Goals

• Understand and apply for-loops
• Use counting variables meaningfully
• Program more complex geometric patterns

For-Loops

You surely remember how you sent the robot Karol through the small worlds. Often Karol had to execute certain instructions several times in a row. Instead of typing each instruction individually, we used loops. You surely remember the following example:

repeat 5 times 
Step
TurnRight
endrepeat

Loops exist not only with Karol, but also with Tobi the turtle. Here they look somewhat different but serve the same purpose. If you want to repeat instructions a fixed number of times (Karol: repeat 5 times), you use the for i in range(start, end): command in Python.

import turtle
t = turtle.Turtle()

for i in range(1, 10):
    t.write(i)
    t.forward(10)
t.forward(10)

Here i is a variable that can be used just like the variables from Block 1. Important: it's always increased by 1 at the end of the loop. It's often called a counting variable.

Important Note

The counting variable i equals the first number in range(start, end) at the beginning of the loop. However, i never becomes end, because the loop's termination condition is reached as soon as i reaches end. In our example, i counts up to 9, then is increased by 1 and thus breaks the loop.

Different range() Variants

# Only one number: from 0 to n-1
for i in range(5):      # 0, 1, 2, 3, 4
    t.write(i)

# Two numbers: from start to end-1  
for i in range(2, 8):   # 2, 3, 4, 5, 6, 7
    t.write(i)

# Three numbers: from start to end-1 in steps
for i in range(0, 10, 2):  # 0, 2, 4, 6, 8
    t.write(i)

Geometric Patterns with For-Loops

Simple Shapes

import turtle
t = turtle.Turtle()

# Equilateral triangle
for i in range(3):
    t.forward(100)
    t.left(120)

# Square  
for i in range(4):
    t.forward(80)
    t.left(90)

More Complex Patterns

import turtle
t = turtle.Turtle()

# Spiral
for i in range(50):
    t.forward(i * 2)
    t.left(90)

# Concentric circles
for i in range(1, 6):
    t.circle(i * 20)
    t.penup()
    t.right(90)
    t.forward(20)
    t.left(90)
    t.pendown()

Tasks

Task 1: Zebra Stripes

Draw 5 short, parallel lines next to each other that look like zebra stripes using a loop.

Tip

Use t.forward() and t.penup() / t.pendown() so Tobi can jump without drawing.

Task 2: Staircase Steps

Draw a staircase with 6 steps, each step e.g. 10 steps high and 10 steps wide.

Task 3: Star

Write a loop that draws a 5-pointed star. A star is created when you do the following 5 times: go forward, then turn 144 degrees to the right.

Task 4: Flower Pattern

Draw 8 circles around a center point so they look like a flower.

Tip

After each circle: t.left(45) for even distribution.

Task 5: Rainbow Rectangles

Draw 7 rectangles in different colors next to each other. Each rectangle should have a different color.

colors = ["red", "orange", "yellow", "green", "blue", "indigo", "violet"]

Preparation for Block 5

Think about which parts of your code you use more often and how you could pack them into reusable building blocks.