Control Statements in Java: Loops, Conditionals & Decision-Making

Core Concepts & Definitions

Control statements determine the flow of execution in a program by:

1. Making decisions (if-else, switch-case).
2. Repeating tasks (loops: for, while, do-while).
3. Terminating loops early (break, continue).

1. Decision-Making Statements

A. if Statement

• Executes a block if a condition is true.
• Syntax:

  if (condition) {
      // code to execute if true
  }
  

• Example: Check if a number is positive.

  int num = 5;
  if (num > 0) {
      System.out.println("Positive");
  }
  

  Trace:

  Step	num	Condition (num > 0)	Output
  1	5	true	"Positive"

B. if-else Statement

• Executes one block if true, another if false.
• Syntax:

  if (condition) {
      // true block
  } else {
      // false block
  }
  

• Example: Check even/odd (real-world tie: eSewa transaction validation).

  int amount = 1000;
  if (amount % 2 == 0) {
      System.out.println("Even: Valid transaction");
  } else {
      System.out.println("Odd: Invalid amount");
  }
  

  Trace:

  Step	amount	amount % 2	Condition	Output
  1	1000	0	true	"Even: Valid transaction"

C. Nested if-else

• Multiple conditions checked sequentially.
• Example: Grade calculator (real-world tie: NTC exam grading).

  int marks = 85;
  if (marks >= 90) {
      System.out.println("A");
  } else if (marks >= 80) {
      System.out.println("B");
  } else {
      System.out.println("C");
  }
  

  Trace:

  Step	marks	Condition 1 (>=90)	Condition 2 (>=80)	Output
  1	85	false	true	"B"

D. switch-case

• Efficient for multiple discrete choices.
• Syntax:

  switch (variable) {
      case value1: // code; break;
      case value2: // code; break;
      default:     // fallback
  }
  

• Example: Day-of-week printer (real-world tie: Pathao driver shifts).

  int day = 3;
  switch (day) {
      case 1: System.out.println("Monday"); break;
      case 2: System.out.println("Tuesday"); break;
      case 3: System.out.println("Wednesday"); break;
      default: System.out.println("Invalid day");
  }
  

  Trace:

  Step	day	Matched Case	Output
  1	3	case 3	"Wednesday"

2. Looping Statements

Loops repeat code blocks until a condition is met.

A. for Loop

• Used for known iterations (e.g., arrays, fixed counts).
• Syntax:

  for (initialization; condition; update) {
      // loop body
  }
  

• Example: Print numbers 1–5 (real-world tie: Daraz order IDs).

  for (int i = 1; i <= 5; i++) {
      System.out.println("Order ID: " + i);
  }
  

  Trace:

  Iteration	i	Condition (i <= 5)	Output
  1	1	true	"Order ID: 1"
  2	2	true	"Order ID: 2"
  ...	...	...	...
  5	5	true	"Order ID: 5"
  6	6	false	Loop ends

B. while Loop

• Runs while a condition is true (unknown iterations).
• Syntax:

  while (condition) {
      // loop body
  }
  

• Example: Sum digits of a number (real-world tie: Khalti transaction fees).

  int num = 1234, sum = 0;
  while (num > 0) {
      sum += num % 10;
      num /= 10;
  }
  System.out.println("Sum: " + sum); // Output: 10
  

  Trace:

  Step	num	num % 10	sum	num /= 10	Condition (num > 0)
  1	1234	4	4	123	true
  2	123	3	7	12	true
  3	12	2	9	1	true
  4	1	1	10	0	true
  5	0	0	10	0	false (loop ends)

C. do-while Loop

• Guarantees at least one execution (condition checked after).
• Syntax:

  do {
      // loop body
  } while (condition);
  

• Example: Password retry (real-world tie: Ncell login system).

  String password = "";
  do {
      System.out.print("Enter password: ");
      password = "1234"; // Assume user input
  } while (!password.equals("1234"));
  System.out.println("Access granted!");
  

  Trace:

  Iteration	password	Condition (!="1234")	Output
  1	"1234"	false	"Access granted!"
  (Note: Loop runs once even if condition is false initially.)

