2. Binary Search Example CSC 172 SPRING 2004 LECTURE 5

3. Reminder Project Due Friday 5PM

4. Lookup: Array public boolean lookup(Object o){ for (int j = 0 ; j < length;j++) if (datum[j].equals(o)) return true; return false; }

5. So, suppose we kept the list sorted  Each insert now costs “n”  But what happens to lookup?

6. Guessing Game  I’m thinking of a number 1<=x<=64  I’ll tell you “higher” or “lower”  How many guesses do you need?  29  47 33 You know with “higher” or “lower” you can divide The remaining search space in half (i.e. log 2 (64)==6)

7. Binary Search  Recursively divide the array half until you find the item (if it is sorted).

8. Lookup on sorted array public boolean lookup(Object o) { return lookupRange(Object o,0,length-1); }

9. LookupRange: Binary Search public static void lookupRange(Object o, int lower,int higher){ if (lower > higher) return false; else { int mid = (low + high)/2; if (datum[mid].compareTo(o) < 0) return lookupRange(o,lower,mid); else if (dataum[mid].compareTo(o) > 0) return lookupRange(o,mid+1,higher); else return true; // o == datum[mid] }

10. All very well and good  But can it help me be politically aware?  YES!  In order to be aware of a candidate’s position we have to call them and talk.  So, you have to look up someone’s phone number in the phone book.  You can save time by using binary search.

11. VOTE FOR PRESIDENT  John Kerry  Howard Dean  Wesley Clark  John Edwards  Joe Lieberman  George Bush  (Write in)