2]}"
   show f"Row 0, column 2 is empty: {board[0][2]}"
   
   board[1][1] = "-"
   show "Center cleared, then filled again:"
   board[1][1] = "O"
   show_board(board)
   
   x_count = 0
   o_count = 0
   for row in board:
       for cell in row:
           if cell == "X":
               x_count = x_count + 1
           if cell == "O":
               o_count = o_count + 1
   show f"The board holds {x_count} X marks and {o_count} O marks"
   

   copyrun it →

4. Run it:

   nme r grid
   

   copy

   Empty board:
   ['-', '-', '-']
   ['-', '-', '-']
   ['-', '-', '-']
   After four marks:
   ['X', 'O', '-']
   ['-', 'X', '-']
   ['-', '-', 'X']
   Top-left corner: X
   Center: X
   Bottom-right corner: X
   Row 0, column 2 is empty: -
   Center cleared, then filled again:
   ['X', 'O', '-']
   ['-', 'O', '-']
   ['-', '-', 'X']
   The board holds 2 X marks and 2 O marks
   

   copy

   Writing then reading back a cell gives exactly what was stored — that is the whole point of a grid. Guide 52 turns this board into a playable game.

#Try it yourself

Place marks so the first column reads X top to bottom, print the board, and read board[2][0] back. Then make the counting loop print how many cells are still empty.

#What you learned

• A grid is a list of rows, and each row is a list of cells.
• board[row][col] reads a cell; board[row][col] = "X" writes one.
• for row in board: visits each row; a second loop visits each cell.
• A - marks an empty cell, so the board can tell a mark from a gap.