3. Loop Control Statements

A. break

• Exits the loop immediately.
• Example: Stop at first even number.

  for (int i = 1; i <= 5; i++) {
      if (i % 2 == 0) break;
      System.out.println(i);
  }
  

  Trace:

  Iteration	i	i % 2	Condition	Output	Action
  1	1	1	false	1	Continue
  2	2	0	true	-	Break

B. continue

• Skips current iteration.
• Example: Print odd numbers only.

  for (int i = 1; i <= 5; i++) {
      if (i % 2 == 0) continue;
      System.out.println(i);
  }
  

  Trace:

  Iteration	i	i % 2	Condition	Output	Action
  1	1	1	false	1	Continue
  2	2	0	true	-	Continue
  3	3	1	false	3	Continue

## In the Real World

1. eSewa Transaction Validation

   • Uses if-else to check amount validity (e.g., if (amount > 0 && amount <= balance)).
   • Switch-case for payment method selection (e.g., switch (method) { case "Khalti": ... }).

2. Pathao Driver Shifts

   • for loops iterate through daily ride requests (for (Ride r : rides) { ... }).
   • break exits the loop if the driver’s shift ends (if (currentTime > shiftEnd) break;).

3. NTC Exam Grading

   • Nested if-else determines grade letters based on marks:

     if (marks >= 80) grade = 'A';
     else if (marks >= 60) grade = 'B';
     else grade = 'C';
     

4. Khalti Transaction Fees

   • while loop calculates total fees by summing percentages:

     while (amount > 0) {
         fee += amount * 0.02; // 2% fee
         amount -= 100;        // Process in chunks
     }
     

5. Daraz Order Queue

   • do-while ensures at least one order is processed before checking stock:

     do {
         processOrder();
     } while (inventory > 0);
     

## Visualizing Control Flow

1. Decision Tree for Even/Odd Check

flowchart TD
    A["Start"] --> B["Is number % 2 == 0?"]
    B -->|"Yes"| C["Print 'Even'"]
    B -->|"No"| D["Print 'Odd'"]
    C --> E["End"]
    D --> E

2. for Loop Execution (1–5)

3. Stack Simulation with break

4. Linked List Traversal with continue

## Comparison Table: Control Statements

## Common Pitfalls & Best Practices

1. Off-by-One Errors

   • Problem: for (int i=0; i<=10; i++) runs 11 times (0–10).
   • Fix: Use < for upper bounds: i < 10.

2. Infinite Loops

   • Cause: Missing update in while or for (e.g., while (true) without break).
   • Fix: Always update loop variables (e.g., i++, num--).

3. Switch-Case Fallthrough

   • Problem: Missing break causes unintended execution of next cases.
   • Fix: Add break after each case (except default).

4. Nested Loops Clarity

   • Problem: Complex nested loops are hard to debug.
   • Fix: Use meaningful variable names (e.g., for (User u : users)).

## Exam Tip

1. Code Tracing Questions

   • Expect: "Trace the output of the following code."
   • Strategy:
     • Write a step-by-step table (like above).
     • Highlight loop counters and conditions.
     • Example:

       for (int i=2; i<=6; i+=2) System.out.print(i);
       

       Trace:

       Iteration	i	Condition (i<=6)	Output
       1	2	true	2
       2	4	true	4
       3	6	true	6
       4	8	false	-

2. GUI-Based Questions

   • Expect: "Design a form to check even/odd."
   • Strategy:
     • Use if-else with JOptionPane or JTextField.
     • Example:

       int num = Integer.parseInt(JOptionPane.showInputDialog("Enter number:"));
       if (num % 2 == 0) JOptionPane.showMessageDialog(null, "Even");
       else JOptionPane.showMessageDialog(null, "Odd");
       

3. Real-World Scenarios

   • Expect: "How would you use loops in [eSewa/Khalti]?"
   • Answer:
     • eSewa: while loop for transaction validation (check balance, retry on failure).
     • Khalti: for loop to process multiple payments in a batch.

4. Short-Answer Definitions

   • Key Terms to Memorize:
     • Loop: Repeats code until a condition is false.
     • Nested if: if inside another if for hierarchical checks.
     • Infinite loop: Loop with no exit condition (e.g., while (true)).

## Practice Problems

1. Trace the Output:

   int x = 5;
   while (x > 0) {
       System.out.print(x--);
   }
   

   Answer: 54321

2. Fix the Code:

   for (int i=0; i<5; i++) {
       if (i == 2) continue;
       System.out.print(i);
   }
   

   Output: 0134 (skips 2).

3. Real-World Application:

   • Write a switch-case to handle Ncell recharge options (e.g., case "100": charge(100); break;).

4. Debugging:

   • Why does this loop run infinitely?

     int i = 0;
     while (i < 5) {
         System.out.println(i);
     }
     

   Answer: Missing i++ (counter never updates). Fix: while (i < 5) { ...; i++; }.