Python Loops and Selections: Chapter 3

Objectives

• Chapter 3 focuses on loops and selection statements in Python.
• The objectives cover writing different types of loops (fixed number, character traversal, counting up/down).
• Also covered are entry-controlled loops, selection statements, Boolean expressions, and exiting loops.

Definitive Iteration: The for Loop

• Repetition statements, also known as loops, allow for repeating an action.
• Each instance of an action being repeated is a pass or iteration.
• Loops can be definite (repeating a predefined number of times).
• Loops can be indefinite (performing an action until the program determines it needs to stop).

Executing Statements a Given Number of Times

• The for loop is a control statement in Python that easily facilitates definite iteration.
• Basic structure: `for in range(): ...
• The first line of code in a for loop is the loop header.
• The rest of the code, known as the loop body, must be indented and aligned in the same column.

Example: Exponentiation Loop

• Computes the exponentiation for a non-negative exponent.
• If the exponent is 0, the loop body does not execute and the value of product remains 1.

Count-Controlled Loops

• Loops can count through a range of numbers using the range function.
• Explicit lower bound can be specified in the range.

Example: Bound-Delimited Summation

• Summing numbers within a specified range.

Augmented Assignment

• The assignment symbol can be combined with arithmetic and concatenation operators to provide augmented assignment operations.
• a += 3 is equivalent to a = a + 3.
• Format: <variable> <operator>= <expression>.
• This is equivalent to: <variable> = <variable> <operator> <expression>.

Loop Errors: Off-by-One Error

• Example of counting from 1 through 4.
• The loop actually counts from 1 through 3 if range(1,4) is used.
• This represents a logic error, not a syntax error.

Traversing Data Sequence Contents

• range returns a list.
• Strings consist of character sequences.
• Values in a sequence can be visited with a for loop: for <variable> in <sequence>: <do something with variable>.

Specifying Steps in the Range

• range expects a third argument to specify a step value.
• theSum gives the sum, based on the specified condition.

Loops that Count Down

• Example given for a for loop that counts from 10 to 1.

Formatting Text Output

• Many data-processing applications output data in tabular format.
• Field width refers to the total number of data characters and additional spaces for a datum in a formatted string.
• It automatically starts in the first column

Formatting Text Output Examples using format % operator

• This version uses format strings and format operators but a single data value will be generated
• d is used for integer formatting, instead of s
• Can also be used to show a sequence of calculations

Formatting Text Output using 'Float" Type

• Use %.f to format as a float and . is optional

Selection: If and If-Else Statements

• Selection statements enable computers to make choices.
• The choices are based on a condition.

Boolean Type, Comparisons, Boolean Expressions

• Boolean data types include two values: true and false.
• Comparison Operators ( e.g. ==, !=, <, >, <=, >=) provide a "Meaning"

If-Else Statements (1 of 2)

• A two-way selection statement used to check inputs for errors.

If-Else Statements (Syntax)

• if :
• else:

One-Way Selection Statements

• Simplest form of selection is the if statement:
• if <condition>: <sequence of statements>

Multi-Way If Statements

• Programs test conditions that entail more than two alternative courses of action.
• In programming, referred to often as a multi-way selection statement.

Multi-Way If Statements(2 of 3)

• Example of the program to convert number grades to letter grades
• if condition 1 is met do first
• elif condition 2 is met, do second
• else do the default statements if nothing else works

Logical Operators and Compound Boolean Expressions

• If either of two courses of action is true, below are two approaches to resolve
• Youcan use elif to go through each statement but it can become complex
• Or you can simplify and use or in the comparison statement

Logical Operators and Compound Boolean Expressions (Truth Table)

• Some claims can be made from previous tables.
• Use A = True, B= False to show AND, NOT, OR statement
• Can be used with variable assignment

Logical Operators and Compound Boolean Expressions (Operator Precendence)

• Remember not has higher precedence than and, oror

Short-Circuit Evaluation

• In (A and B), if A is false, the expression is false
• There is no need to run B
• In (A or B), if A is true, the expression is true
• Example is given to count a number, and show the evaluation if a condition is or is not completed

Conditional Iteration: The "While" Loop

• while loop describes the conditional iteration code
• Example is given to take any string input until the use enters a sentinel value "" to quit and end the loop

The Structure and Behavior of a While Loop

• Called a continuation loop when it should continue with another condition
• Improper use may lead to an infinite loop
• Can test conditions at the top of the loop and only execute 0 or more times

The Structure and Behavior of a While Loop(Data)

• The first input statement initializes a variable to an initial value that the loop condition can test

Count Control with a While Loop

• Can create a count-controlled loop instead of a for loop to iterate through something

The While True Loop and The Break Statement

• The while loop can and should be written correctly for readability
• Example is given with the true as the initial condition to prompt the statements
• It then tests a condition to break from the loop
• Use if data == "": (no input)

The While Loop and the Break Statement (Grade conversion with Loop)

• Used to continuous take grades from user, until the user enters in an acceptable value
• You must break from the loop, or else it will continue indefinitely

The While True Loop and the Break Statement(Bool Loop)

• A boolean variable can be used to control the loop instead of a direct conditional

Random Numbers

• Programming has generating resources for generating random numbers
• the random module provides functions for this

Random Numbers (simple guessing example function)

• used to create a simple random number game
• random.randint* creates a random integer