Speed up searches with hash tables

Speed up searches with hash tables

You probably know that there are basically two methods to search a value in an array: the brute force approach (i.e. linear searching) and the binary search. Both of them have disadvantages: when the array counts N items, linear searching requires N/2 comparisons on the average for successful searches, and N+1 comparisons for unsuccessful searches; binary search is much more efficient, requiring no more than Log2(N)+1 comparisons, but adds the overhead of keeping the array sorted.

Hash search is a third method that is about as efficient as binary sort, without requiring the added overhead of sorting. You generally build a hash table that counts M items, with M > N, then start adding the original values into this table. To know where each item should go in the hash table, you have to evaluate the so-called hash function, which accepts as an argument the value of the key and returns an Integer value in the range [1,M]. When the key is an integer value, a simple hash function is

hashCode = (key Mod M) + 1

when the key is a string, we can always convert it to a number, for instance by summing the ASCII codes of individual characters. The following routine evaluates the checksum of a string and works better for our purposes:

Function Checksum(text As String) As Long    Dim sum As Long, i As Long    Dim bArray() As Byte    ' move to a byte array    bArray() = StrConv(text, vbFromUnicode)    ' evaluate the sum of array items    For i = 0 To UBound(bArray)        sum = sum + bArray(i) * i    Next    Checksum = sumEnd Function

Of course, no one can guarantee that all keys deliver different hash codes, therefore we must account for collisions. In order to minimize the number of collisions we can choose M (the size of the hash table) to be at least twice as large as N (the number of array items). Better yet, according to sorting theory M should be a prime number. But whatever value we pick for M, we still have to account for collisions. The simplest method to solve them is the so-called linear probing: when a slot in the hash table is already taken, we try with the following slot, until one free slot is found (since M > N, it is impossible that all slots are occupied). When we reach the last item in the hash table, we wrap-around to the first one, and continue the search.

Say we wish to perform hash searches on a dictionary of ten thousand words, so we start with declaring a hash table of exactly twenty thousand items. While many manuals suggest to store the key data directly in the hash table, we will use that table to store the index to the original data:

numEls = 10000ReDim words(numEls) As StringReDim hashTable(2 * numEls) As Integer' read words from a fileOpen "words.dat" For Input As #1For index = 1 To numEls    Line Input#1, words(index)NextClose #1

Next, find the slot corresponding to each word in the original array, and store the corresponding index in the hash table

For index = 1 To numEls    ' search the correct hash index    hashIndex = Checksum(words(index))    ' loop until we find an empty slot    Do        hashIndex = (hashIndex Mod numEls) + 1    Loop While hashTable(hashIndex)    ' store the index in the hash table    hashTable(hashIndex) = indexNext

Now we are finally ready to search any string. The search procedure is very similar to the insert procedure:

search = Text1.TexthashIndex = Checksum(search)Do     hashIndex = (hashIndex Mod numEls) + 1Loop While hashTable(hashIndex) And words(hashTable(hashIndex)) <> searchIf hashTable(hashIndex) Then    Print "Item found at index " & hashTable(hashIndex)Else    Print "Item not found"End If

Note that these routines rely on the assumption that empty slots in the hashTable() have a null value; this works correctly because the words() array includes the zero-th element, even if it is not used for storing any word, therefore the words(hashTable(hashIndex)) subexpression does not raise any error.

It is easy to confirm that well-designed hash searches are more efficient that any other kinds of searches. Researches show that you can even reduce the number of collisions by incrementing the hashIndex variable by a value K different from one, but which be relative prime to M (this condition is automatically enforced if K < M and M is prime). In the previous example we could set K = 17 (which is relatively prime to 20,000) and replace the statement within both Do...Loop blocks as follows:

hashIndex = ((hashIndex + 16) Mod numEls) + 1

devx-admin

devx-admin

Share the Post:
Software Development

Top Software Development Companies

Looking for the best in software development? Our list of Top Software Development Companies is your gateway to finding the right tech partner. Dive in

India Web Development

Top Web Development Companies in India

In the digital race, the right web development partner is your winning edge. Dive into our curated list of top web development companies in India,

USA Web Development

Top Web Development Companies in USA

Looking for the best web development companies in the USA? We’ve got you covered! Check out our top 10 picks to find the right partner

Clean Energy Adoption

Inside Michigan’s Clean Energy Revolution

Democratic state legislators in Michigan continue to discuss and debate clean energy legislation in the hopes of establishing a comprehensive clean energy strategy for the

Chips Act Revolution

European Chips Act: What is it?

In response to the intensifying worldwide technology competition, Europe has unveiled the long-awaited European Chips Act. This daring legislative proposal aims to fortify Europe’s semiconductor

Revolutionized Low-Code

You Should Use Low-Code Platforms for Apps

As the demand for rapid software development increases, low-code platforms have emerged as a popular choice among developers for their ability to build applications with

Software Development

Top Software Development Companies

Looking for the best in software development? Our list of Top Software Development Companies is your gateway to finding the right tech partner. Dive in and explore the leaders in

India Web Development

Top Web Development Companies in India

In the digital race, the right web development partner is your winning edge. Dive into our curated list of top web development companies in India, and kickstart your journey to

USA Web Development

Top Web Development Companies in USA

Looking for the best web development companies in the USA? We’ve got you covered! Check out our top 10 picks to find the right partner for your online project. Your

Clean Energy Adoption

Inside Michigan’s Clean Energy Revolution

Democratic state legislators in Michigan continue to discuss and debate clean energy legislation in the hopes of establishing a comprehensive clean energy strategy for the state. A Senate committee meeting

Chips Act Revolution

European Chips Act: What is it?

