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CS 206 Introduction to Computer Science II 11 / 24 / 2008 Instructor: Michael Eckmann

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Michael Eckmann - Skidmore College - CS 206 - Fall 2008 Today’s Topics Questions/comments? AVL trees –implementation

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An AVL tree is a BST with the added restriction that –for all nodes, the height of its left subtree is at most 1 different than the height of its right subtree. The height of an empty subtree is -1. –A node in an AVL tree contains the data item, reference to left child node reference to right child node height of the node Balanced BSTs

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When we insert a node into an AVL tree we first insert as we would in a BST and update the height information in all the nodes on its path back to the root but there are several situations that we can encounter: a) the tree retains the properties of an AVL tree (that is, no node has left and right subtrees that differ in height by more than 1) b) or some node, alpha, along the path back to the root has subtrees that differ in height by 2 (stop there with the updating of height information) --- there are 4 cases 1) node inserted into LST of the LC of alpha 2) node inserted into RST of the LC of alpha 3) node inserted into LST of the RC of alpha 4) node inserted into RST of the RC of alpha Note: only nodes on the path back to the root could possibly be effected by the insertion. LST = left subtree, RST = right subtree, LC = left child, RC = right child Balanced BSTs

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Red/Black trees are another form of balanced binary search tree, that we will not discuss. There are others too. What I'd like to introduce now is an idea of a tree that worls well for large amounts of data, too big to all fit in memory (RAM). The data will have to partially reside in memory and the rest (the bulk of it) on disk. Why do we care? Any ideas? Other Balanced Trees

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We care because disk accesses take so much longer than main memory accesses. Some numbers on the board. Leading up to B-Trees

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We would rather do many many calculations in order to save us from having to access the disk. The worst case height for an AVL tree is 1.44 log n which is close to optimal (least) height for a set of nodes, but we'll see that for large amounts of data that reside on disk, it will not perform as well as we'd like. The worst case height of an optimal BST is log n. Log n is the best we can do with binary trees and that's not good enough. We want a small constant number of disk accesses. So, instead of using a Binary tree, we'll use an M-ary tree (where M>2.) The least height of an M-ary tree is log M n. Example on the board for M=5 B-Trees are discussed in section 19.8 in your text. Leading up to B-Trees

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B-Trees were created to take that discrepancy in processing time into account (when we have large volumes of data to process.) A B-tree can guarantee only a few disk accesses. An M-ary B-Tree has the following properties –data items stored in leaves only –interior (non-leaf) nodes store a maximum of M-1 keys –root is a leaf or has from 2 to M children –all non-leaf nodes have between the ceil(M/2) up to M children –all leaves at same depth –all leaves have between the ceil(L/2) up to L children L means... (see next few slides) all non leaf nodes have at least half of M children, so for large M this will guarantee that the M-ary tree will not approach anything close to a binary tree (why is that a good thing?) B-Trees

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Each node represents a disk block (the amount of data read in one disk access). –example: if a disk block is 8k each node holds M-1 keys and M branches (links) –let's say our keys are of size 32 bytes each –and a link is 4 bytes How would we determine M? –the largest value such that a node doesn't hold more than 8k Determine L by the number of records we can store in one block. –if our records are 256 bytes, how many could we store in one block of 8k? B-Trees

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Each node represents a disk block (the amount of data read in one disk access). –example: if a disk block is 8k (=8192 bytes) each node holds M-1 keys and M branches (links) –let's say our keys are of size 32 bytes each –and a link is 4 bytes How would we determine M? –the largest value such that a node doesn't hold more than 8k 32*(M-1) + 4*M = 8192 M = 228 (is the largest M such that we don't go over 8192) Determine L by the number of records we can store in one block. –if our records are 256 bytes, how many could we store in one block of 8k? 8192/256 = 32 From the rules of our B-Tree, each leaf then has to have between 16 and 32 records and each non-leaf node (except the root) has to have at least 114 children (up to 228 children). B-Trees

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Example: –From the rules of our B-Tree, each leaf then has to have between 16 and 32 records and each non-leaf node (except the root) has to have at least 114 children (up to 228 children). –Picking the worst case B-tree for our example, that is the one with the least number of children per node to give us our highest possible B-Tree for 10,000,000 records, we'll have at most 625,000 leaves which means the height of our B-tree is 4. The worst height of an M-ary tree is approx. log M/2 n. Why? B-Trees

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