在 Python 的 for 循环中使用 range(len(x)) 时添加索引有什么影响？ 例如范围（len（x [0]）

Question

I'm pretty new to coding, and am using Python. In an online lesson, I'm creating a minimax function to try and programme AI to play Connect 4. The following code makes up the evaluation function to tally whether player "X" and player "O" have any 'two streaks' (in one direction for now) by counting whether, given a position on the board, the piece in the column to its right, has the same symbol.

def my_evaluate_board(board):
  if has_won(board, "X"):
    return float("Inf")
  elif has_won(board, "O"):
    return -float("Inf")
  else:
    x_two_streak = 0
    o_two_streak = 0
    for col in range(len(board)-1):
      for row in range(len(board[0])):
        if board[col][row] == "X" and board[col + 1][row] == "X":
          x_two_streak += 1
    for col in range(len(board)-1):
      for row in range(len(board[0])):
        if board[col][row] == "O" and board[col + 1][row] == "O":
          o_two_streak += 1
    return x_two_streak - o_two_streak

I understand that in the for loop:

for col in range(len(board)-1):

the -1 is in place because the code is looking to the right of each column to look for two pieces in a row and this prevents the loop from trying to look to the right of the final column.

What I don't understand is why the addition of an index in next line of code is doing what it is doing:

for row in range(len(board[0])):

Elsewhere in my script, I've printed the output of this line, with and without the index, but no matter what index I put, I get the same result. The board has 7 columns, each with 6 rows. See below:

print(range(len(new_board[0])))
# this returns: range(0, 6)

print(range(len(new_board)))
# this prints: range(0, 7)

and to demonstrate what I mean about the value of the index not making any difference:

print(range(len(new_board[4])))
# this prints: range(0, 6)

I guess the purpose of this is to reduce the number of times the function loops through the rows by 1 because there is 1 less row than there are columns.

Please could someone explain why the addition of an index to the for loop makes the function behave this way?

Answer 1

looks like your data is organized as a matrix that is this case is implemented as a list of lists. This means that each element of board is a list representing a column. The length of a column represents the number of rows of your matrix. Each column will have the same length, so you can pick any of them to find the number of rows. Best pick is the first column (the one with index [0]).

To be more clear you could use

n_cols = len(board)
n_rows = len(board[0])

and rewrite your loops as

for row_index in range(n_rows):
    for column_index in range(n_cols):

Trying to understand better this structure you can create your own sample matrix

l = [[1, 2, 3], [4, 5, 6]]

thus you created a matrix with shape 2x3

print(len(l))

will return 2, while

print(l[0])
print(l[1])

will return [1,2, 3] and [3, 4, 5]. Each of them has length 3 (so print(len(l[0]) or print(len(l[1]) will both return 3). Even better way to visualize your matrix would be casting it to a numpy array

import numpy
print(numpy.array(l))

will show you the matrix in a more familiar shape

[[1 2 3]
 [2 3 4]]