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Unformatted text preview: 1 HashBased Improvements to APriori ParkChenYu Algorithm Multistage Algorithm Approximate Algorithms 2 PCY Algorithm Hashbased improvement to APriori. During Pass 1 of Apriori, most memory is idle. Use that memory to keep counts of buckets into which pairs of items are hashed. Just the count, not the pairs themselves. Gives extra condition that candidate pairs must satisfy on Pass 2. 3 Picture of PCY Hash table Item counts Bitmap Pass 1 Pass 2 Frequent items Counts of candidate pairs 4 PCY Algorithm  Before Pass 1 Organize main memory: Space to count each item. One (typically) 4byte integer per item. Use the rest of the space for as many integers, representing buckets, as we can. 5 PCY Algorithm  Pass 1 FOR (each basket) { FOR (each item) add 1 to items count; FOR (each pair of items) { hash the pair to a bucket; add 1 to the count for that bucket } } 6 PCY Algorithm  Between Passes Replace the buckets by a bitvector: 1 means the bucket count the support s ( frequent bucket ); 0 means it did not. Integers are replaced by bits, so the bit vector requires little secondpass space. Also, decide which items are frequent and list them for the second pass. 7 PCY Algorithm  Pass 2 Count all pairs { i , j } that meet the conditions: 1. Both i and j are frequent items. 2. The pair { i , j }, hashes to a bucket number whose bit in the bit vector is 1....
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 Spring '09
 Algorithms

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