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The Array is Not Enough

The Array is Not Enough. Lists. Arrays. Key Insight. The array is our friend (sort of) but… turn all values into numbers (smaller than the array’s size). 0. 4. 8. 9. 7. 2. 3. 5. 6. 1. Let’s assume an array of 10 elements…. 4. 0. 4. 8. 9. 7. 2. 3. 5. 6. 1. 7. 4.

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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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