B2.3.2 Constructing Programs Utilizing Appropriate Selection Structures Notes

Understanding Selection Structures

1. Selection structures are fundamental programming constructs that allow you to make decisions within your code.
2. They enable your program to execute different blocks of code based on specific conditions.

Types of Selection Structures

1. If Statements: Executes a block of code if a specified condition is true.
2. If-Else Statements: Provides an alternative block of code to execute if the condition is false.
3. Else If (Elif) Statements: Allows multiple conditions to be checked in sequence.
4. Boolean and Relational Operators: Enhance the flexibility of selection structures by allowing complex conditions.

The Role of Selection Structures in Programming

1. Selection structures are essential for controlling the flow of a program.
2. They enable your code to:
   1. Respond to User Input
      1. For example, display different messages based on a user's age.
   2. Handle Different Scenarios
      1. For example, determine the next action in a game based on the player's performance.
   3. Implement Business Logic
      1. For example, calculate discounts based on purchase amounts.

If Statements: The Building Block of Decision-Making

1. If statements are the simplest form of selection structure.
2. They execute a block of code only if a specified condition is true.

Syntax and Structure

Python:

if condition:
    # code executes if condition is true

Java:

if (condition) {
    // code executes if condition is true
}

age = 16

if age < 18:
    print("Recommended activity: Play video games.")

int age = 16;

if (age < 18) {
    System.out.println("Recommended activity: Play video games.");
}

If-Else Statements: Providing Alternatives

If-else statements extend the basic if structure by providing an alternative block of code to execute if the condition is false.

Python:

if condition:
    # code executes if condition is true
else:
    # code executes if condition is false

Java:

if condition:
    # code executes if condition is true
else:
    # code executes if condition is false

age = 16

if age < 18:
    print("Recommended activity: Play video games.")
else:
    print("Recommended activity: Go to the gym.")

int age = 16;

if (age < 18) {
    System.out.println("Recommended activity: Play video games.");
} else {
    System.out.println("Recommended activity: Go to the gym.");
}

Else If (Elif) Statements: Handling Multiple Conditions

1. Else if (Java) / elif (Python) allow checking multiple conditions in sequence.
2. They are used when there are more than two possible outcomes.
3. The program evaluates each condition in order until one is true.If no condition is true, the final else (if present) runs.

Syntax and Structure

Python:

if condition1:
    # code executes if condition1 is true
elif condition2:
    # code executes if condition2 is true
else:
    # code executes if none of the above are true

Java:

if (condition1) {
    // code executes if condition1 is true
} else if (condition2) {
    // code executes if condition2 is true
} else {
    // code executes if none of the above are true
}

age = 20

if age < 13:
    print("Recommended activity: Play with toys.")
elif age < 18:
    print("Recommended activity: Play video games.")
elif age < 30:
    print("Recommended activity: Go to the gym.")
else:
    print("Recommended activity: Take a relaxing walk.")

int age = 20;

if (age < 13) {
    System.out.println("Recommended activity: Play with toys.");
} else if (age < 18) {
    System.out.println("Recommended activity: Play video games.");
} else if (age < 30) {
    System.out.println("Recommended activity: Go to the gym.");
} else {
    System.out.println("Recommended activity: Take a relaxing walk.");
}

Boolean and Relational Operators in Selection Structures

Boolean and relational operators enhance the power of selection structures by allowing you to create complex conditions.

Relational Operators

1. == : Equal to
2. != : Not equal to
3. > : Greater than
4. < : Less than
5. >= : Greater than or equal to
6. <= : Less than or equal to

Boolean Operators

1. AND ( && ; in Java, and in Python): True if both conditions are true
2. OR ( || in Java, or in Python): True if at least one condition is true
3. NOT ( ! in Java, not in Python): Inverts the truth value of a condition

Constructing Programs with Selection Structures

To effectively use selection structures, follow these steps:

1. Define the Problem
   1. Clearly understand the decision-making requirements of your program.
2. Identify Conditions
   1. Determine the specific conditions that will guide your program's decisions.
3. Choose the Appropriate Structure
   1. Use if, if-else, or else if statements based on the complexity of the decision.
4. Test Your Code
   1. Ensure that all possible scenarios are covered and that the program behaves as expected.