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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.

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How to select hash functions for cuckoo hashing?

In cuckoo hashing, an insertion failure occurs when there is an infinite loop of displacements. This is detected by a “max iterations” count on the insert function. When the insert fails, the table is ...
haz's user avatar
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What is the probability of no collision in two hash tables, where a key is hashed to both tables and chained to the shortest chain from the tables?

can someone help me with this problem? Consider a hashing scheme that uses r hash tables T1, . . . , Tr of size m, where collisions in each table are resolved by chaining. When a key k arrives, it is ...
Barud_77's user avatar
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Create a class or structure like union of ranges

How to create a structure which acts as union of ranges. In that structure new ranges can be inserted beforehand and then some queries are asked to find out if the given point is covered by any of the ...
user166204's user avatar
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Trouble Understanding Yao's "Basic Model" for Hashing

I am trying to understand the probing model proposed by Yao in the paper "Should Tables Be Sorted?". Yao suggests a "Basic Model" (in the section appropriately titled "Basic ...
Dair's user avatar
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Find a substring length $k$ with maximum occurrences

Given a string length $S$, find a substring length $k$ that has the most occurrences in the given string. We want $O(S)$ time complexity in an average case. I think the solution lies in sophisticated ...
popcorn's user avatar
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Effect of Insertion order on number of collisions in Double Hashing

How is the number of collisions affected by the order of insertion of elements in a hash table using double hashing? I've tried uing different hash function for double hashing with different set of ...
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Collisions in hash table - Is this table wrong?

I have the following hash table with hashcodes being quadratic numbers. Why do 0, 16, and 64 have different number of collisions when they map to the same number (line %16). Also how is it possible ...
Rubus's user avatar
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How to properly save Zobrist Keys?

In a project of mine, I want to represent states as efficiently as possible. Initially, I went ahead with a BitBoard representation and some clever tricks to speed up the evaluation of the game tree. ...
J. M. Arnold's user avatar
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Chord Ring with limited table size of 3

In the normal case of a chord ring the big O notation of the look up is O(logn) because of long haul pointers of the Finger Table (or Routing Table). In this question what if the Finger Table has a ...
DarkArtistry's user avatar
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1 answer
179 views

Simple Uniform hashing with chances of no collision

I know that if I have $n$ different values in an array of size $m$ (where $m>n$) under simple uniform hashing, the average probability of the total number of collisions is: $$\sum_{i=1}^{n-1}\frac{...
Mike's user avatar
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Order-preserving hashtable for integer tuples

There are integer tuples which index cells of a sparse multi-dimensional array (points inside n-parallelepiped), $n \le 32$. The array itself is a BST with keys formed as $key = (...((a_0 * S_1 + a_1) ...
Andrey Godyaev's user avatar
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Slot implementation of LRU cache

Usually LRU cache is implemented with double linked list to fast remove oldest visited, push to front, and move from any place to front; also uses map to fast find element by key. In 64bit systems ...
Saku's user avatar
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How to deal with a very big hash table?

I'm building an implementation of the dynamo paper, yottastore. Given a key, I need to find which NVMe block stores the data. To do that I hash the key to find the shard where I have an in memory ...
Mascarpone's user avatar
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Universal family of hash functions — dependent on table size?

Given the following family of hash functions: $$ \mathbb{H} = \{h_c(x) = (12x + c) \bmod m \mid c \in \mathbb{N} \}, $$ where $m$ is the key size. Prove that $\mathbb{H}$ is not a universal ...
blox2212's user avatar
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Is there a hash-map implementation optimized to favor key lookups based on frequency (if key is referenced the most then it is searched first)?

I have started to play around with HTTP3 which relies on QUIC for transport. I have noticed that I very often have a finite number of Web Transport sessions stream data continuously along side burst-y ...
Liam Kelly's user avatar
1 vote
3 answers
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What is a good load factor for seperate chaining (closed addressing)?

