in reply to Re: Re: A short meditation about hash search performance
in thread A short meditation about hash search performance
You obviously don't understand what O(1) means.Let's see. The definition of big O is:
I don't have any problem understanding with it. In layman terms, it means that a function f of n is in the order of g of n, if, and only if, there's a constant, such that if n gets large enough, the value of f is at most the value of g times said constant.f(n) = O (g (n)) iff there are a M > 0 and a c > 0 such that for all m > M, 0 <= f(m) <= c * g (m). [1] [ +2] [3]
Search from beginning to end, going thru each element one by one, until hit what you are searching for. In the worst case (the element is at the end of the array), you have to hit 1 billion elements, but according to you, that's O(1). I say it is O(n). We never put a restriction saying that an array can at most contain 1 billion elements (so the size of an array in general is not a constant, although it is a constant for a given array at one given observation point.)Hello? We never put a restriction on the size? Come again. What do you call:
And still O(1) is not reachable, unless each element resolve a unique key ;)That's a restriction of 1. You started out by putting restrictions on it, claiming that only if there's a restriction of a size of 1, the search algorithm is O (1). I on the other hand pointed out that as long as there is a restriction on the limit of the chain, it doesn't matter what the restriction is, 1, 14 (for 5.8.2), or a billion. If there's a restriction on the size, even with a linear search it's O (1). Here's a proof:
Suppose the chain is limited to length K, where K is a constant, independent of the amount of keys in the hash. Searching for a key is a two step process: first we need to find the bucket the key hashes to, then we need to find the key in the associated chain. Finding the right bucket takes constant time. Traversing the chain takes at most K * e time, for some constant e. So, searching for the element takes at most:I won't deny the performance will be rather lousy, but it's still O (1). Which proves that bigOh doesn't say everything.e * K + O (1), e >= 0 {definition of O()} <= e * K + d * 1, e >= 0, d >= 0 {arithmetic} == (e * K + d) * 1, e >= 0, d >= 0 {c == e * K + d} == c * 1 {c > 0} == O (1). q.e.d.
Abigail


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Re: Re: A short meditation about hash search performance
by demerphq (Chancellor) on Nov 17, 2003 at 08:58 UTC  
by AbigailII (Bishop) on Nov 17, 2003 at 09:34 UTC 