Changing Colors with If-Else

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

• Write an if/else statement to make a choice in your code
• Use the modulo operator % to test whether a number is odd or even
• Draw a square with alternating red and blue sides

What if you want the turtle to draw some sides in red and others in blue — automatically? An if/else statement makes a choice every time the loop runs, so each side can be a different color without any extra work from you.

Welcome to This Lesson!

Loops repeat, but what if you need something to be different on some repetitions? That's what if/else is for! Today you'll teach Monty to pick colors automatically. Let's code it together!

If-Else: Making Choices

An if/else statement runs one block of code when a condition is true and a different block when it's false:

if condition:
    # runs when condition is True
else:
    # runs when condition is False

To alternate colors every other side, you need to know whether the loop counter i is odd or even. The modulo operator % gives you the remainder after division. i % 2 is: - 0 when i is even (0, 2, 4, …) - 1 when i is odd (1, 3, 5, …)

Scratch Bridge

In Scratch you used the "If … then / else" block to make a choice. In Python, if condition: / else: does exactly the same thing — just written as text!

Sample Code

import turtle
monty = turtle.Turtle()
monty.shape('turtle')

distance = 100
angle = 90

for i in range(4):
    if i % 2 == 0:
        monty.color('red')
    else:
        monty.color('blue')
    monty.forward(distance)
    monty.right(angle)

What Do You Think Will Happen?

i goes 0, 1, 2, 3. Which values make i % 2 == 0 true? Will the first side be red or blue? Make your guess — then click Run!

Try It Now

Edit the code below and click Run to see the result right on this page. No account needed — everything runs in your browser.

▶ Run ↻ Reset

Were you right? When i is 0 or 2, i % 2 == 0 is true, so those sides are red. When i is 1 or 3, it's false, so those sides are blue — alternating all the way around the square.

How It Works

The loop counter i starts at 0. On each repetition the if condition is checked: is i % 2 equal to 0? If yes, the color becomes red. If no, the color becomes blue. After the if/else block, forward and right always run — they're at the same indentation level as the if, not inside either branch.

Explanation Table

Line	What it does
for i in range(4):	Repeats 4 times; i is 0, 1, 2, then 3
if i % 2 == 0:	True when i is even (0, 2); false when i is odd (1, 3)
monty.color('red')	Runs only when the condition is true (indented 8 spaces — inside the if)
else:	What to do when the condition is false
monty.color('blue')	Runs only when the condition is false
monty.forward(distance)	Always runs — at the same level as if, not inside either branch

Indentation Is the Key

Lines inside the if branch need 8 spaces (4 for the loop + 4 for the if). Lines inside the else branch also need 8 spaces. monty.forward(distance) needs only 4 (it's in the loop but not in either branch). Mix these up and the wrong lines will be inside the wrong blocks!

Learning Check

Your Turn — Add the Missing Lines

The program below draws a hexagon where every third side should be green and the other sides should be orange. The condition uses i % 3 == 0. Add the two missing color lines — one inside the if and one inside the else!

▶ Run ↻ Reset

Experiments

Try these changes to the original alternating-color square. Predict first!

1. Swap the colors — put 'blue' in the if branch and 'red' in the else branch. You'll know it worked when the first side is blue instead of red.

2. Change monty.color('red') to monty.pensize(6) (remove the color line and add pensize). You'll know it worked when alternating sides are thick while the others stay thin.

3. Change range(4) to range(6) and angle to 60 to draw a hexagon. You'll know it worked when you see a 6-sided shape with alternating colors.

4. Change the condition from i % 2 == 0 to i > 1. You'll know it worked when the first two sides are one color and the last two are another.

5. Add a third color: use elif i % 3 == 1: between the if and else blocks. You'll know it worked when sides cycle through three colors.

Great Work!

You can now make decisions inside a loop — that's the combination most real programs are built on! Next up: you'll create your own functions to organize your drawing code. Let's code it together!