Lecture 8: Arrays

Introduction to Arrays

• What is an array?
  • A collection of data types (primitives or objects)
  • Syntax: dataType[] indicating an array
• Why Arrays?
  • Efficient data storage and management
  • Example: Storing multiple roll numbers
• Syntax
  • Declaration: dataType[] arrayName;
  • Initialization: arrayName = new dataType[size]; or dataType[] arrayName = {value1, value2, ...};

Key Points

• Data in arrays must be of the same type
• Declaration and initialization
  • Declaration assigns reference variable
  • Initialization creates the actual object in heap memory
• Dynamic Memory Allocation
  • Memory allocated at runtime
  • Java uses a new keyword for creating objects
  • Example: int[] rollNumbers = new int[5];

Memory Management

• Arrays stored in heap memory
• Objects in heap memory are not necessarily continuous
• Primitives stored in stack, objects in heap
• Array Indices
  • Arrays start at index 0
  • Example: arr[0] accesses the first element

Null Value

• By default, array elements of objects are initialized to null
• null is a special literal value

Input and Output of Arrays

• Input elements using loops
  • Example: Using Scanner for taking input
• Output elements
  • Using loops for printing
  • Using Arrays.toString() for easy printing

Enhanced For Loop

• Syntax:

  for(dataType element : arrayName) { 
    // code 
  }
  

Multi-Dimensional Arrays (2D Arrays)

• Definition and Initialization
  • Example: int[][] arr = new int[3][4];
  • Input using nested loops
  • Variable number of columns allowed
• Internal Representation
  • Arrays of arrays stored in heap memory

ArrayList

• Dynamic arrays that allow dynamic resizing
• Syntax:

  ArrayList<dataType> list = new ArrayList<>();
  

• Supports various operations: add, set, remove, contains
• Internally handled using copying and doubling the size of the array

Questions and Examples

• Swapping elements in an array
• Finding the maximum element in an array or a range
• Reversing an array
  • Using two-pointer method
• More complex problems and methods like rotation, Armstrong number, palindrome

Plan for Next Sessions

• Sorting techniques
• Detailed questions on arrays
• Searching techniques and related questions
• Advanced patterns: Sliding window, two-pointer method

Conclusion

• Comprehensive understanding of arrays
• Importance of practice and applying concepts