Taking Decisions using Code (Conditional Statements)

Table of contents
  1. Why Take Decisions in Code?
  2. Boolean Expressions in Conditional Statements
  3. How and Where?
  4. When?
  5. Common Confusions for Absolute Beginners

Why (Motivation to Use Conditional Statements)

Why Use Conditional Statements?

Scenario 1: Making Decisions Based on User Input Imagine you’re building a program that interacts with users, such as a simple ATM system. The user might choose to check their balance, withdraw money, or deposit funds. To handle these different actions, your program needs to make decisions based on the user’s input. This is where conditional statements, like if, else if, and else, become essential. They allow your program to choose different paths of execution based on certain conditions.

Scenario 2: Validating Data Consider a scenario where you need to validate user input, such as checking if a password meets certain criteria (like length and character requirements). Using conditional statements, you can evaluate each criterion and provide appropriate feedback to the user, ensuring that the input meets the required standards.

Scenario 3: Handling Different Cases in a Switch In situations where you have multiple possible values for a variable (such as a menu selection or a status code), a switch statement can be more efficient and readable than multiple if-else statements. The switch statement allows your program to jump directly to the relevant case, improving performance and clarity.

Got it! I’ll provide the summary at the end. Let’s move on to the next step.

Boolean Expressions in Conditional Statements

Boolean expressions are the foundation of conditional logic in programming. They evaluate to either true or false and are commonly used to control the flow of a program by making decisions. Boolean expressions are built using relational operators (e.g., >, <, ==) and logical operators (e.g., &&, ||, !). Here are specific ways to use Boolean expressions, including checking ranges and combining multiple conditions, with examples from various domains.

1. Checking Ranges of Values

Scenario: Education

  • Use Case: Determining Grade Bands
  • Example: Suppose you’re developing a system to assign grades based on a student’s score. You need to check if the score falls within certain ranges to assign the appropriate grade.
int score = 85;
if (score >= 90 && score <= 100) {
    System.out.println("Grade: A");
} else if (score >= 80 && score < 90) {
    System.out.println("Grade: B");
} else if (score >= 70 && score < 80) {
    System.out.println("Grade: C");
} else {
    System.out.println("Grade: F");
}

In this example, the Boolean expression score >= 90 && score <= 100 checks if the score is within the A grade range. Similar expressions are used for other grades.

Scenario: Health

  • Use Case: Monitoring Blood Pressure
  • Example: In a healthcare application, you might need to check if a patient’s blood pressure is within a normal range.
int systolic = 120;
int diastolic = 80;

if (systolic >= 90 && systolic <= 120 && diastolic >= 60 && diastolic <= 80) {
    System.out.println("Blood pressure is normal.");
} else {
    System.out.println("Blood pressure is outside the normal range.");
}

Here, multiple Boolean expressions are combined to check if both systolic and diastolic pressures are within healthy ranges.

2. Combining Multiple Conditions with && (Logical AND)

Scenario: Finance

  • Use Case: Loan Eligibility
  • Example: In a loan application, you might check if an applicant meets several criteria to determine eligibility, such as a minimum credit score and sufficient income.
int creditScore = 700;
double annualIncome = 50000;

if (creditScore >= 650 && annualIncome >= 40000) {
    System.out.println("Loan approved.");
} else {
    System.out.println("Loan not approved.");
}

This example uses the && operator to ensure that both the credit score and income criteria are met for loan approval.

Scenario: Housing

  • Use Case: Rental Application Approval
  • Example: A rental property system might check if an applicant has both a good rental history and sufficient income before approving the rental application.
boolean goodRentalHistory = true;
double monthlyIncome = 3000;

if (goodRentalHistory && monthlyIncome >= 2500) {
    System.out.println("Rental application approved.");
} else {
    System.out.println("Rental application denied.");
}

Here, both conditions must be true for the application to be approved.

3. Using || (Logical OR) for Alternatives

Scenario: Retail

  • Use Case: Promotional Discounts
  • Example: In a retail system, you might want to apply a discount if a customer is either a loyalty program member or if they have a coupon code.
boolean isLoyaltyMember = true;
boolean hasCoupon = false;

if (isLoyaltyMember || hasCoupon) {
    System.out.println("Discount applied.");
} else {
    System.out.println("No discount available.");
}

The || operator checks if at least one of the conditions is true to apply the discount.

Scenario: Machines and Factories

  • Use Case: Machine Safety Checks
  • Example: In a factory, a machine might need to be stopped if either an emergency stop button is pressed or if a safety sensor detects an issue.
boolean emergencyStopPressed = false;
boolean safetySensorTriggered = true;

if (emergencyStopPressed || safetySensorTriggered) {
    System.out.println("Machine stopped for safety.");
} else {
    System.out.println("Machine running normally.");
}

This ensures that the machine stops if any critical safety condition is met.

