Introduction

Learning Outcomes

Digital Data

Digital Data

• Numbers, text, images, sound: any information stored as 0s and 1s.
• Examples: text documents, image files, databases, audio recordings.

Analog sound → ADC → 0s and 1s → computer → DAC → analog sound

Digital Data

Why Digital?

Digital Data

Representing Digital Data

Digital Data

Kinds of Digital Data

Introduction to Data Structures

Data Structures and Algorithms

• Good algorithms and data structures make software efficient.
• Together they solve problems systematically.

Introduction to Data Structures

Data Structures

Introduction to Data Structures

Basic Building Blocks

• Objects group related attributes into one unit with named fields.
• Arrays store items side by side in consecutive memory slots.
• Links let items point to one another in any directed pattern.

Introduction to Data Structures

Primitive Data Types

Introduction to Data Structures

Data Types

Introduction to Data Structures · The Java memory model

Memory in Java: Stack and Heap

int x = 5;               // primitive: the value
int[] a = {7, 2, 9};     // reference → array
String s = "data";       // reference → object
int[] p = null;          // refers to nothing

Introduction to Data Structures · Step through the code

Code Trace: Value or Reference?

int x = 5;
int y = x;
y = 7;
int[] a = {1, 2, 3};
int[] b = a;
b[0] = 9;
b = new int[2];
System.out.println(x + " " + a[0]);

Introduction to Data Structures · Java

Generics: One Structure for Every Type

public class Box<T> {
  private T item;
  public void put(T x) { item = x; }
  public T get() { return item; }
}

Box<String> b = new Box<>();
b.put("data");
String s = b.get();     // no cast needed
Box<Integer> m = new Box<>();
m.put(42);              // int → Integer
// m.put("x");          // COMPILE ERROR

Introduction to Data Structures

Non-Primitive Data Structures

More complex structures, such as:

• More complex; can combine several primitive types.
• Most can grow and shrink (arrays cannot).
• Built for specialized operations.

Introduction to Data Structures

Data Structures at a Glance

The same four values (7, 2, 9, 5) arranged eight ways, each with its own strengths and weaknesses:

Introduction to Data Structures

Abstract Data Types vs. Data Structures

Introduction to Data Structures

From ADT to Java: The interface

public interface Stack<E> {
  int size();         // number of elements
  boolean isEmpty();  // is it empty?
  void push(E e);     // add on top
  E top();            // return the top element
  E pop();            // remove and return the top
}

Introduction to Data Structures · From ADT to data structure

One Implementation: ArrayStack

public class ArrayStack<E> implements Stack<E> {
  private E[] data;         // the elements
  private int t = -1;       // index of the top element
  public ArrayStack(int capacity) {
    data = (E[]) new Object[capacity];
  }
  public int size() { return t + 1; }
  public boolean isEmpty() { return t == -1; }
  public E top() {
    if (isEmpty()) return null;
    return data[t];
  }
  // push and pop
}

Introduction to Data Structures · Step through the code

Code Trace: push / pop

public void push(E e) {
  if (size() == data.length)
    throw new IllegalStateException("full");
  data[++t] = e;
}
public E pop() {
  if (isEmpty()) return null;
  E e = data[t];
  data[t--] = null;     // let GC reclaim it
  return e;
}
s.push(5); s.push(8);
System.out.println(s.pop());
s.push(3);

Introduction to Data Structures

Organizing Data

Introduction to Data Structures

Choosing a Data Structure

Fundamental Data Structures

Fundamental Data Structures: An Overview

Summary

Summary

Summary

Sample Problems

1. Which data structure fits "undo" in a text editor? Why?
2. Which structure lets a printer print documents in the order they arrive?
3. Which structure gives fast lookup of a student's record by student ID?
4. How would you model a city map to find the shortest route between two points?
5. Which structure would you use to remove duplicates from a list?
6. In Java, is the stack ADT written as an interface or a class? Which is ArrayStack?

Summary

Exercises

1. After int[] a = {1, 2}; int[] b = a; b[1] = 7;, what is a[1]? Why?
2. Why doesn't Box<int> b; compile? What should you write instead?
3. Write the character 'B' and the number 13 as 8-bit patterns.
4. After String s = "data"; String t = s; t = t + "!";, what is s? Why?
5. Starting from an empty ArrayStack with capacity 3, run push(1), push(2), pop(), push(3), push(4). What are t and the array at the end?
6. A music player needs a playlist in order, a "recently played" list, and fast search by song title. Which structures fit?

Summary

Further Reading

• Goodrich, Tamassia, and Goldwasser, Data Structures and Algorithms in Java, 6th ed., 2014.
• Wirth, Algorithms + Data Structures = Programs, 1976.
• Oracle, Java SE 17 API documentation.