Checkerboard Grid

By the end of this lab you'll be able to:

• Use nested for loops to iterate over rows and columns
• Compute the (x, y) position of each square from its row and column index
• Choose a color based on whether row + col is even or odd

An 8×8 grid of black and white squares fills the canvas — just like a chessboard — built entirely from loops and arithmetic, not hard-coded coordinates.

Welcome to the Checkerboard!

In this lab you'll learn how two loops — one for rows, one for columns — can place 64 squares on the screen with just a few lines of code. Let's code it together!

How the Grid Works

The outer loop counts rows (0–7, top to bottom). The inner loop counts columns (0–7, left to right). For each combination, the position is:

• x = -160 + col * 40 — starts at the left, shifts right by 40px per column
• y = 160 - row * 40 — starts at the top, shifts down by 40px per row

The color alternates because (row + col) % 2 is 0 on even squares and 1 on odd squares — exactly like the black and white pattern of a chessboard.

Sample Code

import turtle
monty = turtle.Turtle()
monty.speed(0)
monty.hideturtle()

size = 40

for row in range(8):
    for col in range(8):
        x = -160 + col * size
        y = 160 - row * size
        color = 'black' if (row + col) % 2 == 0 else 'white'
        monty.penup()
        monty.goto(x, y)
        monty.pendown()
        monty.color(color)
        monty.begin_fill()
        for _ in range(4):
            monty.forward(size)
            monty.right(90)
        monty.end_fill()

What Do You Think Will Happen?

The top-left square is at row=0, col=0. (0 + 0) % 2 = 0 → black. What color is the square at row=2, col=3? Work it out, then click Run to check!

Try It Now

▶ Run ↻ Reset

(2 + 3) % 2 = 1 → white. Were you right?

How It Works

The innermost for _ in range(4) draws one square: forward 40, turn right 90°, four times. (The _ is a throwaway variable name used when you don't need the loop counter.)

Before each square, monty.goto(x, y) jumps to the correct position with the pen up — without this, Monty would drag lines across the canvas between squares.

Explanation Table

Line	What it does
for row in range(8)	Outer loop — 8 rows top to bottom
for col in range(8)	Inner loop — 8 columns left to right
x = -160 + col * size	Column 0 starts at x = -160; each column is 40px to the right
y = 160 - row * size	Row 0 starts at y = 160; each row is 40px lower
(row + col) % 2	0 on even sum → black; 1 on odd sum → white
for _ in range(4)	Draw the four sides of a square

Learning Check

Spot the Bug!

The program below draws all 64 squares black — no alternating pattern! Fix the one line that decides the color so the chessboard pattern appears.

▶ Run ↻ Reset

Replace color = 'black' with color = 'black' if (row + col) % 2 == 0 else 'white'.

Experiments

1. Make it colorful. Replace 'black' and 'white' with 'navy' and 'gold'. You'll know it worked when the board shows two bold colors instead of grayscale.

2. Resize the squares. Change size = 40 to size = 30. The board stays 8×8 but is smaller. Try adjusting the x/y formulas to keep it centered. You'll know it worked when all 64 squares still appear.

3. Go bigger. Change range(8) to range(10) in both loops and adjust starting positions. You'll know it worked when you have a 10×10 grid (100 squares).

4. Use random colors. Add import random and replace the color line with color = random.choice(['red','blue','green','purple','orange']). You'll know it worked when each square is a different random color.

Excellent Work!

You mastered nested loops, coordinate arithmetic, and modulo-based color selection — three of the most powerful tools in all of programming! Up next: Diagonal Stripe Field to explore line drawing with setheading().