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This module is one in a collection of modules on Python designed for teaching ITSE 1359 Introduction to Scripting Languages: Python at Austin Community College in Austin, TX.
You have learned how to construct while loops, for loops, and if statements in recent modules on control flow in Python.
In this module, you will learn that loops can be nested and how to nest them.
I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand the Listings while you are reading about them.
(Note to blind and visually impaired students: most of the Figures and all of the Listings inthis module are presented in plain text format and should be accessible using an audio screen reader or a braille display. Note however that the requiredindentation may not be properly represented by an audio screen reader.)
Python allows you to nest while loops and for loops inside of while loops and for loopsto a greater depth than you are likely to need.
Figure 1 shows the general syntax for nested while loops.
| Figure 1 . Syntax for nested while loops. |
|---|
while expression:
while expression:while expression:
statement(s)statement(s)
statement(s) |
Figure 2 shows the general syntax of nested for loops.
| Figure 2 . Syntax for nested for loops |
|---|
for var in sequence:
for var in sequence:statements(s)
statements(s) |
The syntax can become more complicated than that shown in either Figure 1 or Figure 2 . This is particularly true when you mix while loops and for loops in a nested construct.
Listing 1 shows a program with a while loop nested inside of a for loop which in turn is nested inside of another while loop.
| Listing 1 . An example of nested while and for loops. |
|---|
# Illustrates nested loops--------------------------------------------------
leftDigit = 0rightDigit = 0
while leftDigit<3:
for middleDigit in range(3):rightDigit = 0
print("") #blank linewhile rightDigit<3:
print(str(leftDigit) + "-" + str(middleDigit) + "-" + str(rightDigit))rightDigit += 1
#end inner while loop#end for loop
leftDigit += 1#end outer while loop |
Figure 3 shows the output from the code in Listing 1 . Note that the blank lines were inserted to make the material in Figure 3 easier to read.
| Figure 3 . Output from the code in Listing 1. |
|---|
0-0-0
0-0-10-0-2
0-1-00-1-1
0-1-20-2-0
0-2-10-2-2
1-0-01-0-1
1-0-21-1-0
1-1-11-1-2
1-2-01-2-1
1-2-22-0-0
2-0-12-0-2
2-1-02-1-1
2-1-22-2-0
2-2-12-2-2 |
This program simulates a scaled-down version of the odometer in your car. This odometer, however, only has three digits and each digit is limited tothe values of 0, 1, and 2. In other words, Each digit of the odometer rolls over to 0 when its value exceeds 2.
The program begins by creating two working variables named leftDigit and rightDigit and initializing their values to zero. These are essentially counter variables for the outermost while loop and the innermost while loop in Listing 1 . They are also the left-most and right-most digits shown in the columns of digits in Figure 3 .
It isn't necessary to create a separate working variable for the middle digit. That variable, named middleDigit is created as part of the for loop statement in Listing 1 .
If you compare the code in Listing 1 with the output in Figure 3 , you will see that the for loop goes through its full range of iterations (3) for each iteration of the outermost while loop.
You will also see that the innermost while loop goes through its full range of iterations (3) for every iteration of the for loop. As a result, the innermost while loop goes through nine iterations for every iteration of the outermost while loop.
When you think about the behavior of nested loops, think of an odometer. That may help you to remember how they behave.
I recommend that you create a visualization for the code in Listing 1 and step through the program one instruction at a time. As you do that, pay attention to the movements of thered and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to better understand the behaviorof nested loops.
I also encourage you to copy the code from Listing 1 . Execute the code and confirm that you get the same results as those shown in Figure 3 . Experiment with the code, making changes, and observing the results of your changes. Make certain that youcan explain why your changes behave as they do. For example, rewrite the program using all while loops, all for loops, or some other combination of while loops and for loops.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
-end-
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