04.MakingDecisions

Course Information

• Course Title: Computer Science I

• Course Code: COSC 1020

• Institution: Georgetown University

Chapter 4: Making Decisions

• Overview: Understanding flow charts and decision structures.

Key Topics

• Making Decisions

  • Relational operators (4.1)

  • Logical operators (4.7)

  • The if statement (4.2)

  • The if/else statement (4.3)

  • The if/else if statement (4.4)

  • Nested if statements (4.6)

  • More about blocks and scope (4.9)

• User Interfaces

  • Menu-driven programs (4.5)

  • Validating user inputs (4.8)

  • Validating chars and strings (4.10)

  • The conditional operator (4.11)

  • The switch statement (4.12)

  • Enumerated data types (4.13)

Relational Operators

• Function: Test the relationship between two values, often referred to as comparison operators.

• Types of Relational Operators:

  • Testing Equality:

    • Equals: ==

    • Not Equals: !=

  • Testing Order (Inequality Operators):

    • Strict Inequality: > (greater than), < (less than)

    • Nonstrict Inequality: >= (greater than or equal to), <= (less than or equal to)

• Returns: Boolean value (true/false)

Common Errors with Relational Operators

• Assignment vs. Comparison:

  • Single equal sign = is for assignment.

  • Double equal sign == is for comparison.

• Floating-point Comparison:

  • Precision issues due to rounding can lead to unexpected false results.

  • Use ranges to check closeness: n is close to 1.1 if ((n – 1.1) <= 1e-5).

Logical Operators

• Usage: Act on boolean values and expressions.

• Types:

  • AND (&&)

  • OR (||)

  • NOT (!)

Writing Logical Expressions

• Must check complete Boolean expressions.

• Example: To check a range, you use:

  • not (a < x < b) but instead do (a < x && x < b).

Logical Operator Precedence

• Favor clarity with parentheses.

• Precedence sequence:

  1. NOT - evaluated first

  2. AND - evaluated left to right

  3. OR - evaluated left to right

The

if Statement

• Structure:

  • A condition (boolean expression).

  • A block of statements executed if the condition is true.

• Syntax:

if (boolean_expression) {
  // statements if true
}

The if/else Statement

• Concept: Mutually exclusive execution: either block 1 or block 2 runs, but never both.

• Structure:

  • If condition is true, execute block 1.

  • If false, execute block 2.

The if/else if Statement

• Allows checking multiple mutually exclusive conditions.

• Example:

if (first_condition) {
  // true block
} else if (second_condition) {
  // second block
} else {
  // fallback block
}

Nested if Statements

• Used for follow-up questions based on prior answers.

• Example: Check user ID for age verification.

The Switch Statement

• A control structure for making choices among integer options.

• Includes cases and can have a default case.

• Usage Example:

switch(variable) {
  case 'a':
    //...
    break;
  default:
    //...
}

Enumerated Data Types

• Definition: A programmer-defined data type with specific values.

• Allows for distinct variables that are meaningful.

• Declaration Example:

enum Color {red, green, blue};  
Color favorite = green;

User Inputs and Enumerations

• User inputs are translated into enumerated values for processing.

• Offers flexibility in implementing logic based on user preferences.

Conclusion

• Decision-making structures like if, switch, and enumerations help design interactive and flexible programs.