Lecture on Decision Structures and Boolean Logic

The if Statement

• Controls the order of execution of statements.

• Control Structures:

  • Sequence structure: Executes statements in order.

  • Decision structure: Executes actions based on conditions (Also known as selection structure).

Boolean Expressions and Relational Operators

• Boolean Expression: Tested to check if it is true or false (e.g., a > b).

• Relational Operators:

  • > - greater than

  • < - less than

  • >= - greater than or equal to

  • <= - less than or equal to

  • == - equal

  • != - not equal

The if-else Statement

• Structure:

  • Syntax: if condition: statements else: other statements

  • Executes one of two paths based on condition (true/false).

Comparing Strings

• Strings compared using ==, !=, >, < based on ASCII values.

• Case sensitive: e.g., 'abc' > 'Abc'.

Nested Decision Structures

• A structure containing another decision structure.

• Syntax: Nested if statements should maintain proper indentation.

The if-elif-else Statement

• A simplified version of nested structures allowing multiple conditions.

• Syntax:

if condition_1:  
  statements  
elif condition_2:  
  statements  
else:  
  statements

Logical Operators

• Logical Operators: Create complex Boolean expressions.

  • and: True if both operands are true.

  • or: True if at least one operand is true.

  • not: Reverses Boolean value of the operand.

Checking Numeric Ranges

• Within a Range: Use and (e.g., x \geq 10 \text{ and } x \leq 20).

• Outside a Range: Use or (e.g., x < 10 \text{ or } x > 20).

Boolean Variables

• Represents two values: True or False.

• Often used as flags to signal conditions in programs.

  • Flag set to False: Condition does not exist.

  • Flag set to True: Condition exists.