全局变量未更新：10*22二维列表矩阵调用函数后未修改

Hey there! Let's dig into why your matrix isn't updating after calling your step function—this is a super common gotcha with Python lists, so we'll fix it in no time.

First, let's recap your goal: you have a 10x22 matrix initialized with dots (.), and you want to convert any row that has no dots at all into a full row of dots. The problem is that after running your step function, the original matrix stays unchanged.

The Most Likely Culprit: Incorrect Row Modification #

In Python, when you loop through a list like for row in matrix, the row variable is just a reference to the original row in the matrix. If you reassign row to a new list (like row = ['.'] * WIDTH), you're only changing what that local variable points to—not the actual row stored in the matrix.

Example of the Wrong Approach #

This is probably what your step function looks like right now, and it won't modify the original matrix:

def step(mat):
    for row in mat:
        if '.' not in row:  # Check if the row has no dots
            row = ['.'] * WIDTH  # This only updates the local 'row' variable, not the matrix!

The Fix: Modify the Matrix via Indexes #

Instead of reassigning the loop variable, access each row by its index in the matrix and replace it directly. This ensures you're modifying the original matrix object.

Working Step Function #

def step(mat):
    # Loop through each row index
    for i in range(len(mat)):
        current_row = mat[i]
        # Check if this row has no dots
        if '.' not in current_row:
            # Replace the row in the original matrix with a full dot row
            mat[i] = ['.'] * WIDTH

Full Working Code Example #

Here's how it all fits together, including initialization and verification:

WIDTH, HEIGHT = 10, 22
# Initialize matrix with all dots
matrix = [['.' for x in range(WIDTH)] for y in range(HEIGHT)]

# Simulate some rows without dots (replace this with your actual middle logic)
matrix[3] = ['X'] * WIDTH
matrix[7] = ['#'] * WIDTH

# Your fixed step function
def step(mat):
    for i in range(len(mat)):
        if '.' not in mat[i]:
            mat[i] = ['.'] * WIDTH

# Run the step function
step(matrix)

# Print the updated matrix to verify
for row in matrix:
    print(''.join(row))

Double-Check Your Logic #

If the fix above doesn't work, make sure your condition is correct:

• You want if '.' not in row to target rows with no dots
• If you accidentally wrote if '.' in row, you'd be modifying rows that do have dots, which won't match your goal

内容的提问来源于stack exchange，提问作者Karan Singh