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Table of contents
1.
Introduction
2.
How to Work with Hash Function in C++
2.1.
C++
3.
Examples to Understand the Working of Hashing Function in C++
3.1.
Example 1: Simple Hash Function for Integers
3.2.
C++
3.3.
Example 2: Hash Function for Strings
3.4.
C++
3.5.
Example 3: Custom Hash Function for a User-Defined Data Type
3.6.
C++
3.7.
Example 4: Hash Function for Complex Data Structures
3.8.
C++
4.
Frequently Asked Questions
4.1.
Why are hash functions important in C++?
4.2.
Can hash functions guarantee unique results for different inputs?
4.3.
How do hash functions improve program performance?
5.
Conclusion
Last Updated: Apr 2, 2024
Easy

Hash Function in C++

Author Rinki Deka
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Prerita Agarwal
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23 Jul, 2024 @ 01:30 PM

Introduction

Hash functions in C++ are like the secret codes that keeps our data safe and easy to access. Imagine you've got a bunch of keys, but instead of labeling them, you use a special code to know which key opens what. That's sort of what hash functions do with our data. They take our information, no matter how big, and turn it into a shorter, fixed-size value or key. This makes storing, searching, and retrieving data super quick and efficient.

Hash Function in C++

In this article, we're going to learn about the magic of hash functions. We'll explore how they work in the C++ world and why they're so important for keeping our data organized.

How to Work with Hash Function in C++

Working with hash functions in C++ might sound complicated, but it's really just about turning your data into a smaller, more manageable form without losing its essence. It's like when you summarize a long story into a few key sentences. In C++, hash functions take your data (like numbers, strings, or any other kind of information), crunch it down, and spit out a shorter, fixed-size piece of data called a 'hash value'. This hash value is unique to the original data. If you change just one tiny bit of the original data, the hash function gives you a completely different hash value.

Hash functions in C++ are quite handy. They help in quickly comparing large sets of data, securing information, and even in building data structures like hash tables.

Here's a simple way to start using hash functions in C++:

  • Choose Your Hash Function: C++ doesn't have a built-in hash function like some other languages, but it offers a great library called <functional> that includes a generic hash function. You can use this for basic types like integers and strings.
     
  • Apply the Hash Function: You take your data, pass it through the hash function, and get your hash value. This process is the same whether your data is a number, a sentence, or something else.
     
  • Use the Hash Value: You can use this hash value for various purposes like storing your data in a hash table, checking if two pieces of data are the same, or even encrypting your data.


Let's see a simple code example where we use a hash function on a string:

  • C++

C++

#include <iostream>
#include <functional>
#include <string>

int main() {
std::string myData = "Hello, hash functions!";
std::hash<std::string> hashFunction; // Create a hash function object for strings
size_t hashValue = hashFunction(myData); // Apply the hash function

std::cout << "The hash value for '" << myData << "' is: " << hashValue << std::endl;

return 0;
}

Output

The hash value for 'Hello, hash functions!' is: 17195561476320278156


In this code, we first include the necessary libraries. Then, we create a string called myData that we want to hash. We use the std::hash function from the <functional> library to hash myData. Finally, we print out the hash value.

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Examples to Understand the Working of Hashing Function in C++

To really understand how hashing functions work in C++, let's look into some examples. These will help you see the practical side of hashing and how it's used in coding.

Example 1: Simple Hash Function for Integers

Let's start with a basic example. We'll write a simple hash function that works with integers. This function will take an integer, do some operations, and give back a hash value.

  • C++

C++

#include <iostream>

// A simple hash function for integers
unsigned int simpleHash(int key) {
return key % 100; // Let's say our hash table size is 100
}

int main() {
int number = 123456;
unsigned int hashValue = simpleHash(number);

std::cout << "The hash value for " << number << " is: " << hashValue << std::endl;

return 0;
}

Output

The hash value for 123456 is: 56

 

In this code, our hash function takes an integer and uses the modulo operator % with 100. This means our hash table could have 100 slots, and this function places each integer into one of those slots based on the remainder when divided by 100. It's a simple way to distribute integers across a fixed number of buckets.

