Lecture 4

Objectives for today

1. Explain when loops are used
2. Describe the execution of a for loop
3. Use a for loop to iterate through a range

Turtle

In the second programming assignment we are going to use the turtle module. turtle mimics an old programming language Logo (Logo was actually my first programming language).

from turtle import *


Recall this imports all the functions, etc. from the turtle module into our symbol table.

turtle works by moving a “pen” around the screen, e.g.

forward(100)
right(90)
forward(100)
right(90)
forward(100)
right(90)
forward(100)
right(90)


for loops

That repetition is very tedious (and not very DRY). We know we want move and turn 4 times. Can we loop over those statements 4 times? Yes. With a for loop. A for loop does exactly what it sounds like. For a specific set of iterations, execute the enclosed statements. Note that we also convert that fixed side length into function parameter that we can change.

def draw_square_with_loop(side_length):
for i in range(4):
forward(side_length)
right(90)


Note that “for” and “in” are reserved words (just like def) and therefore they cannot be used as variable or function names.

A for-loop generally:

1. Defines a loop variable that is set each iteration. For indexes, the convention is to use i, j, and k although you can and should use more descriptive variable names when relevant.
2. The loop variable is set from some sequence. In this case the range function generates a sequence of integers starting at 0 up to but not including the supplied stop parameter, e.g. 0, 1, 2, 3. So there will be 4 iterations. As we will see in future classes there are other kinds of sequences.
3. Each iteration the loop variable is set to the next value from the sequence and the for-loop body is executed.

Let’s look at another example using loops, this time to print numbers:

def print_loop(n):
""" Print numbers from 0 until (but not including) n """
print("Begin list of numbers")
for i in range(n):
print(i)
print("End list of numbers")

>>> print_loop(5)
Begin list of numbers
0
1
2
3
4
End list of numbers


We can use the above output to remind us that there are three “regions” in and around our loop:

1. Before the loop
2. Inside the loop body
3. After the loop

The statements inside the loop body will execute on every loop iteration, while the “after” statements will only execute once and only after all of the loop iterations are complete.

PI Questions (For loops)1

Recall in the first lecture that we said the computers were the right tool for the job when we needed to do something more than once. As you might imagine for-loops are one of our key tools for doing anything more than once. For example, when performing a computation “for each” data point in a file, or simulating a process “for” a set of different inputs.

Warning about turtle errors

You may encounter errors with turtle, including rendering again after a bye() call. The graphics system that underlies turtle can only be “fired” once per Python console session. You can restart the Python console by clicking the “stop” sign to restart the Shell. Another warning, don’t name your file “turtle.py”, doing so will prevent Python from finding the built-in turtle functions.

Turtle Enhancements

We saw we can control the direction and angle of the drawing pen. What else can we control to enhance our square? Lets check out the docs. Here we change the color of the line to be red and set the color of the interior of our square to be yellow. The key for the “fill” is invoking begin_fill before you draw the shape and then end_fill after you draw the shape.

pencolor("red")
fillcolor("yellow")
begin_fill()

draw_square_with_loop(100)

end_fill()


What about more interesting shapes? Specifically a Fibonacci spiral. A Fibonacci spiral is created by inscribing quarter circles inside squares whose edge length increases as the Fibonacci sequence. (Show code.)

def golden_spiral(radius, segments):
"""
Draw a Fibonacci spiral using Turtle. turtle package must be imported into namespace.

Args:
segments: Number of quarter circle segments to draw after initial quarter circle.
Must be >= 2.

Returns:
None
"""
for i in range(segments-2):
c = a + b
circle(c, 90)
a = b
b = c


Note that if we invoke the golden_spiral function several times, each new spiral starts from where the last one finished. This reminds us that the pen has “state”, specifically an x and y position and a heading. Controlling that state is a key part of the programming assignment.

Here are many more turtle examples for you to review in preparation for the programming assignment.

Summary

1. Turtle
2. Basic for loops
3. Turtle in more detail