Unit 4: Iteration

This unit covers different types of loops and iteration structures in Java programming.

Key Concepts

  • while Loops: Condition-based iteration
  • for Loops: Counter-based iteration
  • Enhanced for Loops: Iterating over collections
  • Nested Loops: Loops within loops
  • Loop Control: break and continue statements
  • Infinite Loops: Causes and prevention
  • Loop Invariants: Understanding loop behavior

Example Code: Basic Iteration


// Example: Different Types of Loops in Java
public class LoopExamples {
  public static void main(String[] args) {
    // while loop
    int count = 0;
    while (count < 5) {
      System.out.println("While loop count: " + count);
      count++;
    }
    
    // for loop
    for (int i = 0; i < 5; i++) {
      System.out.println("For loop count: " + i);
    }
    
    // enhanced for loop
    String[] fruits = {"Apple", "Banana", "Orange"};
    for (String fruit : fruits) {
      System.out.println("Fruit: " + fruit);
    }
    
    // nested loops
    for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
        System.out.println("i=" + i + ", j=" + j);
      }
    }
    
    // break statement
    for (int i = 0; i < 10; i++) {
      if (i == 5) {
        break;
      }
      System.out.println("Count before break: " + i);
    }
    
    // continue statement
    for (int i = 0; i < 5; i++) {
      if (i == 2) {
        continue;
      }
      System.out.println("Count with continue: " + i);
    }
  }
}

Example Code: Advanced Iteration


// Example: Advanced Loop Patterns and Algorithms
public class AdvancedLoopExamples {
    public static void main(String[] args) {
        // Pattern printing
        printPyramid(5);
        
        // Array manipulation with loops
        int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        reverseArray(numbers);
        printArray(numbers);
        
        // Finding prime numbers
        findPrimes(20);
        
        // Matrix operations
        int[][] matrix = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
        transposeMatrix(matrix);
        printMatrix(matrix);
    }
    
    private static void printPyramid(int rows) {
        for (int i = 1; i <= rows; i++) {
            // Print spaces
            for (int j = 0; j < rows - i; j++) {
                System.out.print(" ");
            }
            // Print stars
            for (int k = 0; k < 2 * i - 1; k++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
    
    private static void reverseArray(int[] arr) {
        for (int i = 0; i < arr.length / 2; i++) {
            int temp = arr[i];
            arr[i] = arr[arr.length - 1 - i];
            arr[arr.length - 1 - i] = temp;
        }
    }
    
    private static void findPrimes(int limit) {
        for (int num = 2; num <= limit; num++) {
            boolean isPrime = true;
            for (int i = 2; i <= Math.sqrt(num); i++) {
                if (num % i == 0) {
                    isPrime = false;
                    break;
                }
            }
            if (isPrime) {
                System.out.print(num + " ");
            }
        }
        System.out.println();
    }
    
    private static void transposeMatrix(int[][] matrix) {
        for (int i = 0; i < matrix.length; i++) {
            for (int j = i + 1; j < matrix[i].length; j++) {
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
        }
    }
    
    private static void printArray(int[] arr) {
        for (int num : arr) {
            System.out.print(num + " ");
        }
        System.out.println();
    }
    
    private static void printMatrix(int[][] matrix) {
        for (int[] row : matrix) {
            for (int num : row) {
                System.out.print(num + " ");
            }
            System.out.println();
        }
    }
}

Iteration and Loops

Common Loop Types:

Loop Type Description Example
for Fixed number of iterations for (int i = 0; i < 10; i++)
while Condition-based iteration while (x > 0)
do-while Executes at least once do { ... } while (condition);
for-each Iterate over collections for (String s : strings)

Loop Examples:


// For loop example
for (int i = 0; i < 5; i++) {
    System.out.println("Count: " + i);
}

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

// Do-while loop example
int num = 0;
do {
    System.out.println("Number: " + num);
    num++;
} while (num < 5);

// For-each loop example
String[] fruits = {"Apple", "Banana", "Orange"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

Loop Control Statements:


// Break statement
for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;  // Exit the loop
    }
    System.out.println(i);
}

// Continue statement
for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    System.out.println(i);
}

Loop Tips:

  • Choose the right loop type for your needs
  • Ensure loops have a valid termination condition
  • Use meaningful loop variable names
  • Be careful with nested loops
  • Consider using break and continue when appropriate

Common Pitfalls and Debugging Tips

Common Mistakes:

  • Infinite Loops: Missing or incorrect termination condition 
    
while (x > 0) {  // Wrong! x never changes
    System.out.println(x);
}
while (x > 0) {  // Correct! x is decremented
    System.out.println(x);
    x--;
}
  • Off-by-One Errors: Incorrect loop bounds 
    
for (int i = 0; i <= array.length; i++) {  // Wrong! ArrayIndexOutOfBounds
for (int i = 0; i < array.length; i++) {   // Correct! Proper bounds
  • Modifying Loop Variables: Changing loop counter inside loop 
    
for (int i = 0; i < 10; i++) {
    i++;  // Wrong! Skips every other iteration
    System.out.println(i);
}

Debugging Tips:

  • Use System.out.println() to track loop variables
  • Check loop termination conditions
  • Verify loop bounds with array lengths
  • Test edge cases (empty arrays, single elements)
  • Use breakpoints in your IDE

AP Exam Tips:

  • Know when to use each type of loop
  • Understand loop control statements
  • Be able to trace loop execution
  • Know how to handle array iteration
  • Understand nested loops

AP Exam-Style Practice Problems

Problem 1: Loop Execution

Consider the following code segment:


int sum = 0;
for (int i = 1; i <= 5; i++) {
    sum += i;
}
System.out.println(sum);

What is printed as a result of executing the code segment?

15
10
5
6
The correct answer is A. The loop sums the numbers from 1 to 5, which equals 15.
Problem 2: Nested Loops

Consider the following code segment:


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

What is printed as a result of executing the code segment?

0 1 1 2 2 3
0 1 1 2 2 3
0 1 1 2 2 3
0 1 1 2 2 3
The correct answer is A. The nested loops print the sum of i and j for each combination, with a new line after each inner loop completes.
Problem 3: While Loop

Consider the following code segment:


int x = 10;
while (x > 0) {
    x -= 2;
    System.out.print(x + " ");
}

What is printed as a result of executing the code segment?

8 6 4 2 0
10 8 6 4 2
8 6 4 2
10 8 6 4 2 0
The correct answer is A. The loop decrements x by 2 each iteration until it reaches 0, printing each value.
Problem 4: Loop Control

Consider the following code segment:


int sum = 0;
for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) {
        continue;
    }
    sum += i;
}
System.out.println(sum);

What is printed as a result of executing the code segment?

37
55
27
45
The correct answer is A. The loop sums all numbers from 1 to 10 except those divisible by 3 (3, 6, 9), resulting in 37.

Interactive Code Playground

Try out your own iteration code here!

Output:
Note: This is a simulated environment. For actual Java code execution, please use an IDE or online Java compiler.

For actual code execution, we recommend using:

← Previous Unit: Boolean Expressions
Next Unit: Writing Classes →