Loops

Introduction

Something you might have noticed: our programs often feature repetition. Python has a powerful concept it makes use of called looping (jargon: iteration), which we can use to cut out our reptitive code! For now, try this easy example:

for name in "John", "Sam", "Jill":
    print("Hello " + name)

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 that action once. Here’s another version of a loop:

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

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

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.

You can also loop over elements of your choice:

total = 0
for i in 5, 7, 11, 13:
    print(i)
    total = total + i

print(total)

Write this example out and run it with python, to check it works how you might expect.

Sometimes you want to repeat some code a number of times, but don’t care about the value of the i variable; so it can be good practice to replace it with _ instead. This signifies that we don’t care about its value, or don’t wish to use it. Here’s a simple example:

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

You may or may not be wondering about the variable i - why is it used all the time above? Well, it simply stands for “index” and is one of the most common variable names ever found in code. But if you are looping over something other than just numbers, be sure to name it something better! For instance:

for drink in list_of_beverages:
    print("Would you like a " + drink + "?")

This is immediately clearer to understand than if we had used i instead of drink.

Drawing a dashed line

Exercise

Draw a dashed line. You can move the turtle without the turtle drawing its movement by using the turtle.penup() function; to tell it to draw again, use turtle.pendown().

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)
    # write more code here

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

Comments

In the example above, the line that starts with a # is called a comment. In Python, anything that goes on a line after # is ignored by the computer. Use comments to explain what your program does, without changing the behaviour for the computer. They can also be used to easily and temporarily disable, or “comment out” some lines of code.

Comments can also go at the end of a line, like this:

turtle.left(20)     # tilt our next square slightly

More Efficient Squares

for _ in range(4):
    turtle.forward(100)
    turtle.left(90)

for ...:
    for ...:
        # drawing code inside the inner loop goes here
        ...
    # you can put some code here to move
    # around after!
    ...