4. Negating Conditions with ! (Logical NOT)

Scenario: Self-Care

  • Use Case: Reminders for Uncompleted Tasks
  • Example: In a self-care app, you might want to send a reminder if a user hasn’t completed their daily meditation session.
boolean meditationCompleted = false;

if (!meditationCompleted) {
    System.out.println("Reminder: Don't forget to meditate today!");
}

The ! operator negates the condition, triggering the reminder only if the session hasn’t been completed.

Scenario: Design and Engineering

  • Use Case: Checking Design Constraints
  • Example: In an engineering design tool, you might need to check if a design does not exceed certain constraints before approving it.
double designWeight = 150.0; // in kilograms
double maxAllowedWeight = 200.0;

if (!(designWeight > maxAllowedWeight)) {
    System.out.println("Design approved.");
} else {
    System.out.println("Design rejected due to excessive weight.");
}

The ! operator ensures the design is only approved if it does not exceed the maximum allowed weight.

5. Combining &&, ||, and !

Scenario: Biotechnology

  • Use Case: Genetic Testing Conditions
  • Example: In a genetic testing application, you might need to evaluate a combination of conditions to determine the likelihood of a certain genetic trait being present.
boolean hasGeneA = true;
boolean hasGeneB = false;
boolean hasGeneC = true;

if ((hasGeneA && hasGeneB) || (!hasGeneB && hasGeneC)) {
    System.out.println("Trait likely present.");
} else {
    System.out.println("Trait unlikely to be present.");
}

This example combines multiple logical operators to evaluate complex genetic conditions.

Summary

Boolean expressions are powerful tools in programming, enabling you to create complex decision-making logic. By combining relational and logical operators, you can evaluate conditions ranging from simple comparisons to complex multi-condition checks. The examples provided span various domains such as education, finance, health, retail, and engineering, demonstrating the versatility of Boolean expressions in real-world scenarios.

These foundational concepts are essential for building robust and dynamic applications across different industries.

How (Syntax Specifics + Tester Code) and Where (Logical Placement of the Code)

How to Use Conditional Statements

1. if Statement

  • Syntax: The if statement evaluates a condition. If the condition is true, the code block inside the if statement is executed.
  • Where: Use the if statement when you need to execute a block of code based on a specific condition.
public class ATM {
    public static void main(String[] args) {
        double balance = 5000.00;
        double withdrawalAmount = 300.00;

        if (withdrawalAmount <= balance) {
            balance -= withdrawalAmount;
            System.out.println("Withdrawal successful. New balance: " + balance);
        }
    }
}

2. else Statement

  • Syntax: The else statement follows an if statement. If the if condition is false, the code block inside the else statement is executed.
  • Where: Use else to define an alternative action if the if condition is not met.
public class ATM {
    public static void main(String[] args) {
        double balance = 5000.00;
        double withdrawalAmount = 6000.00;

        if (withdrawalAmount <= balance) {
            balance -= withdrawalAmount;
            System.out.println("Withdrawal successful. New balance: " + balance);
        } else {
            System.out.println("Insufficient funds. Withdrawal denied.");
        }
    }
}

3. else if Statement

  • Syntax: The else if statement allows you to check multiple conditions. If the first if condition is false, the else if condition is evaluated. If it is true, its block is executed.
  • Where: Use else if when you need to evaluate multiple conditions in sequence.
public class GradeCalculator {
    public static void main(String[] args) {
        int score = 85;

        if (score >= 90) {
            System.out.println("Grade: A");
        } else if (score >= 80) {
            System.out.println("Grade: B");
        } else if (score >= 70) {
            System.out.println("Grade: C");
        } else {
            System.out.println("Grade: F");
        }
    }
}

4. switch Statement

  • Syntax: The switch statement is used for handling multiple possible values of a variable. Each possible value is called a “case”. The code inside the matching case block is executed.
  • Where: Use a switch statement when you have a variable that can take on multiple discrete values, and you want to execute different code based on those values.
public class VendingMachine {
    public static void main(String[] args) {
        int itemNumber = 2;

        switch (itemNumber) {
            case 1:
                System.out.println("You selected a soda.");
                break;
            case 2:
                System.out.println("You selected a snack.");
                break;
            case 3:
                System.out.println("You selected water.");
                break;
            default:
                System.out.println("Invalid selection.");
                break;
        }
    }
}

Where to Place Conditional Statements in Your Code

  • In Methods: Conditional statements are often placed inside methods to control the flow of logic based on parameters or user input.
  • In Constructors: You might use conditional statements in constructors to validate or set up initial values based on certain conditions.
  • In Event Handlers: In GUI applications or games, conditional statements are often used within event handlers to respond to user actions (e.g., button clicks, key presses).