Example 2: Hash Function for Strings

Now, let's tackle something a bit more complex - a hash function for strings. We'll use a common technique where each character's ASCII value contributes to the final hash.

  • C++

C++

#include <iostream>
#include <string>

// A basic hash function for strings
unsigned long hashString(const std::string &str) {
unsigned long hash = 5381; // Starting value
int c;

for (auto s : str) {
c = s;
hash = ((hash << 5) + hash) + c; // hash * 33 + c
}

return hash;
}

int main() {
std::string text = "HelloWorld";
unsigned long hashValue = hashString(text);

std::cout << "The hash value for \"" << text << "\" is: " << hashValue << std::endl;

return 0;
}

Output

The hash value for "HelloWorld" is: 8244914940577085761

 

In this string hash function, we start with a hash of 5381. Then, for each character in the string, we multiply the current hash by 33 (done efficiently as (hash << 5) + hash) and then add the ASCII value of the character. This gives us a hash value that's unique for different strings.

Example 3: Custom Hash Function for a User-Defined Data Type

In this example, we'll create a custom hash function for a user-defined data type. Imagine we have a simple Point class representing a point in 2D space with x and y coordinates. We want to create a hash function that uniquely identifies each point.

  • C++

C++

#include <iostream>

class Point {
public:
int x, y;

Point(int x, int y) : x(x), y(y) {}

// Custom hash function for Point
size_t hash() const {
return std::hash<int>()(x) ^ std::hash<int>()(y);
}
};

int main() {
Point p(5, 10);
size_t pointHash = p.hash();

std::cout << "The hash for Point(" << p.x << ", " << p.y << ") is: " << pointHash << std::endl;

return 0;
}

Output

The hash for Point(5, 10) is: 15


In this code, the Point class has a hash method that combines the hash values of x and y using the bitwise XOR operator ^. This method uses std::hash<int>(), which is a standard hash function for integers, to generate hash values for x and y before combining them. This approach provides a simple way to generate a unique hash for each Point object.

Example 4: Hash Function for Complex Data Structures

For our fourth example, let's consider hashing a more complex data structure. Suppose we have a Book class that includes a title, author, and publication year. We want to create a hash function that considers all these attributes to generate a unique hash value for each book.

  • C++

C++

#include <iostream>
#include <string>
#include <functional>

class Book {
public:
std::string title;
std::string author;
int year;

Book(std::string title, std::string author, int year) : title(title), author(author), year(year) {}

// Custom hash function for Book
size_t hash() const {
return std::hash<std::string>()(title) ^ std::hash<std::string>()(author) ^ std::hash<int>()(year);
}
};

int main() {
Book myBook("C++ Primer", "Stanley B. Lippman", 2012);
size_t bookHash = myBook.hash();

std::cout << "The hash for \"" << myBook.title << "\" by " << myBook.author << " (" << myBook.year << ") is: " << bookHash << std::endl;

return 0;
}

Output

The hash for "C++ Primer" by Stanley B. Lippman (2012) is: 2195925761031829699

 

This Book class includes a hash method that generates a hash value by combining the hash values of the title, author, and year. It uses std::hash<std::string>() for the strings and std::hash<int>() for the integer, combining these values using the bitwise XOR operator. This method ensures that books with different titles, authors, or publication years will have unique hash values.

Frequently Asked Questions

Why are hash functions important in C++?

Hash functions help organize data in a way that makes searching and retrieving it super fast. They're key in data structures like hash tables, making them essential for efficient coding in C++.

Can hash functions guarantee unique results for different inputs?

Not always. While hash functions aim to produce unique hash codes, sometimes two different inputs might produce the same hash code. This is known as a collision, and handling collisions is a big part of designing a good hash function.

How do hash functions improve program performance?

By converting large or complex data into fixed-size hash codes, hash functions allow data to be stored, searched, and retrieved quickly. This efficiency boost is especially noticeable in tasks involving large amounts of data.

Conclusion

In this article, we've learned about the hash functions in C++. We started with the basics, showing you what hash functions are and how they turn our data into manageable, fixed-size pieces. Through examples, we explored how to implement simple hash functions for both integers and strings, giving you a better understanding at how hashing works in C++.

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