CS 228 meeting -*- Outline -*-

* introduction to searching and hashing (HR 12.4-6)

	This section of chapter 12
		studies the problem of associative retrieval,
		or searching in random access data structures.

** motivating question

	Q: How to find info associated with a key in a (online) database?

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   SEARCHING AND HASHING (HR 12.4-6)

          MOTIVATING PROBLEM

Given a collection of records,
retrieve the record with a given key.

Example:
   A poison control hotline.
   Records are

      struct Poison {
        String name;      // the key
        String treatment;
      };
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	The hotline takes calls from people,
	after finding out the substance's name, reads the remedy to the caller.
	Have *lots* of poisons listed (hundreds of thousands).
	Since it's a nonprofit organization, it has an old, slow computer (:-)
	Want the lookup of a treatment to be fast.

	Less dramatic examples are also very common:
		getting information associated with a file name
			from a directory
		looking up telephone numbers
		etc.

** overview
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     SEARCHING AND HASHING
           OVERVIEW

Techniques	     Time cost
 A. linear search    O(N)
 B. binary search    O(log N)
 C. hashing          O(1) on average

Key insight:

    Changing the data structure
    leads to qualitative improvements
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	the insight has very wide application!

	this is also the outline

	Caveat: because these use different data structures,
		need to use all of them at different times,
		so can't just, say, use hashing.