Now that you’ve seen how to use and place conditional statements.

When (Scenarios of Situations When Conditional Statements Can Be Used)

When to Use Conditional Statements

1. if Statement

  • Scenario: Use an if statement when you need to check a single condition and execute a block of code only if that condition is true.
  • Example: In a login system, you might check if a user’s password matches the stored password. If it does, you allow the login; if not, you deny access.

2. else Statement

  • Scenario: Use an else statement when you want to define an alternative action if the if condition is false.
  • Example: In an e-commerce application, if an item is in stock, you proceed to checkout; otherwise, you notify the user that the item is out of stock.

3. else if Statement

  • Scenario: Use else if when you have multiple conditions to evaluate, and you want to perform different actions based on which condition is true.
  • Example: In a grading system, you might use else if to assign letter grades based on a student’s score: A for scores above 90, B for scores above 80, and so on.

4. switch Statement

  • Scenario: Use a switch statement when you have a single variable that can take on multiple values, and you want to execute different blocks of code depending on the specific value.
  • Example: In a menu-driven application, where each menu option corresponds to a different action (e.g., view account, deposit money, withdraw money), a switch statement is ideal for handling the selection.

How to Decide Which Conditional Statement to Use

1. if vs. else if vs. else

  • Use if: When there’s only one condition to check.
  • Use else if: When there are multiple conditions to check in sequence.
  • Use else: When you want to handle all cases not covered by if or else if.

2. if-else vs. switch

  • Use if-else: When the conditions involve complex logic or ranges (e.g., checking if a number is within a range).
  • Use switch: When you’re dealing with a single variable that can have a limited number of discrete values (e.g., day of the week, menu selection).

How to Decide - Any General Confusions for Absolute Beginners

How to Decide the Structure of Conditional Statements

1. Deciding Between if and else if

  • Question: Do you have multiple conditions to check, and only one of them should trigger?
  • Guideline: Start with an if statement for the first condition. Use else if for additional conditions if you have multiple possibilities. The else block should handle the case where none of the if or else if conditions are true.

Example:

  • Scenario: You want to assign a letter grade based on a student’s score.
  • Structure: 
    if (score >= 90) {
    grade = "A";
} else if (score >= 80) {
    grade = "B";
} else if (score >= 70) {
    grade = "C";
} else {
    grade = "F";
}

2. Deciding When to Use else

  • Question: Do you need to handle all cases not covered by if or else if?
  • Guideline: Use else when you want a default action if none of the specified conditions are met. This is useful for catching unexpected inputs or providing a fallback action.

Example:

  • Scenario: You’re checking if a user input is valid, but you want to display an error message if the input doesn’t match any expected values.
  • Structure: 
    if (input.equals("Option1")) {
    // Do something
} else if (input.equals("Option2")) {
    // Do something else
} else {
    System.out.println("Invalid input. Please try again.");
}

3. Deciding Between if-else and switch

  • Question: Are you checking a single variable against multiple specific values?
  • Guideline: Use a switch statement if you have a single variable and a defined set of possible values. Use if-else when your conditions involve ranges or complex logic.

Example:

  • Scenario: You want to display different messages based on the day of the week.
  • Structure: 
    switch (day) {
    case "Monday":
        System.out.println("Start of the work week.");
        break;
    case "Friday":
        System.out.println("Almost the weekend!");
        break;
    default:
        System.out.println("Midweek grind.");
        break;
}

Common Confusions for Beginners

1. Mixing Up if and else if

  • Explanation: Beginners sometimes use multiple if statements where else if should be used. This leads to all conditions being checked, even when one is already true. Remember, else if only checks its condition if the previous if (or else if) was false.

2. Forgetting to Use else for a Default Action

  • Explanation: Beginners might omit the else block, which can result in unhandled cases or unexpected behavior. Always consider whether an else is needed to handle any conditions not explicitly checked.

3. Misunderstanding the switch Statement

  • Explanation: A common mistake is not using break in a switch statement, which causes fall-through behavior (where multiple cases are executed). Always include break unless you specifically want to allow fall-through.

4. Using if-else Instead of switch

  • Explanation: Beginners might use multiple if-else statements where a switch would be clearer and more efficient. Use switch when you’re dealing with a single variable and a small, defined set of values.

Are these decision-making guidelines and explanations clear? If so, we can move on to experimenting with conditional statements to see what could go wrong and how to avoid common pitfalls.

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Conditionals Experiments

Experiment with Conditionals exercises.

Practice Exercises

Practice Conditionals with exercises.