Binary Search Example

1
Binary Search Example

• CSC 172
• SPRING 2004
• LECTURE 5

2
Reminder

• Project Due Friday 5PM

3
Lookup Array

• public boolean lookup(Object o)
• for (int j 0 j lt lengthj)
• if (datumj.equals(o)) return true
• return false

4
So, suppose we kept the list sorted

• Each insert now costs n
• But what happens to lookup?

5
Guessing Game

• Im thinking of a number 1ltxlt64
• Ill tell you higher or lower
• How many guesses do you need?
• 29
• 47
• 3
• You know with higher or lower you can divide
• The remaining search space in half (i.e.
  log2(64)6)

6
Binary Search

• Recursively divide the array half until you find
  the item (if it is sorted).

7
Lookup on sorted array

• public boolean lookup(Object o)
• return lookupRange(Object o,0,length-1)

8
LookupRange Binary Search

• public static void
• lookupRange(Object o, int lower,int higher)
• if (lower gt higher) return false
• else
• int mid (low high)/2
• if (datummid.compareTo(o) lt 0)
• return lookupRange(o,lower,mid)
• else if (dataummid.compareTo(o) gt 0)
• return lookupRange(o,mid1,higher)
• else return true // o datummid

9
All very well and good

• But can it help me be politically aware?
• YES!
• In order to be aware of a candidates position we
  have to call them and talk.
• So, you have to look up someones phone number in
  the phone book.
• You can save time by using binary search.

10
VOTE FOR PRESIDENT

• John Kerry
• Howard Dean
• Wesley Clark
• John Edwards
• Joe Lieberman
• George Bush
• (Write in)