For open addressing, I know that once you have around 70% table being filled you should resize because more than that you get collision. But for closed addressing, I read that load factor should not ...
Bam's user avatar
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Consistent Hashing Algorithm Without Distribution / Load Balancing

I need some help finding or creating a consistent hashing algorithm with the following properties: Given N buckets, only distributes keys to bucket N. When number of buckets are increased from N to N+...
scroobius's user avatar
1 vote
2 answers
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Averege time complexity of open addressing

I get that it depends from the number of probes, so by how many times the hash code has to be recalculeted, and that in the best case there will only be one computation of the hash code and the ...
Spyromancer's user avatar
11 votes
2 answers
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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 ...
Bender's user avatar
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Confused about the appropriate slot in linear probing

The following paragraph is from the book CLRS: Given an ordinary hash function $h' : U \rightarrow \{0, 1, ..., m - 1\}$, which we refer to as an auxiliary hash function, the method of linear probing ...
Emad's user avatar
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Iterable hash table

I'm currently working on implementing all the major data structures from C++ to better understand them and I arrived at the unordered_map, which should be a hash table implementation (it's actually ...
Petok Lorand's user avatar
1 vote
2 answers
201 views

Hash maps vs extending structs / classes

A hash map that maps keys of type $K$ into values of type $V$, is essentially equivalent to "extending" this type $K$ to also contain an Option<V> ...
nir shahar's user avatar
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Unusual approach to tabulation algorithm

Consider simple tabulation algorithm firstly for Fibonacci numbers. We will use the dictionary as a cache (and Python as example PL): ...
lesobrod's user avatar
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1 answer
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Analysis of a calculation of expected number of collisions in hashing

For a formal problem statement, I quote from the text Introduction to Algorithms by Cormen et. al Suppose we use a hash function $h$ to hash $n$ distinct keys into an array $T$ of length $m$. ...
Abhishek Ghosh's user avatar
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worst case time for adding values to a hash table

A certain hash table uses external chaining, and is resized each time its load factor exceeds 3. Also, if an addition does not cause the load factor to exceed 3, but does cause some bucket to exceed 6 ...
daniel's user avatar
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Does DHT (Distributed Hash Table) allow to list ALL (not only nearest) nodes?

As I know any local DHT keeps only some of nodes - if the network is small/very small then all nodes, otherwise only some of them (nearest?). So, when I checked different Golang implementations of DHT ...
RandomB's user avatar
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14 votes
4 answers
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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. ...
Caleb Stanford's user avatar
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3 answers
183 views

How do i delete one single unequal element from an array of equal elements without using hashing?

The rules of the question state that: Only one element is different. Rest are all same. Array A size is 8. I need to find the different element and remove it (Hashing cannot be used). I have not ...
Swarnava Chandra Bose's user avatar
1 vote
1 answer
191 views

A hash function for a 2D hash table with a scattering property?

I have an $n\times n$ matrix, and want to find a bijective function $h:[n^2] \to [n]\times [n]$ that can act as a hash function to map the numbers 1 through $n^2$ to row/column indices in my matrix. ...
RJL's user avatar
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1 answer
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Build a priority queue using dictionary

QUESTION: Can we Implementing priority queue with hash table please? As I understand, dictionary/hash table does not have sorting mechanism inside. It adds pairs of $<key,value>$ to hash table. ...
Avv's user avatar
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2 answers
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How does a hash table not require all the keys to be looked through to find a value, i.e. O(N)?

Before starting, I want to say that I understand time complexity, and I understand how a hash table is considered O(1) vs an array having a time comp. of O(N) in terms of what you learn. What I don't ...
Jake Jackson's user avatar
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1 answer
512 views

hash-tables - Expected-time for an unsuccessful search

The following question is from MIT-OCW 6.006, Spring-2008, Problem-Set 2, Q-3.c. Suppose you have a hash table where the load-factor $\alpha$ is related to the number $n$ of elements in the table by ...
x.projekt's user avatar
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1 vote
1 answer
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How does the Go language implements maps?

To what extent is the MAP data type implementation language dependant??? If we know that C++ implements maps as Red Black Trees, unordered maps as hash tables, How C++ and alike maps are actually ...
ShAr's user avatar
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1 answer
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The interpretation of expected time bound for searches in a hash table

As CLRS book,page 260 stated, Thus, the total time required for a successful search is $\Theta{\left(2+\alpha/2-\alpha/2n\right)}=\Theta{(1+\alpha)}$ I wouldn't have any problem if the author says ...
absuu's user avatar
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1 answer
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If $(h_2(k),m) = 1$ then $h_1(k)+ih_2(k) \bmod{m}$ is a permutation of $0,\ldots,m-1$

