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.