java_module_003.md

Java Module 003: Week 3 - Control Flow (Loops)

This module covers Java's loop constructs: for, while, do-while, break, and continue. Designed for beginners building on Weeks 1 and 2, it includes explanations, code examples, and practice exercises for each day of Week 3 in the 3-month Java fundamentals plan. By the end, you'll write programs that repeat tasks efficiently and create a prime number checker as a mini-project.

---

Day 1: for Loop

Objective: Learn to use the for loop for controlled repetition.

Explanation

• For Loop: Executes a block of code a specific number of times.
• Syntax:

  for (initialization; condition; update) {
      // code
  }

• Components:
  • Initialization: Sets loop variable (e.g., int i = 0).
  • Condition: Checked before each iteration (e.g., i < 10).
  • Update: Modifies loop variable (e.g., i++).
• Use for known iteration counts (e.g., iterating 1 to 10).

Code Example

public class ForLoopExample {
    public static void main(String[] args) {
        // Print numbers 1 to 5
        for (int i = 1; i <= 5; i++) {
            System.out.println("Number: " + i);
        }
    }
}

Practice

1. Write a program to print numbers 1 to 10 using a for loop.
2. Calculate and print the sum of numbers 1 to 10.
3. Print even numbers from 2 to 20.

Practice Code:

public class ForLoopPractice {
    public static void main(String[] args) {
        // Print 1 to 10
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }
        
        // Calculate sum
        int sum = 0;
        for (int i = 1; i <= 10; i++) {
            sum += i;
        }
        System.out.println("Sum: " + sum);
    }
}

---

Day 2: while Loop

Objective: Use the while loop for condition-based repetition.

Explanation

• While Loop: Executes code as long as a condition is true.
• Syntax:

  while (condition) {
      // code
  }

• Requires manual variable update to avoid infinite loops.
• Best for unknown iteration counts (e.g., waiting for user input).

Code Example

public class WhileLoopExample {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 5) {
            System.out.println("Number: " + i);
            i++; // Update to prevent infinite loop
        }
    }
}

Practice

1. Print even numbers from 2 to 20 using a while loop.
2. Calculate the sum of numbers until the sum exceeds 50.
3. Print numbers in reverse (10 to 1).

Practice Code:

public class WhileLoopPractice {
    public static void main(String[] args) {
        // Print even numbers
        int num = 2;
        while (num <= 20) {
            System.out.println(num);
            num += 2;
        }
        
        // Sum until > 50
        int sum = 0, i = 1;
        while (sum <= 50) {
            sum += i;
            i++;
        }
        System.out.println("Sum: " + sum);
    }
}

---

Day 3: do-while Loop

Objective: Understand the do-while loop for guaranteed execution.

Explanation

• Do-While Loop: Executes code at least once, then repeats if condition is true.
• Syntax:

  do {
      // code
  } while (condition);

• Useful when code must run before checking the condition (e.g., user input prompts).

Code Example

public class DoWhileExample {
    public static void main(String[] args) {
        int i = 1;
        do {
            System.out.println("Number: " + i);
            i++;
        } while (i <= 5);
    }
}

Practice

1. Write a program to print numbers 1 to 5 using do-while.
2. Simulate a user input loop that continues until a positive number is entered (use a hardcoded value for now).
3. Compare while and do-while by rewriting a Day 2 practice with do-while.

Practice Code:

public class DoWhilePractice {
    public static void main(String[] args) {
        // Print 1 to 5
        int i = 1;
        do {
            System.out.println(i);
            i++;
        } while (i <= 5);
        
        // Simulate input until positive
        int input = -1;
        do {
            System.out.println("Input: " + input);
            input++; // Simulate changing input
        } while (input <= 0);
    }
}

---

Day 4: break and continue

Objective: Control loop flow with break and continue.

Explanation

• Break: Exits the loop immediately.
• Continue: Skips the current iteration and continues with the next.
• Use break to stop early (e.g., when a condition is met).
• Use continue to skip specific iterations (e.g., skip odd numbers).