In response to the intensifying worldwide technology competition, Europe has unveiled the long-awaited European Chips Act. This daring legislative proposal aims to fortify Europe’s semiconductor supply chain and enhance its

Revolutionized Low-Code

You Should Use Low-Code Platforms for Apps

As the demand for rapid software development increases, low-code platforms have emerged as a popular choice among developers for their ability to build applications with minimal coding. These platforms not

Cybersecurity Strategy

Five Powerful Strategies to Bolster Your Cybersecurity

In today’s increasingly digital landscape, businesses of all sizes must prioritize cyber security measures to defend against potential dangers. Cyber security professionals suggest five simple technological strategies to help companies

Global Layoffs

Tech Layoffs Are Getting Worse Globally

Since the start of 2023, the global technology sector has experienced a significant rise in layoffs, with over 236,000 workers being let go by 1,019 tech firms, as per data

Huawei Electric Dazzle

Huawei Dazzles with Electric Vehicles and Wireless Earbuds

During a prominent unveiling event, Huawei, the Chinese telecommunications powerhouse, kept quiet about its enigmatic new 5G phone and alleged cutting-edge chip development. Instead, Huawei astounded the audience by presenting

Cybersecurity Banking Revolution

Digital Banking Needs Cybersecurity

The banking, financial, and insurance (BFSI) sectors are pioneers in digital transformation, using web applications and application programming interfaces (APIs) to provide seamless services to customers around the world. Rising

FinTech Leadership

Terry Clune’s Fintech Empire

Over the past 30 years, Terry Clune has built a remarkable business empire, with CluneTech at the helm. The CEO and Founder has successfully created eight fintech firms, attracting renowned

The Role Of AI Within A Web Design Agency?

In the digital age, the role of Artificial Intelligence (AI) in web design is rapidly evolving, transitioning from a futuristic concept to practical tools used in design, coding, content writing

Generative AI Revolution

Is Generative AI the Next Internet?

The increasing demand for Generative AI models has led to a surge in its adoption across diverse sectors, with healthcare, automotive, and financial services being among the top beneficiaries. These

Microsoft Laptop

The New Surface Laptop Studio 2 Is Nuts

The Surface Laptop Studio 2 is a dynamic and robust all-in-one laptop designed for creators and professionals alike. It features a 14.4″ touchscreen and a cutting-edge design that is over

5G Innovations

GPU-Accelerated 5G in Japan

NTT DOCOMO, a global telecommunications giant, is set to break new ground in the industry as it prepares to launch a GPU-accelerated 5G network in Japan. This innovative approach will

AI Ethics

AI Journalism: Balancing Integrity and Innovation

An op-ed, produced using Microsoft’s Bing Chat AI software, recently appeared in the St. Louis Post-Dispatch, discussing the potential concerns surrounding the employment of artificial intelligence (AI) in journalism. These

Savings Extravaganza

Big Deal Days Extravaganza

The highly awaited Big Deal Days event for October 2023 is nearly here, scheduled for the 10th and 11th. Similar to the previous year, this autumn sale has already created

Cisco Splunk Deal

Cisco Splunk Deal Sparks Tech Acquisition Frenzy

Cisco’s recent massive purchase of Splunk, an AI-powered cybersecurity firm, for $28 billion signals a potential boost in tech deals after a year of subdued mergers and acquisitions in the

Iran Drone Expansion

Iran’s Jet-Propelled Drone Reshapes Power Balance

Iran has recently unveiled a jet-propelled variant of its Shahed series drone, marking a significant advancement in the nation’s drone technology. The new drone is poised to reshape the regional

Solar Geoengineering

Did the Overshoot Commission Shoot Down Geoengineering?

The Overshoot Commission has recently released a comprehensive report that discusses the controversial topic of Solar Geoengineering, also known as Solar Radiation Modification (SRM). The Commission’s primary objective is to

Remote Learning

Revolutionizing Remote Learning for Success

School districts are preparing to reveal a substantial technological upgrade designed to significantly improve remote learning experiences for both educators and students amid the ongoing pandemic. This major investment, which

Revolutionary SABERS Transforming

SABERS Batteries Transforming Industries

Scientists John Connell and Yi Lin from NASA’s Solid-state Architecture Batteries for Enhanced Rechargeability and Safety (SABERS) project are working on experimental solid-state battery packs that could dramatically change the

Build a Website

How Much Does It Cost to Build a Website?

Are you wondering how much it costs to build a website? The approximated cost is based on several factors, including which add-ons and platforms you choose. For example, a self-hosted

Battery Investments

Battery Startups Attract Billion-Dollar Investments

In recent times, battery startups have experienced a significant boost in investments, with three businesses obtaining over $1 billion in funding within the last month. French company Verkor amassed $2.1