02-assoc

G occurrences of pairs of items the number of

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Unformatted text preview: g counts in/out is a disaster (why?) 1/5/2011 Jure Leskovec, Stanford C246: Mining Massive Datasets 21 The hardest problem often turns out to be finding the frequent pairs of items {i1, i2} Often frequent pairs are common, frequent triples are rare We’ll concentrate on pairs, then extend to larger sets The game: We always need to generate all the itemsets But we would only like to count/keep track only of those that at the end turn out to be frequent 1/5/2011 Jure Leskovec, Stanford C246: Mining Massive Datasets 22 Read file once, counting in main memory the occurrences of each pair: From each basket of n items, generate its n(n-1)/2 pairs by two nested loops Fails if (#items)2 exceeds main memory Remember: #items can be 100K (Wal-Mart) or 10B (Web pages) 1/5/2011 Jure Leskovec, Stanford C246: Mining Massive Datasets 23 Approach 1: Store triples [i, j, c], where count(i , j) = c If integers and item ids are 4 bytes, we need approximately 12 bytes for pairs with count > 0 Plus some additional overhead for the hashtable What if most pairs occur, even if infrequently? 1/5/2011 Jure Leskovec, Stanford C246: Mining Massive Datasets 24 Approach 2: Count all pairs Number items 1, 2, 3,…, n Count {i, j} only if...
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