COP 1000c Lab Assignment 7

amunoz1030

GaddisPythonChapter3DecisionStructuresandBoolean.ppt

Home >Computer Science homework help >COP 1000c Lab Assignment 7

C H A P T E R 3

Decision Structures and Boolean Logic

Topics

The if Statement
The if-else Statement
Comparing Strings
Nested Decision Structures and the if-elif-else Statement
Logical Operators
Boolean Variables
Turtle Graphics: Determining the State of the Turtle

The if Statement

Control structure: logical design that controls order in which set of statements execute
Sequence structure: set of statements that execute in the order they appear
Decision structure: specific action(s) performed only if a condition exists
Also known as selection structure

The if Statement (cont’d.)

In flowchart, diamond represents true/false condition that must be tested
Actions can be conditionally executed
Performed only when a condition is true
Single alternative decision structure: provides only one alternative path of execution
If condition is not true, exit the structure

The if Statement (cont’d.)

Python syntax:

if condition:

Statement

First line known as the if clause
Includes the keyword if followed by condition
The condition can be true or false
When the if statement executes, the condition is tested, and if it is true the block statements are executed. otherwise, block statements are skipped

Boolean Expressions and Relational Operators

Boolean expression: expression tested by if statement to determine if it is true or false
Example: a > b
true if a is greater than b; false otherwise
Relational operator: determines whether a specific relationship exists between two values
Example: greater than (>)

Boolean Expressions and Relational Operators (cont’d.)

>= and <= operators test more than one relationship
It is enough for one of the relationships to exist for the expression to be true
== operator determines whether the two operands are equal to one another
Do not confuse with assignment operator (=)
!= operator determines whether the two operands are not equal

Boolean Expressions and Relational Operators (cont’d.)

Using a Boolean expression with the > relational operator

Boolean Expressions and Relational Operators (cont’d.)

Any relational operator can be used in a decision block
Example: if balance == 0
Example: if payment != balance
It is possible to have a block inside another block
Example: if statement inside a function
Statements in inner block must be indented with respect to the outer block

The if-else Statement

Dual alternative decision structure: two possible paths of execution

One is taken if the condition is true, and the other if the condition is false

Syntax: if condition:

statements

else:

other statements

if clause and else clause must be aligned
Statements must be consistently indented

The if-else Statement (cont’d.)

Comparing Strings

Strings can be compared using the == and != operators
String comparisons are case sensitive
Strings can be compared using >, <, >=, and <=
Compared character by character based on the ASCII values for each character
If shorter word is substring of longer word, longer word is greater than shorter word

Comparing Strings (cont’d.)

Nested Decision Structures and the if-elif-else Statement

A decision structure can be nested inside another decision structure
Commonly needed in programs
Example:
Determine if someone qualifies for a loan, they must meet two conditions:
Must earn at least $30,000/year
Must have been employed for at least two years
Check first condition, and if it is true, check second condition

Nested Decision Structures and the if-elif-else Statement (cont’d.)

Important to use proper indentation in a nested decision structure
Important for Python interpreter
Makes code more readable for programmer
Rules for writing nested if statements:
else clause should align with matching if clause
Statements in each block must be consistently indented

The if-elif-else Statement

if-elif-else statement: special version of a decision structure
Makes logic of nested decision structures simpler to write
Can include multiple elif statements
Syntax:

if condition_1:

statement(s)

elif condition_2:

statement(s)

elif condition_3:

statement(s)

else

statement(s)

Insert as many elif clauses

as necessary.

The if-elif-else Statement (cont’d.)

Alignment used with if-elif-else statement:
if, elif, and else clauses are all aligned
Conditionally executed blocks are consistently indented
if-elif-else statement is never required, but logic easier to follow
Can be accomplished by nested if-else
Code can become complex, and indentation can cause problematic long lines

Logical Operators

Logical operators: operators that can be used to create complex Boolean expressions
and operator and or operator: binary operators, connect two Boolean expressions into a compound Boolean expression
not operator: unary operator, reverses the truth of its Boolean operand

The and Operator

Takes two Boolean expressions as operands
Creates compound Boolean expression that is true only when both sub expressions are true
Can be used to simplify nested decision structures
Truth table for

the and operator

Expression	Value of the Expression
false and false	false
false and true	false
true and false	false
true and true	true

The or Operator

Takes two Boolean expressions as operands
Creates compound Boolean expression that is true when either of the sub expressions is true
Can be used to simplify nested decision structures
Truth table for

the or operator

Expression	Value of the Expression
false and false	false
false and true	true
true and false	true
true and true	true

Short-Circuit Evaluation

Short circuit evaluation: deciding the value of a compound Boolean expression after evaluating only one sub expression
Performed by the or and and operators
For or operator: If left operand is true, compound expression is true. Otherwise, evaluate right operand
For and operator: If left operand is false, compound expression is false. Otherwise, evaluate right operand

The not Operator

Takes one Boolean expressions as operand and reverses its logical value
Sometimes it may be necessary to place parentheses around an expression to clarify to what you are applying the not operator
Truth table for the not operator

Expression	Value of the Expression
true	false
false	true

Checking Numeric Ranges with Logical Operators

To determine whether a numeric value is within a specific range of values, use and

Example: x >= 10 and x <= 20

To determine whether a numeric value is outside of a specific range of values, use or

Example: x < 10 or x > 20

Boolean Variables

Boolean variable: references one of two values, True or False
Represented by bool data type
Commonly used as flags
Flag: variable that signals when some condition exists in a program
Flag set to False  condition does not exist
Flag set to True  condition exists