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Re: A short meditation about hash search performance

by Abigail-II (Bishop)
on Nov 16, 2003 at 02:58 UTC ( #307409=note: print w/ replies, xml ) Need Help??


in reply to Re: A short meditation about hash search performance
in thread A short meditation about hash search performance

Well, if you can't do queries in O(1) time, you can't do deletes in O(1) time (because to delete something, you first need to find it), and you can only do inserts in O(1) if you accept duplicates - which Perl hashes don't.

Abigail


Comment on Re: A short meditation about hash search performance
Re: Re: A short meditation about hash search performance
by Anonymous Monk on Nov 16, 2003 at 05:19 UTC
    Perhaps I need to re-review the analysis of CLR-1990*, because they clearly state on page 223:
    The worst-case running time for insertion is O(1). For searching, the worst-case running time is proportional to the length of the list: we shall analyze this more closely below. Deletion of an element x can be accomplished in O(1) time if the lists are doubly linked.

    Note, I wasn't referring to perl's hashes in particular, just hashes in general.

    * Cormen, Leiserson, Rivest 1990: Introduction to Algorithms. MIT Press, Cambridge Massachusetts.

      Deletion of an element x can be accomplished in O(1) time if the lists are doubly linked.

      It still means you need to find an object before you can delete it.
      If you read page 223 carefully, you see that the arguments to the function Chained-Hash-Delete are (T, x) and not (T, k). Now x is here the node to be deleted. That is, you have already found the node with key k to be deleted. So, yes, for a double linked list, the actual deletion is in constant time, but that does not include the time to search for the node to be deleted.

      Abigail

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