Hash Function for String data in C# - GeeksforGeeks

01 Feb, 2019
Question: Write code in C# to Hash an array of keys and display them with their hash code.
Answer: Hashtable is a widely used data structure to store values (i.e. keys) indexed with their hash code. Hash code is the result of the hash function and is used as the value of the index for storing a key. If two distinct keys hash to the same value the situation is called a collision and a good hash function minimizes collisions.
Question: How to choose a hash function appropriate for the data?
Answer: If your data consists of integers then the easiest hash function is to return the remainder of the division of key and the size of the table. It is important to keep the size of the table as a prime number. But more complex functions can be written to avoid the collision. If your data consists of strings then you can add all the ASCII values of the alphabets and modulo the sum with the size of the table (the code below depicts the same).
Example 1:
// C# Program to create a Hash // Function for String datausing System;  class Geeks {          // Main Method    public static void Main(String []args)    {                  // Declaring the an string array        string[] values = new string[50];        string str;          // Values of the keys stored        string[] keys = new string[] {"Alphabets",               "Roman", "Numbers", "Alphanumeric",                                   "Tallypoints"};          int hashCode;          for (int k = 0; k < 5; k++) {                          str = keys[k];                          // calling HashFunction            hashCode = HashFunction(str, values);              // Storing keys at their hashcode's index            values[hashCode] = str;        }          // Displaying Hashcodes along with key values        for (int k = 0; k < (values.GetUpperBound(0)); k++) {                          if (values[k] != null)                Console.WriteLine(k + " " + values[k]);        }    }      // Defining the hash function    static int HashFunction(string s, string[] array)    {        int total = 0;        char[] c;        c = s.ToCharArray();          // Summing up all the ASCII values         // of each alphabet in the string        for (int k = 0; k <= c.GetUpperBound(0); k++)            total += (int)c[k];          return total % array.GetUpperBound(0);    }}
11 Tallypoints
16 Alphanumeric
19 Roman
34 Alphabets
46 Numbers

Example 2:
// C# Program to create a Hash // Function for String datausing System;  class Geeks {          // Main Method    public static void Main(String []args)    {                  // Declaring the an string array        string[] values = new string[50];        string str;          // Values of the keys stored        string[] keys = new string[] {"C", "C++",                  "Java", "Python", "C#", "HTML"};          int hashCode;          for (int k = 0; k < 5; k++) {                          str = keys[k];            hashCode = HashFunction2(str, values);              // Storing keys at their hashcode's index            values[hashCode] = str;        }          // Displaying Hashcodes along with key values        for (int k = 0; k < (values.GetUpperBound(0)); k++) {                          if (values[k] != null)                Console.WriteLine(k + " " + values[k]);        }    }      // Defining the hash function    static int HashFunction2(string s, string[] array)    {        long total = 0;        char[] c;        c = s.ToCharArray();          // Horner's rule for generating a polynomial         // of 11 using ASCII values of the characters        for (int k = 0; k <= c.GetUpperBound(0); k++)                      total += 11 * total + (int)c[k];          total = total % array.GetUpperBound(0);                  if (total < 0)            total += array.GetUpperBound(0);          return (int)total;    }}
6 C#
15 C++
18 C
19 Python
28 Java

Explanation: In HashFunction, we pass the arguments as a string to be hashed and the string data ‘values‘. The method ToCharArray converts the string to the character array and then we start a for loop from the start to the end of the character array. Inside the for loop we calculate the sum total of the ASCII values of each character in the array. The method GetUpperBound returns the value of the highest index of the array. Then hash function returns modulo of the sum total by the upper bound of the array ( in this case 49 since string[] values = new string[50] ). Whereas in HashFunction2, we are passing the same arguments but this function is less likely to have collisions. Everything is basically the same except here we use Horner’s Rule to compute the polynomial function of 11.
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