Code Example

public class BreakContinueExample {
    public static void main(String[] args) {
        // Break example: Stop at 5
        for (int i = 1; i <= 10; i++) {
            if (i == 5) {
                break;
            }
            System.out.println("Number: " + i);
        }
        
        // Continue example: Skip odd numbers
        for (int i = 1; i <= 10; i++) {
            if (i % 2 != 0) {
                continue;
            }
            System.out.println("Even: " + i);
        }
    }
}

Practice

1. Use break to stop a loop when a number exceeds 50.
2. Use continue to print only odd numbers from 1 to 20.
3. Combine break and continue in a loop to print numbers divisible by 3 until 30.

Practice Code:

public class BreakContinuePractice {
    public static void main(String[] args) {
        // Continue: Print odd numbers
        for (int i = 1; i <= 20; i++) {
            if (i % 2 == 0) {
                continue;
            }
            System.out.println("Odd: " + i);
        }
        
        // Break: Stop when divisible by 3 and > 30
        for (int i = 1; i <= 50; i++) {
            if (i % 3 != 0) {
                continue;
            }
            System.out.println("Divisible by 3: " + i);
            if (i > 30) {
                break;
            }
        }
    }
}

---

Day 5: Combining Loops and Conditionals

Objective: Integrate loops with conditionals for complex programs.

Explanation

• Combine if statements with loops to filter or process data.
• Example: Use a loop to iterate numbers and if to check conditions (e.g., prime numbers).
• Ensure proper loop termination to avoid infinite loops.

Code Example

public class LoopConditionalExample {
    public static void main(String[] args) {
        // Print multiplication table for 5
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) { // Only even multipliers
                System.out.println("5 * " + i + " = " + (5 * i));
            }
        }
    }
}

Practice

1. Write a program to print a multiplication table for a given number (e.g., 7).
2. Use a loop and if to print numbers divisible by both 2 and 3 (up to 50).
3. Create a program to count how many numbers from 1 to 100 are divisible by 5.

Practice Code:

public class LoopConditionalPractice {
    public static void main(String[] args) {
        // Multiplication table for 7
        int num = 7;
        for (int i = 1; i <= 10; i++) {
            System.out.println(num + " * " + i + " = " + (num * i));
        }
        
        // Count numbers divisible by 5
        int count = 0;
        for (int i = 1; i <= 100; i++) {
            if (i % 5 == 0) {
                count++;
            }
        }
        System.out.println("Numbers divisible by 5: " + count);
    }
}

---

Weekend: Review and Mini-Project

Objective: Consolidate Week 3 learning with a prime number checker.

Review

• Revisit:
  • for loop for fixed iterations.
  • while and do-while for condition-based loops.
  • break and continue for loop control.
  • Combining loops with conditionals.
• Debug common errors (e.g., infinite loops, off-by-one errors).

Mini-Project: Prime Number Checker

Create a console-based program that:

1. Takes a number (hardcoded for now) and checks if it’s prime.
2. Uses a loop to test divisibility from 2 to the square root of the number.
3. Uses if to determine primality.
4. Prints all prime numbers up to 50 using a loop.

Project Code:

public class PrimeNumberChecker {
    public static void main(String[] args) {
        // Check if a single number is prime
        int num = 17;
        boolean isPrime = true;
        
        if (num <= 1) {
            isPrime = false;
        } else {
            for (int i = 2; i <= Math.sqrt(num); i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;
                }
            }
        }
        System.out.println(num + " is prime: " + isPrime);
        
        // Print all primes up to 50
        System.out.println("Primes up to 50:");
        for (int n = 2; n <= 50; n++) {
            isPrime = true;
            for (int i = 2; i <= Math.sqrt(n); i++) {
                if (n % i == 0) {
                    isPrime = false;
                    break;
                }
            }
            if (isPrime) {
                System.out.print(n + " ");
            }
        }
    }
}

Weekend Practice

1. Run and test the prime number checker with different inputs.
2. Modify to count how many primes are found up to 50.
3. Add a feature to check if a number is composite (not prime).

---

Tips for Week 3

• Practice: Run each code example in an IDE or terminal.
• Debug: Watch for infinite loops (e.g., missing i++) or incorrect conditions.
• Resources: Refer to Oracle’s Java Tutorials or W3Schools for loop details.
• Experiment: Modify loop bounds or conditions to see how output changes.