The following question appears in Introduction to Algorithms (CLRS): Suppose that we use double hashing to resolve collisions; that is, we use the hash function $ h(k, i) = (h_1(k) + ih_2(k)) \bmod{m}...
Avv's user avatar
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Confused about a corollary in MIT.Introduction.to.Algorithms(CLRS), seek help

Desc' I was reading CLRS Book's chapter 11 Hash Tables, and encountering some problems. There is an corollary in CLRS book, in Page 275 (quoted on the bottom of post), which discussed the expected ...
absuu's user avatar
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1 vote
1 answer
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Detecting and correcting collisions in (Zorbist) hashing to avoid errors in transposition table

Context Say I have a transposition table that uses keys produced by (e.g. Zorbist) hashing game positions. The table has a finite recycled memory (key % p is the <...
Vepir's user avatar
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1 vote
1 answer
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Number of unique values in array in $\theta(n)$ average expected time

My idea is to initialize a hash table (with chaining) with $n$ cells, having load factor $\alpha = 1$ hence having $\theta(1)$ expected number of values in each cell in the hash table, then go cell by ...
user183748292's user avatar
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0 answers
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The Most efficient algorithm for a program that returns true if the two strings only differ by one character

We have a list of strings with the same length. Each string only contains a, b and c.(For instance : aabcc) The program gets a string and returns true if the two strings(input and the list of strings) ...
emma funkhouser's user avatar
2 votes
2 answers
83 views

What kind of data structure should be used for working with dynamic sets where elements follow "normal distribution" and why?

I am wondering if Hash tables would be a good choice as it mentions dynamic sets to store data which follows normal distribution. I have doubt in this because normal distribution has bell shaped curve ...
Yukti Kumari's user avatar
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2 answers
130 views

Find distinict elements in an array in $O(n)$ time

Given this pseudo-code that finds the number of distinct elements in the given array: ...
Omri Braha's user avatar
2 votes
0 answers
118 views

Finding the longest power subsequence

A power sequence is a sequence containing consecutive powers of a number starting from power one. for example $3^1, 3^2, 3^3$ is a power sequence. The question is to find the length of the longest ...
user136515's user avatar
2 votes
1 answer
1k views

Time complexity of checking whether a string exists in a HashSet

From what I know hash sets generally have complexity of $O(1)$ (unless the hash function is bad, but let's just ignore that for this question). However, sets need to either read the full data so as to ...
asharpharp's user avatar
2 votes
1 answer
86 views

How do we pick and switch hash functions from a universal family?

When we have a universal family of hash functions, it gives us a few useful mathematical guarantees. But, if we pick a specific function from the family and use it all the time it's effectively like ...
snatchysquid's user avatar
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0 answers
33 views

Hash tables for storing data in an application that supports partial search

Is a hash table a good data structure for storing data in an application that support partial search such as "select * from MyTable where name like 'John%'. If not, what is? I would have thought ...
Trajan's user avatar
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0 answers
106 views

Bloom filter variant(using only half of it)

A Bloom filter is an array $B[1..m]$ of bits, together with a collection of $k$ independent ideal random hash functions $h_1,h_2,...,h_k$. To insert an item $x$ into a Bloom filter, we set $B[h_i(x)] ←...
kings's user avatar
  • 141
2 votes
1 answer
163 views

find the strong component containing the vertex v

I'm trying to solve this question for practising purposes: "Describe a linear-time algorithm for computing the strong component containing a given vertex v. On the basis of that algorithm, ...
Mika2019's user avatar
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175 views

Why is LRU cache not implemented using doubly linked list with just two nodes? Do we even need double linked list?

I don't understand why do we need to have more than two nodes in doubly linked list when all we need to maintain is the most recent and least recently accessed nodes. I was going through solutions for ...
SamuraiJack's user avatar
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0 answers
222 views

Algorithm question: Given a binary array, find the maximum length of a contiguous subarray with equal number of 0 and 1

Algorithm question: Given a binary array, find the maximum length of a contiguous subarray with equal number of 0 and 1. Example 1: Input: [0,1] Output: 2 Explanation: [0, 1] is the longest ...
ayuken's user avatar
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-1 votes
1 answer
106 views

Find best value for size of the table in hashing

Given a set of keys $\{i^2|0\leq i\leq 100\}$, and a hash table such that collision resolve by chaining. Our hash function is $ k \mod m$ . What is option is best for $m$ to decrease our search for ...
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