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Searching the Web

Searching the Web. The web can be considered a graph “ the web graph ” Web pages are the graph nodes Hyperlinks on pages are graph edges The web graph is huge (way over 8 billion nodes) - even infinite (pages are created on the fly)

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Searching the Web

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  1. Searching the Web • The web can be considered a graph “the web graph” • Web pages are the graph nodes • Hyperlinks on pages are graph edges • The web graph is huge (way over 8 billion nodes) - even infinite (pages are created on the fly) • For the web graph, DFS is not a good strategy (you get lost quickly) • You need to search your neighborhood before going deeper

  2. Syllabus Assignments Documentation CS Dept. Brian Text PLTC ... ... ... ... ... ... The Web Graph - Starting at CS230 Home CS230

  3. S A H I J C B D E G F Breadth-First Search (BFS): Neighbors First • In DFS you keep the nodes you visited on a stack so that you can find your way back • In BFS you keep the nodes* you visited on a queue so that you can explore them in the order you found them • At every visited node you also keep track of the direct path that took you there from S in a list* actually the paths to the nodes, not just the nodes Initialization While you have not reached G remove path from BFS queue and check at the last node L in the path extend the path to unvisited neighbors of L and add extended paths to back of queue

  4. S[S] A [SA] H [SH] I [SHI] J [SHJ] C [SAC] B [SAB] D [SACD] E [SACE] G[SHJG] F BFS: Queue of Paths S SA SH SH SAB SAC SAB SAC SHI SHJ SAC SHI SHJ SHI SHJ SACD SACE SHJ SACD SACE Initialization While you have not reached G remove path from BFS queue and check at the last node L in the path extend the path to unvisited neighbors of L and add extended paths to back of queue SACD SACE SHJG SACE SHJG … in two more steps it will reach the goal

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