CS 261 Spring 2005

1
CS 261 Spring 2005

• Vector Review
• (Read chapter 6)

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Arrays, Pro and Con

• Arrays have nice feature that they are randomly
  accessible - can quickly get to any element
• Dark side - size must be fixed when created.
• Often you dont know much much space you need
  until you are done

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Vector (ArrayList)

• Vector (and ArrayList, same thing, different API)
  get around this by encapsulating a partially
  filled array.
• Hide memory management details behind a simple
  API
• Is still randomly accessible, but now it grows as
  necessary

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Partially Filled Array
5
Size vs Capacity

• The size is the logical size - The number of
  elements. What the programmer thinks. Managed by
  an internal data value.
• The capacity is the size of the physical array.
  Number of elements it can hold.

6
Adding an element

• Adding an element to end is sometimes easy. Just
  increase the (logical) size, and put new value at
  end.
• But what happens when the size reaches the
  capacity?
• Must reallocate new data array - but this detail
  is hidden from user.

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Reallocation and copy
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Adding to Middle

• Adding an element to middle can also force
  reallocation (if the current size is equal to
  capacity)
• But will ALWAYS require elements to moved up to
  make space
• Is therefore O(n) worst case

9
Picture of Adding to Middle

• Must use a loop to make space for new value.
  Careful! Loop from top down

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Removing an Element

• Removing an Element will also require sliding
  over to delete the value
• Therefore is O(n) worst case

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Picture of Remove Element

• Remove also requires loop. This time should it be
  from top or bottom?

12
A word on the future

• Our Vector (and Java lt 1.4) stores values as
  Object - requires a cast when values are moved.
• Java 5.0 and the future vectors will be generic,
  will not require cast, but declaration is
  different. Watch for this soon.
• VectorltBankRecordgt foo
• new VectorltBankRecordgt()

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Your chance

• Anti-Quiz time. Give implementation
• Public class Vector
• Object get(int index)
• void put(int index, Object value)
• void add(Object value)
• void add(int index, Object value)
• void remove(int index)