COP 1000c Lab Assignment 3

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GaddisPythonChapter3DecisionStructuresandBoolean.pdf

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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 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

Value of the Expression

Expression

falsefalse and false falsefalse and true falsetrue and false truetrue and 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

Value of the Expression

Expression

falsefalse and false truefalse and true truetrue and false truetrue and 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

Value of the Expression 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