Monday, October 12, 2009

Probing and Probing Sequence

It should be obvious that when we need to locate an element in a hashed list, we must use the same algorithm that we used to insert it into the list. Consequently, we first hash the key and check the home address to determine whether it contains the desired element. If it does, the search is complete. If not, we must use the collision resolution algorithm to determine the next location and continue until we find the element or determine that it is not in the list. Each calculation of an address and test for success is known as a probe. In case of collisions, method of open addressing produces alternate lists of addresses. The process of probing produces a probing sequence which is a sequence of locations that we examine when we attempt to insert a new key into the hashed table, T. The first location in the probe sequence is the hash address, H (K). The second and successful locations in the probe sequence are determined by Collision Resolution Policy. To guarantee availability of always an empty location in every probe sequence, we define a “Full” Table T, to be a table having exactly one empty table entry.

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