9.1.7 Checkerboard V2 Answers -

Even with the correct code, students often hit frustrating roadblocks. Here’s a quick troubleshooting table:

| Error Message / Symptom | Likely Cause | Solution | |-------------------------|--------------|----------| | Square is not filled | Missing setFilled(true) | Add the line before setting the color. | | All squares are the same color | Incorrect modulus logic | Use (row + col) % 2 == 0. Check your starting color. | | ArrayIndexOutOfBoundsException | Using <= instead of < in loop | Ensure loops run 0 to 7 (not 0 to 8). | | Nothing appears on screen | Forgot to call add(square) | After creating and coloring, call add(square). | | Squares overlap or have gaps | Incorrect coordinate math | x = col * size, y = row * size. Don’t add extra offset. | | Autograder fails on style | Missing constants or comments | Use private static final int for magic numbers (8, 50). |

Let’s outline a generic solution in pseudocode: 9.1.7 checkerboard v2 answers

function draw_checkerboard(rows, cols, colorA, colorB, top_left_is_colorA):
    for r from 0 to rows-1:
        for c from 0 to cols-1:
            if (r + c) % 2 == 0:
                color = colorA if top_left_is_colorA else colorB
            else:
                color = colorB if top_left_is_colorA else colorA
            draw_square_at(r, c, color)

Deep point: The condition (r + c) % 2 naturally alternates. The top_left_is_colorA flag just swaps which color gets the even sum.

The checkerboard problem usually requires creating a program that can: Even with the correct code, students often hit

import java.awt.Color;
import acm.graphics.*;
import acm.program.*;
public class CheckerboardV2 extends GraphicsProgram 
private static final int ROWS = 8;
private static final int COLS = 8;
private static final int SQUARE_SIZE = 50;
public void run() 
    // Create a 2D array to store colors
    Color[][] checkerboard = new Color[ROWS][COLS];
// Fill the array with alternating colors
    for (int row = 0; row < ROWS; row++) 
        for (int col = 0; col < COLS; col++) 
            if ((row + col) % 2 == 0) 
                checkerboard[row][col] = Color.RED;
             else 
                checkerboard[row][col] = Color.BLACK;
// Draw the checkerboard using the stored colors
    for (int row = 0; row < ROWS; row++) 
        for (int col = 0; col < COLS; col++) 
            int x = col * SQUARE_SIZE;
            int y = row * SQUARE_SIZE;
            GRect square = new GRect(x, y, SQUARE_SIZE, SQUARE_SIZE);
            square.setFilled(true);
            square.setFillColor(checkerboard[row][col]);
            add(square);

Here is a simple Python solution to generate a checkerboard pattern: Let’s outline a generic solution in pseudocode: function

def print_checkerboard():
    for row in range(8):
        for col in range(8):
            # Use the sum of row and column indices to determine the color
            if (row + col) % 2 == 0:
                print('\033[40m  ', end='')  # Black
            else:
                print('\033[47m  ', end='')  # White
        print('\033[0m')  # Reset color
print_checkerboard()

This script prints a simple text-based checkerboard to the console. The colors are represented using ANSI escape codes.

To make each square stand out, add:

square.setBorderColor(Color.WHITE);