9.1.7 Checkerboard V2 Answers

The 9.1.7: Checkerboard, v2 exercise is a common challenge in introductory Python courses, specifically on platforms like CodeHS. While version 1 typically asks you to fill specific rows with 1s, version 2 requires a true alternating checkerboard pattern across the entire 8x8 grid. The Objective

You need to create an 8x8 grid (a list of lists) where the elements alternate between 0 and 1. The key constraint is often that you must use nested loops and assignment statements (board[i][j] = 1) rather than just printing the expected output string. The Solution: Python Implementation

To solve this, you first initialize an 8x8 grid of zeros. Then, use a nested loop to check if the sum of the row index and column index is odd or even to determine where to place the 1s.

# Function to print the board in a readable format def print_board(board): for row in board: print(" ".join([str(x) for x in row])) # 1. Initialize an 8x8 grid filled with 0s board = [] for i in range(8): board.append([0] * 8) # 2. Use nested loops to apply the checkerboard pattern for row in range(8): for col in range(8): # If the sum of row + col is odd, set the value to 1 # This creates the alternating pattern if (row + col) % 2 != 0: board[row][col] = 1 # 3. Output the result print_board(board) Use code with caution. Why This Works

The logic (row + col) % 2 != 0 is the standard mathematical way to create a checkerboard. Row 0, Col 0: Sum is 0 (Even) → stays 0. Row 0, Col 1: Sum is 1 (Odd) → becomes 1. Row 1, Col 0: Sum is 1 (Odd) → becomes 1. Row 1, Col 1: Sum is 2 (Even) → stays 0.

This ensures that no two adjacent squares (horizontal or vertical) have the same value. Common Pitfalls

Indentation Errors: In Python, improper indentation of your nested loops will cause a SyntaxError or logic failure. Ensure your if statement is inside the second loop.

Assignment vs. Printing: Many students try to print the pattern using a string like "0 1 0 1". However, the CodeHS autograder often checks if you actually modified the list values.

Index Out of Range: Ensure your loops run exactly range(8) to match the 8x8 requirement.

For more practice on similar grid-based logic, you can explore the CodeHS Python Curriculum which covers 2D lists and nested iterations in detail.

The fluorescent lights of the computer lab hummed, a soundtrack to the mounting panic of a dozen Intro to Java students. It was Friday, 3:45 PM. The deadline for the "Nested Loops" unit was looming like a storm cloud.

Leo stared at his screen, his eyes blurring. The objective seemed simple enough: 9.1.7 Checkerboard v2. Draw a grid. Black square, white square, black square. But the code on his screen was producing a pattern that looked less like a chessboard and more like a barcode gone wrong.

He ran the code again. Row 1: Black, White, Black, White. (Perfect.) Row 2: Black, White, Black, White. (Disaster.) 9.1.7 checkerboard v2 answers

"It’s offset," Leo muttered, burying his face in his hands. "It’s supposed to be offset."

"You look like you're trying to calculate the trajectory of a Mars rover, but you’re just staring at a black screen."

Leo looked up. It was Maya, the TA. She was holding a mug of tea and looking amused. She pulled up a chair next to him.

"It’s 9.1.7," Leo groaned. "The Checkerboard. I can get the rows to alternate colors, but I can’t get the columns to sync up. My rows are identical. It’s just stripes."

Maya nodded. "Ah, the classic v2 trap. Did you look at the 'answers' in the documentation?"

"I tried," Leo admitted. "I searched '9.1.7 checkerboard v2 answers' online, but I just found a bunch of code blocks with no explanation. If I copy-paste it, I get the points, but I won’t know why it works. And the test is next week."

"Good instinct," Maya said. "Copying the answer key is like eating the menu instead of the meal. Let’s figure it out the hard way."

She pointed to his loop structure on the screen. "Show me your logic."

Leo pointed to the for loop for the rows. "I have i for the rows and j for the columns. If j is even, I draw black. If j is odd, I draw white."

"Right," Maya said. "So, for every row, column 0 is black, column 1 is white. That works for Row 0. But what happens when you jump down to Row 1?"

Leo paused. "Well... the loop restarts. So j starts at 0 again. Column 0 is black again."

"Exactly," Maya said. "So Row 0 and Row 1 look identical. You're forgetting to factor in where you are vertically. You're only looking at the horizontal position." The 9

"So... it’s a math problem?"

"It’s a coordinate problem," Maya corrected gently. "Think of a coordinate plane. You have an X and a Y. The color of a square depends on the sum of its coordinates."

She grabbed a piece of scratch paper and drew a grid. She labeled the top left corner (0,0).

"At (0,0), the sum is 0. Even. So, Black," she said. "At (0,1), the sum is 1. Odd. So, White." "Now, look at the start of the next row. (1,0). The sum is 1. Odd."

Leo’s eyes widened. "So if the sum is odd, it inverts the starting color automatically."

"Right," Maya smiled. "You don't need a complex if-else chain to check the row number separately. You just need to check if (row + column) % 2 == 0."

Leo turned back to his keyboard. He highlighted his clumsy logic: if (j % 2 == 0)

He deleted it and typed the new condition: if ((i + j) % 2 == 0)

He held his breath and hit Run.

The Java console sprang to life. The canvas rendered. Row 0: Black, White, Black, White. Row 1: White, Black, White, Black. Row 2: Black, White...

A perfect checkerboard.

"Yes!" Leo whispered, pumping a fist.

"That," Maya said, standing up, "is the difference between finding the answers and finding the solution. You didn't just pass 9.1.7; you understand how to map a grid."

"Thanks, Maya," Leo said, watching the green "Check Passed" box appear on his screen.

"And hey," she called over her shoulder as she walked away. "Next time, don't look for the answer key. Look at the coordinates. Usually, the logic is simpler than the panic allows you to see."

Leo smiled, saved his file, and closed the lab. The checkerboard was solved, and for the first time all afternoon, the hum of the lights sounded almost like a victory song.

Since I don’t have access to proprietary problem statements or answer keys, I’ll provide a deep, analytical piece on what such a problem typically involves, the patterns behind it, and how to think through a solution — so you can derive the answer yourself, or understand it at a deeper level.

9.1.7 Checkerboard v2 — Solutions & Walkthrough

Checkerboard Problem Overview

The checkerboard problem usually requires creating a program that can:

1. Generate a Checkerboard Pattern: This often involves printing an 8x8 grid that alternates between two different colors (typically black and white) for each square.

2. Place Checkers: A more complex version might require placing checkers (or "pieces") on the board according to certain rules.

3. Handle Moves: Advanced versions might need to manage the movement of these checkers.

Variation C: Clickable Checkers

Extend the program so that clicking on a square changes its color or places a game piece (turning the checkerboard into a functional Checkers game).

Beyond the Answer: Understanding for the Exam

Simply copying the code might get you a checkmark, but CodeHS often includes a quiz or subsequent exercise that requires you to modify the pattern. Here’s how to adapt your solution for different scenarios: