L7: Iteration in Java

Overview of Iteration

• Iteration allows repeating a block of code multiple times with minimal duplication.
• Loops execute a single block of code multiple times, making programs more flexible and efficient.

The while Loop

• Definition: Executes a block of code as long as the specified condition is true.

• Syntax:

  while (condition) {
      // bodyStatement(s);
  }

• How It Works:

  • Checks the condition at the start. If true, executes the loop body.
  • After executing the body, it checks the condition again.
  • If the condition becomes false, the loop exits, and the program moves to the code following the loop.

• Example:

  int count = 0;
  while (count < 5) {
      System.out.println("Count: " + count);
      count++;
  }

• Notes:

  • Curly braces {} are only necessary if the loop body has more than one statement.
  • Be cautious of infinite loops if the condition never becomes false.

The do-while Loop

• Definition: Similar to the while loop, but guarantees at least one execution of the loop body since the condition is evaluated after the body.

• Syntax:

  do {
      // bodyStatement(s);
  } while (condition);

• How It Works:

  • Executes the loop body first.
  • Then, it checks the condition. If true, it repeats the body.
  • If false, the loop ends, and the program continues with the code after the do-while loop.

• Example:

  int count = 0;
  do {
      System.out.println("Count: " + count);
      count++;
  } while (count < 5);

• Notes:

  • The loop body runs at least once, regardless of the condition.
  • Curly braces {} are only required for multiple statements in the loop body.

The for Loop

• Definition: Combines initialization, condition, and update in a single line, providing a concise way to iterate.

• Syntax:

  for (initialization; condition; update) {
      // bodyStatement(s);
  }

• How It Works:

  • Initialization: Runs once at the start of the loop.
  • Condition: Evaluated before each iteration. If true, the loop body executes.
  • Update: Executes after the loop body, typically used to modify the loop counter.
  • Repeats until the condition becomes false.

• Example:

  for (int count = 0; count < 5; count++) {
      System.out.println("Count: " + count);
  }

• Notes:

  • Not all parts (initialization, condition, update) are required. For example, you can have an infinite loop with for (;;) { ... }.
  • Curly braces {} are only necessary if the loop body has more than one statement.

The for-each Loop

• Definition: Designed to iterate over arrays or collections, accessing each element without using an index.

• Syntax:

  for (type element : collection) {
      // bodyStatement(s);
  }

• How It Works:

  • Directly accesses each element in the array or collection.
  • Useful when you don’t need to modify the collection or need to keep track of the index.

• Example:

  int[] numbers = {1, 2, 3, 4, 5};
  for (int num : numbers) {
      System.out.println(num);
  }

• Notes:

  • Works with arrays, ArrayList, and other collections.
  • Cannot modify the collection (e.g., cannot change elements of an array) in a for-each loop.

Nested Loops

• A loop inside another loop is known as a nested loop.

• How It Works:

  • The outer loop runs once, then the inner loop completes all its iterations.
  • Total iterations = (Outer loop iterations) × (Inner loop iterations).

• Example:

  for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 2; j++) {
          System.out.println("i: " + i + ", j: " + j);
      }
  }

Difference Between break and continue

• break: Exits the loop immediately, regardless of the loop’s condition. Useful for terminating the loop when a specific condition is met.

  for (int i = 0; i < 10; i++) {
      if (i == 5) {
          break;  // Exits the loop when i equals 5
      }
      System.out.println(i);
  }

• continue: Skips the remaining statements in the current iteration and jumps to the next iteration of the loop.

  for (int i = 0; i < 10; i++) {
      if (i == 5) {
          continue;  // Skips the rest of the loop body when i equals 5
      }
      System.out.println(i);
  }

Converting Between while and for Loops

• A while loop can be converted to a for loop by placing initialization, condition, and update in the loop’s header.

  // while loop
  int i = 0;
  while (i < 5) {
      System.out.println(i);
      i++;
  }
  
  // Equivalent for loop
  for (int i = 0; i < 5; i++) {
      System.out.println(i);
  }

• Conversely, a for loop can be converted to a while loop by moving initialization outside the loop and update statements inside the loop body.

  // for loop
  for (int i = 0; i < 5; i++) {
      System.out.println(i);
  }
  
  // Equivalent while loop
  int i = 0;
  while (i < 5) {
      System.out.println(i);
      i++;
  }

Tracing Loops

• Tracing: Following the execution of a loop to understand its flow.
• For while and do-while loops, check the condition before (or after, for do-while) each iteration.
• For for loops, follow the sequence: initialization → condition check → body execution → update → condition check.
• For for-each loops, iterate over each element in the collection sequentially.

Key Points to Remember

• Initialization Statement: Sets up the loop counter (e.g., int i = 0 in a for loop).
• Condition: Decides whether to continue executing the loop.
• Update Statement: Changes the loop counter (e.g., i++).
• break vs. continue: break terminates the loop; continue skips to the next iteration.
• for-each Loop: Ideal for iterating over collections when you do not need to modify elements or track indices.