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This module explains the Python loop modifiers: else, continue, break,and pass.

Table of contents

Preface

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.

What you have learned

You have learned how to construct while loops, for loops, and if statements in recent modules on control flow in Python. In addition, you recently learned how to create nested loop structures .

What you will learn

In this module, you will learn how to add an else clause to while loops and for loops. In addition, once you start using nested loops, you need to know how to use the loop modifiers: continue and break . Those loop modifiers will be explained in this module.

Finally, this module will make a brief mention of the pass statement, which does nothing. If you need to write code that does nothing, the pass statement is for you.

Viewing tip

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.)

Figures

  • Figure 1 . Output from the code in Listing 1.
  • Figure 2 . Output from the code in Listing 2.
  • Figure 3 . Output from the code in Listing 3.

Listings

  • Listing 1 . Loops with else clauses.
  • Listing 2 . Nested loops with a continue statement.
  • Listing 3 . Nested loops with a break statement.

Discussion and sample code

The else clause

You learned about the else clause on an if statement in an earlier module titled Itse1359-1230-The if Statement .

Loop statements can also have an else clause. The else clause on a for loop is executed when the loop terminates through exhaustion of the elements in the list.The else clause on a while loop is executed when the loop terminates because the condition becomes False . The else clause is not executed when the loop is terminated by a break statement.

I will illustrate the else clause using a program that is similar to the odometer program that you learned about in the earlier module titled Itse1359-1250-Nested Loops . The program is shown in Listing 1 .

Listing 1 . Loops with else clauses.
# Illustrates nested loops with else clauses #---------------------------------------------------------------------------leftDigit = 0 rightDigit = 0#Begin outer while loop while leftDigit<2: leftDigit += 1#Begin for loop for middleDigit in range(1,3):rightDigit = 0 #Begin innermost while loopwhile rightDigit<3: rightDigit += 1print(str(leftDigit) + "-" + str(middleDigit) + "-" + str(rightDigit)) else:print("In else clause on inner while loop") #end inner while loop with else clauseelse: print("In else clause on for loop")#end for loop with else clause else:print("In else clause on outer while loop") #end outer while loop with else clauseprint("Goodbye")

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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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