Looping turtles

Earlier, our turtle has drawn a square according to our instructions. But our instructions repeated themselves: Walk 100 steps, turn 90 degrees left, and then do the same thing four times, once for each side of the square.
But this repetition must mean one thing: We can use a loop!

Repeated sides

If we can find a pattern in what we're trying to do, we can express it much simpler and more powerful. When we draw the square we're doing the same thing four times:

# Walk forward 100 steps
# Turn 90 degrees to the left
# Walk forward 100 steps
# urn 90 degrees to the left
# Walk forward 100 steps
# Turn 90 degrees to the left
# Walk forward 100 steps
# Turn 90 degrees to the left

That means we'd rather express it as such:

for i in range(0, 4):
    # Walk forward 100 steps
    # Turn 90 degrees to the left

A complete program can therefore be this short::

from turtle import *
for i in range(0, 4):


In science, mathematics, and programming, finding a pattern and writing something general about that is a strength - finding one formula that expresses a deep connection.

Earlier, we wrote a program to pick what shape the turtle should draw and had square and triangle as alternatives. But what if we wanted more?

The angle sum of a regular geometric shape with n sides can be calculated with the following formula:

$$\texttt{Angles sum}=180\cdot(n-2)$$

That means each inner angle, with n sides, is:

$$\text{Angle} = \frac{\texttt{Angles sum}}{n}=\frac{180\cdot(n-2)}{n}$$

The turtle needs to turn the outer angle of this, so we need to subtract this from 180:

$$\text{Angle} = 180 - \frac{\texttt{Angles sum}}{n}=180 - \frac{180\cdot(n-2)}{n}=180\cdot\frac{2}{n}$$

Using this formula, the turtle can draw any regular geometric shape!

We start by including the parts we need:

import turtle 


Then, we need to ask how many sides to draw:

n = int(input("How many sides?"))

We can even ensure that it's not wrong. For example, the user might enter 2 or 1, or even a negative number. That's not enough for a proper geometric figure. We need at least 3 sides, and we can solve that with an if-statement:

if n < 3:
    print("You must enter at least 3 sides")
    n = 3

Once we know the number of sides we can use the formula above to calculate how much to turn:

angle = 180*(2/n)

What's left? We use the same loop as before, but with the variable angle instead of 90:

for i in range(0, n): 

Here's the complete program:

But this code is missing comments. Where would you add them?

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In this video, we use for-loops to draw more intricate turtle patterns.

  • Formula: Is a mathematical expression that describes a specific law or rule.
  • If-statement: The keyword "if" followed by a logical condition allows our program to make decisions.