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Questions tagged [hash-tables]

A finite map data structure that addresses stored values using a function that maps many values to few addresses.

A hash table is a finite map (i.e. a dictionary), mapping keys to associated values, which relies on a hash function to map keys to a small integer that is typically used as an index in an array.

Hash tables are more efficient than other finite map data structures for many kinds of data.

Further reading

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Why is it best to use a prime number as a mod in a hashing function?

If I have a list of key values from 1 to 100 and I want to organize them in an array of 11 buckets, I've been taught to form a mod function $$ H = k \bmod \ 11$$ Now all the values will be placed one after another in 9 rows. For example, in the…
CodyBugstein
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(When) is hash table lookup O(1)?

It is often said that hash table lookup operates in constant time: you compute the hash value, which gives you an index for an array lookup. Yet this ignores collisions; in the worst case, every item happens to land in the same bucket and the lookup…
Gilles 'SO- stop being evil'
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Hash tables versus binary trees

When implementing a dictionary ('I want to look up customer data by their customer IDs'), the typical data structures used are hash tables and binary search trees. I know for instance that the C++ STL library implements dictionaries (they call them…
Alex ten Brink
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How Does Populating Pastry's Routing Table Work?

I'm trying to implement the Pastry Distributed Hash Table, but some things are escaping my understanding. I was hoping someone could clarify. Disclaimer: I'm not a computer science student. I've taken precisely two computer science courses in my…
Paddy
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For what kind of data are hash table operations O(1)?

From the answers to (When) is hash table lookup O(1)?, I gather that hash tables have $O(1)$ worst-case behavior, at least amortized, when the data satisfies certain statistical conditions, and there are techniques to help make these conditions…
Gilles 'SO- stop being evil'
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Why are graphs represented as adjacency lists instead of adjacency sets?

In answering this question, I was looking for references (textbooks, papers, or implementations) which represent a graph using a set (e.g. hashtable) for the adjacent vertices, rather than a list. That is, the graph is a map from vertex labels to…
Caleb Stanford
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What are the advantages of cuckoo hashing over dynamic perfect hashing?

Dynamic perfect hash tables and cuckoo hash tables are two different data structures that support worst-case O(1) lookups and expected O(1)-time insertions and deletions. Both require O(n) auxiliary space and access to families of hash functions for…
templatetypedef
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What does "non-pathological data" mean?

I took an algorithms class on Coursera. The professor in the video about hash tables said that What's true is that for non-pathological data, you will get constant time operations in a properly implemented hash table. What does "non-pathological…
Alexander Myshov
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Universal Hashing in Practice

A family $H$ of hash functions $h: U \rightarrow \{0,\ldots,M-1\}$ is universal if $$\forall x,y \in U, x \neq y \Rightarrow \Pr_{h \in H}[h(x) = h(y)] \leq \frac{1}{M}$$ You can find more about universal hashing this wikipedia article. The concept…
Dai
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How are hash table's values stored physically in memory?

Question: How are hash table's values stored in memory such that space if efficiently used and values don't have to be relocated often? My current understanding (could be wrong): Let's say I have 3 objects stored in a hash table. Their hash…
Pwner
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Why should one not use a 2^p size hash table when using the division method as a hash function?

I don't understand what is meant by: "m should not be a power of 2, since if m = 2^p, then h(k) is just the p lowest-order bits of k." (pg. 231 of CLRS) Terms defined: m: size of hash table h(k): hash function = k mod m k: key I don't understand…
zallarak
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Hashing using search trees instead of lists

I am struggling with hashing and binary search tree material. And I read that instead of using lists for storing entries with the same hash values, it is also possible to use binary search trees. And I try to understand what the worst-case and…
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Why is the Java HashMap load factor 0.75?

I can't understand why the Java HashMap load factor is 0.75. If I understand well, the formula for the load factor is n/m, where n is the number of key and m is the number of position in the hash table. Since HashMap utilize bucket (i.e., a linked…
Bender
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Why is a (collision-less) hashtable lookup really O(1)?

Disclaimer: I know there are similar sounding questions already here and on Stackoverflow. But they are all about collisions, which is not what I am asking for. My question is: why is collision-less lookup O(1) in the first place? Let's assume I…
Foo Bar
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How to avoid cascading resizes when resizing hash tables?

With conventional collision resolution methods like separate chaining and linear/quadratic probing, the probe sequence for a key can be arbitrarily long - it is simply kept short with high probability by keeping the load factor of the table low.…
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