C# Soundex Utility

C# Soundex Utility
By Jeff Guitard

Description The Soundex algorithm is used to convert a word to a code, based upon the phonetic sound of the word. Converting a word into a phonetic representation is helpful in performing searches that require fuzzy matches for "soundalikes".

Potential Usage I have used this algorithm in the past when constructing a customer management system. In addition to the standard search filters, it was useful for the end-users to have the ability to search for customers by the sound of their name.

For example, the names "Fraser" and "Frazier" both convert to the same soundex code of F626, because they sound phonetically similar.

Implementation I have created a C# implementation of this algorithm, that exists as a public static member of a utility class. I have included the source code below for the utility class, as well as a simple test application.

The algorithm is approximately 10 lines long, but I have commented the code extensively to promote understanding.

Soundex.cs

using System;

using System.Text;

namespace ca.guitard.jeff.utility {

  /// <summary>

  /// Utility class for performing soundex algorithm.

  /// 

  /// The Soundex algorithm is used to convert a word to a

  /// code based upon the phonetic sound of the word.

  /// 

  /// The soundex algorithm is outlined below:

  ///     Rule 1. Keep the first character of the name.

  ///     Rule 2. Perform a transformation on each remaining characters:

  ///                 A,E,I,O,U,Y     = A

  ///                 H,W             = S

  ///                 B,F,P,V         = 1

  ///                 C,G,J,K,Q,S,X,Z = 2

  ///                 D,T             = 3

  ///                 L               = 4

  ///                 M,N             = 5

  ///                 R               = 6

  ///     Rule 3. If a character is the same as the previous, do not include in the code.

  ///     Rule 4. If character is "A" or "S" do not include in the code.

  ///     Rule 5. If a character is blank, then do not include in the code.

  ///     Rule 6. A soundex code must be exactly 4 characters long.  If the

  ///             code is too short then pad with zeros, otherwise truncate.

  /// 

  /// Jeff Guitard

  /// October 2002

  /// </summary>

  public class Soundex {

    private Soundex() {

    }

    /// <summary>

    /// Return the soundex code for a given string.

    /// </summary>

    public static String ToSoundexCode(String aString) {

      String word = aString.ToUpper();

      StringBuilder soundexCode = new StringBuilder();

      int wordLength = word.Length;

      // Rule 1. Keep the first character of the word

      soundexCode.Append(word.Substring(0,1));

      // Rule 2. Perform a transformation on each remaining characters

      for (int i=1; i<wordLength; i++) {

        String transformedChar = Transform(word.Substring(i,1));

        // Rule 3. If a character is the same as the previous, do not include in code

        if (!transformedChar.Equals( soundexCode.ToString().Substring(soundexCode.Length - 1) )) {

          // Rule 4. If character is "A" or "S" do not include in code

          if (!transformedChar.Equals("A") && !transformedChar.Equals("S")) { 

            // Rule 5. If a character is blank, then do not include in code 

            if (!transformedChar.Equals(" ")) {

              soundexCode.Append(transformedChar);

            }

          }  

        }

      }

      // Rule 6. A soundex code must be exactly 4 characters long.  If the

      //         code is too short then pad with zeros, otherwise truncate.

      soundexCode.Append("0000");

      return soundexCode.ToString().Substring(0,4);

    }

    /// <summary>

    /// Transform the A-Z alphabetic characters to the appropriate soundex code.

    /// </summary>

    private static String Transform(String aString) {

      switch (aString) {

        case "A":

        case "E":

        case "I":

        case "O":

        case "U":

        case "Y":

          return "A";

        case "H":

        case "W":

          return "S";

        case "B":

        case "F":

        case "P":

        case "V":

          return "1";

        case "C":

        case "G":

        case "J":

        case "K":

        case "Q":

        case "S":

        case "X":

        case "Z":

          return "2";

        case "D":

        case "T":

          return "3";

        case "L":

          return "4";

        case "M":

        case "N":

          return "5";

        case "R":

          return "6";

      }

      return " ";  

    }

  }

}

SoundexTest.cs

using System;

namespace ca.guitard.jeff.utility.test {

  /// <summary>

  /// Test application for the Soundex class.

  /// 

  /// Jeff Guitard

  /// October 2002

  /// </summary>

  public class SoundexTest {

    public static void Main(String[] args) {

      // The soundex code for "fraser" and "frazier"

      // should be F626, since they sound phonetically

      // the same.

      Console.WriteLine("fraser = " + Soundex.ToSoundexCode("fraser") );

      Console.WriteLine("frazier = " + Soundex.ToSoundexCode("frazier") );

    }

  }

}