Loops

Introduction

One more thing: Our programs often featured repetition. There is a powerful concept in Python called looping, which we will elaborate later on. For now, try this easy example:

for i in range(10):
    print("Hello!")

This is incredibly helpful if we want to do something multiple times — say, drawing the individual border lines of a shape — but only want to write it once. But it gets better:

for i in range(10):
    print(i)

Notice how we write only one line of code using i but it has 10 different values?

You can also loop over elements of your choice:

for i in 5, 7, 11, 13:
    print(i)

The range(n) function can be considered a shorthand for 0, 1, 2, ..., n-1. If you want to know more about it, you can use the help in the Python shell by typing help(range). Use the q key to exit the help again.

Drawing a dashed line

Exercise

Draw a dashed line. You can move the turtle without tracing a line behind you with the turtle.penup() function; put it back on the ground with turtle.pendown().

Solution

for i in range(10):
    turtle.forward(15)
    turtle.penup()
    turtle.forward(5)
    turtle.pendown()

Bonus

Can you make the dashes become larger as the line progresses?

Hint

Feeling lost? Inspect i at every run of the loop:

for i in range(10):
    print(i)
    turtle.forward(15)
    turtle.penup()
    turtle.forward(5)
    turtle.pendown()

Can you utilize i — commonly called the index variable or loop variable — to get increasing step sizes?

Honeycomb loops

Exercise

Take your honeycomb program and make it easier with loops. How small can you get it?

Solution

def hexagon():
    for i in range(6):
        turtle.forward(100)
        turtle.left(60)

for i in range(6):
    hexagon()
    turtle.forward(100)
    turtle.right(60)