Program Control Structures: Conditional Control Structures

Program Control Structures

• Flow of Control: Defines the order in which statements are executed during program runtime.
• Sequential Flow: The default execution mode where instructions execute consecutively in the order written.
• Selection: Alters execution path by choosing specific statements based on whether a condition evaluates to true or false.
• Repetition: Repeats the execution of a block of statements based on loop conditions.
• Invocation: Uses a single statement to call and execute an external block of instructions, such as a function.

Relational Operators and Expressions

• Relational Expression (Condition): An expression comparing two operands using a relational operator, evaluating to boolean true ($1$ or any non-zero value) or false ($0$).
• Relational Operators:
  • <: Less than
  • >: Greater than
  • <=: Less than or equal to
  • >=: Greater than or equal to
  • ==: Equal to
  • !=: Not equal to
• Character Comparisons: Characters are evaluated using their underlying numeric ASCII decimal values (for example, 'B' < 'a' evaluates to true / $1$ because ASCII 'B' is $66$ and 'a' is $97$).

Logical Operators

• Complex Conditions: Combine or invert relational expressions using boolean logic:
  • Logical AND (&&): Evaluates to true ($1$) only if both operands are true; otherwise evaluates to false ($0$).
  • Logical OR (||): Evaluates to true ($1$) if at least one operand is true; evaluates to false ($0$) only if both operands are false.
  • Logical NOT (!): Unary operator that reverses the truth value of an expression (transforms true to false and false to true).

Conditional Selection Statements

• Single if Statement:
  • Executes a single statement or compound block enclosed in braces { } if the condition evaluates to true.
  • Bypasses the controlled block entirely if the condition evaluates to false ($0$).
• if-else Statement:
  • Provides a two-way branch: executes the first statement/block if true, or the else block if false.

• if-else if Ladder:
  • Evaluated sequentially from top to bottom.
  • Once any condition evaluates to true, its corresponding statement executes and all remaining ladder branches are bypassed.
  • If none of the conditions evaluate to true, the trailing else statement executes.
• Nested if Statements:
  • Placement of an if or if-else block inside another if or else branch.
  • Dangling Else Pairing Rule: By default, the compiler pairs an else with the closest preceding unpaired if, unless explicit curly braces { } dictate otherwise.

The Switch Statement

• Multi-Way Equality Branching: Evaluates an integral or character expression against a series of defined case constant labels.
• Data Type Restrictions:
  • Allowed: Integers (int) and individual characters (char).
  • Prohibited: Floating-point types (float, double) and strings/words.
• Control Flow Elements:
  • break: Terminates the switch construct; omitting break causes unintentional fall-through execution into subsequent cases.
  • default: An optional branch that executes if the evaluated expression does not match any specified case constant.

Review Questions & Conceptual Checks

• Divisibility Determination: The modulus operator (%) checks integer divisibility (for example, no % 2 == 0 evaluates whether an integer is even).
• Switch Fall-Through: Removing a break statement causes execution to cascade directly into subsequent case blocks regardless of their match status.
• Omitting Default: If no matching case exists and default is absent, the entire switch statement exits without executing any branch.
• Case Syntax: Removing single quotation marks around a character label (e.g., using case s: instead of case 's':) causes a syntax/compilation error as the compiler attempts to resolve an undeclared identifier.