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The Array is Not Enough

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  1. The Array is Not Enough

  2. Lists Arrays

  3. Key Insight The array is our friend (sort of) but… turn all values into numbers (smaller than the array’s size)

  4. 0 4 8 9 7 2 3 5 6 1 Let’s assume an array of 10 elements…

  5. 4 0 4 8 9 7 2 3 5 6 1 7 4 8

  6. 104 0 4 8 9 7 2 3 5 6 1 104 224 a modulo b = once you divide b into a, what’s left? 5 modulo 2 = ? 4 modulo 2 = ? 37 modulo 17 = ? 104 modulo 10 = ? 463325 modulo 9882 = ?

  7. “Hello” 0 4 8 9 7 2 3 5 6 1

  8. “Hello” 0 4 8 9 7 2 3 5 6 1 combine(72, 101, 108, 108, 111)

  9. 0 4 8 9 7 2 3 5 6 1 class Point { int x; int y; }

  10. 0 4 8 9 7 2 3 5 6 1 class Name { String first; String last; }

  11. 0 4 8 9 7 2 3 5 6 1 class Person { Name n; Address a; Education e; }

  12. The processof converting arbitrary datainto a single numberis called hashing

  13. Good News You don’t need to define these: Java provides hash codes String s = new String("Hello"); s.hashCode() • 69609650

  14. class Point { int x; int y; } Point p1 = new Point(2, 3); Point p2 = new Point(3, 5); p1.hashCode()  1252169911 p2.hashCode()  2101973421 inti = 5; i.hashCode();  error: int cannot be dereferenced

  15. Existing Collection Classes

  16. Arrays Revisited 1 2 3 4 5 0

  17. A New Collection Class 1 1 2 2 3 3 4 4 5 5 0 0 Hash Table

  18. Array: Index determines where the value goes Hash Table: Hash determines where the value goes 1 1 2 2 3 3 4 4 5 5 0 0

  19. A More General Idea: Mapping Person Company String Boolean Number Country A B Person  Employer URL  Visited? Country  +Dialing Code Key  Value

  20. Point p1 = new Point(2, 3); Point p2 = new Point(3, 5); HashMap<Point, String> hmap = new HashMap<Point, String>(); hmap.put(p1, "Red"); hmap.get(p1)  "Red" hmap.put(p1, "Blue"); hmap.get(p1)  "Blue" hmap.get(p2)  null p2 p1 1 2 3 4 5 0 Blue Red

  21. A New Data Structure Primitive The key-value pair In Java, all keys must be the same type, all values the same type, but those two types can be different Basic operations: get and put

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