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CS101 Lecture 25: Python: Repetition Patterns: Counter-Controlled, Interactive, and Sentinel Aaron Stevens 30 March 2009 1 Overview/Questions Review: Flow of Control The idea of iteration: how to conditionally repeat statements

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Aaron Stevens

30 March 2009

CS101 Lecture 25: Python: Repetition

Patterns: Counter-Controlled, Interactive, and Sentinel

Overview/Questions

– Review: Flow of Control – The idea of iteration: how to conditionally repeat statements within a programs. – Strategies for repetition.

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Flow of Control

Sequential Execution Each instruction is executed in order they are written (after the previous one, before the next on). Functions Enable procedural decomposition. Repeat statements by calling functions multiple times.

Flow of Control

Selection Some statements are executed while others are not. Repetition Statements can be repeated some fixed number of time, or else can be repeated until some event signals they should not be repeated any more.

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Review Logical Expressions

Logical Expression Programming statement which achieves a Boolean logical result (True or False). Example: using if-elif-else statements

Repetition in Action

Everybody’s favorite drinking song…

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The Indefinite Loop

The indefinite loop is controlled by a conditional expression.

while statement A structure which evaluates a logical expression, and repeats a controlled block of statements.

while <condition>: <body>

Patterns for Indefinite Loops

Several common patterns:

 Counter-controlled Loop  Interactive Loop  Sentinel-controlled Loop

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Counter-Controlled Pattern

Counter-Controlled Loop Evaluates a counter in loop’s logical expression: Pseudo-code for this pattern: initialize counter while counter > critical-value do controlled block statements decrement (increment) counter

Counter-Controlled Pattern

Example, revisited: Discuss: initialize counter, logical expression, counter update.

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Counter-Controlled Pattern

Example: Calculating factorial

Counter-Controlled Pattern

Example: Calculating an Average

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Interactive Loop Pattern

Interactive Loop Prompts user whether or not to continue. Pseudo-code for this pattern: set moredata to “yes” while moredata is “yes” get next data item process data item ask user if there is moredata

Interactive Loop Pattern

Example: averaging quiz grades

– indefinite loop version, interactive pattern.

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Sentinel Loop Pattern

Sentinel Loop Checks input for a special value to determine whether or not to continue. get first data item while data item is not the sentinel process data item get next data item

Sentinel Loop Pattern

Example: averaging quiz grades

– indefinite loop version, sentinel pattern

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Take-Away Points

– Indefinite Loops – Counter-controlled loops – Interactive loops – Sentinel-controlled loops

Student To Dos

– HW10 due Tuesday 3/31 – Reading: How to Think Like A Computer Scientist: Learning with Python

Available online at: http://openbookproject.net//thinkCSpy/

Ch06 (Monday/Wednesday)
Ch09, 9.1-9.7, 9.13 (Friday)