Decision Structures and Boolean Logic

• Decision Structures and Boolean Logic are covered in the chapter.
• The if statement, if-else statement, comparing strings, nested decision structures and the if-elif-else statement, logical operators, and Boolean variables are discussed.

The if Statement

• A control structure is a logical design that controls the order in which a set of statements executes.
• A sequence structure is a set of statements that execute in the order they appear.
• A decision structure is where specific actions are performed only under certain conditions.
• It is also known as a selection structure.
• In a flowchart, a diamond represents a true or false condition.
• Actions are conditionally executed and performed only if a condition is true.
• A single alternative decision structure provides only one path of execution.
• The structure is exited if the condition is not true.
• Python syntax is "if condition: Statement Statement"
• The first line of the syntax is known as the 'if clause' including the 'if keyword' followed by a condition.
• If a condition executes it gets tested, and if the condition is true, the block statements are executed; otherwise, block statements are skipped.

Boolean Expressions and Relational Operators

• A Boolean expression is determined by the 'if statement' to determine its truth value.
• Relational operators determine whether a specific relationship exists between two values.
• '>=' and '<=' operators test more than one relationship
  • It is enough for one of the relationships to exist for the expression to be true.
• '==' determines whether the two operands are equal to one another.
  • Do not confuse with assignment operator (=).
• '!=' determines whether the two operands are not equal.
• Relational operators can be used in a decision block.
  • Example: If "balance == 0" or "payment != balance".
• A block inside another block is possible.
  • Example: if statement inside a function.
• Statements in inner block must be indented with respect to the outer block.

Conditional Statement in Python Example

• i = 10 is assigned, then the condition i>15 is checked.
  • Since 10 > 15 is False, print("10 is less than 15") statement inside the if block is not executed.
• The next statement print ("Iam not in if") is outside the if block and is always executed, so the system prints "I am not in if".

The if-else Statement

• Dual alternative decision structure has two possible paths of execution, one taken if the condition is true, and the other if the condition is false.
• The syntax "if condition: statements else: other statements"
• The if and else clauses must be aligned, and statements must be consistently indented.
• If the condition is True, the block of statement is executed.
• If the condition is False, this other block of statements is executed.
• In either case the control will jump to the statement following the if-else statement.

Nested Decision Structures and the if-else Statement

• Decision structures can be nested inside one another
  • Commonly used
• A nested decision structure can determine if a customer is eligible for a premium membership at a shopping mall:
  • They must have spent at least $1,000 in total purchases
  • They must have visited the store at least 10 times in the last year
  • The program checks the first condition, and if found to be true, it checks for the second.
• Proper indentation is important in nested decision structures.
  • For the Python interpreter, indentation makes code more readable.
• Rules for writing nested if statements:
  • the else clause should align with matching if clause, and statements in each block must be consistently indented.

The if-elif-else Statement

• An if-elif-else statement is a special version of a decision structure.
• Makes the 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)
• In general, use Alignment is used with the statement if-elif-else
  • If, elif, and else clauses are all aligned, and conditionally executed blocks are consistently indented.
• The if-elif-else statement is never required, but logic is easier to follow.
  • Can be accomplished by nested if-else, but code becomes more complex, and indentation can cause problematic long lines.

Summary of Concepts

• Decision structures
• Single alternative decision structures
• Dual alternative decision structure
• Nested decision structures
• Relational operators and logical operators as used in creating Boolean expressions