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Fuzzy Search

, 2 Jun 2009
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A simple implementation of the fuzzy string search.

Introduction

This article describes a simple implementation of the fuzzy (string) search. It can be used for approximate string matching (for more information, see http://en.wikipedia.org/wiki/Fuzzy_string_searching).

Other algorithms for approximate string searching exist (e.g., Soundex), but those aren't as easy to implement. The algorithm in this article is easy to implement, and can be used for tasks where approximate string searching is used in an easy way.

A List<string> is used for searching, and therefore it's quite easy to search a database.

Background

The algorithm used the Levenshtein-distance for determining how exact a string from a word list matches the word to be found. Information about the Levenshtein-distance can be found at http://en.wikipedia.org/wiki/Levenshtein_distance.

Using the code

The following example will show how simply the class can be used.

static void Main(string[] args)
{
    string word = "Code Project";
    List<string> wordList = new List<string>
    {
        "Code Project",
        "Code project",
        "codeproject",
        "Code Projekt",
        "Kode Project",
        "Other Project"
    };

    List<string> foundWords = FuzzySearch.Search(
        word,
        wordList,
        0.70);

    foundWords.ForEach(i => Console.WriteLine(i));
    Console.ReadKey();
}

Output:

Code Project
Code project
codeproject
Code Projekt
Kode Project

Implementation

A basic approach is shown. Instead of the Levenshtein-distance, a more optimized algorithm could be used - but here, a quite simple implementation is given for clarity reasons.

Levenshtein-distance

For computing the Levenshtein-distance, I use the following algorithm:

public static int LevenshteinDistance(string src, string dest)
{
    int[,] d = new int[src.Length + 1, dest.Length + 1];
    int i, j, cost;
    char[] str1 = src.ToCharArray();
    char[] str2 = dest.ToCharArray();

    for (i = 0; i <= str1.Length; i++)
    {
        d[i, 0] = i;
    }
    for (j = 0; j <= str2.Length; j++)
    {
        d[0, j] = j;
    }
    for (i = 1; i <= str1.Length; i++)
    {
        for (j = 1; j <= str2.Length; j++)
        {

            if (str1[i - 1] == str2[j - 1])
                cost = 0;
            else
                cost = 1;

            d[i, j] =
                Math.Min(
                    d[i - 1, j] + 1,              // Deletion
                    Math.Min(
                        d[i, j - 1] + 1,          // Insertion
                        d[i - 1, j - 1] + cost)); // Substitution

            if ((i > 1) && (j > 1) && (str1[i - 1] == 
                str2[j - 2]) && (str1[i - 2] == str2[j - 1]))
            {
                d[i, j] = Math.Min(d[i, j], d[i - 2, j - 2] + cost);
            }
        }
    }

    return d[str1.Length, str2.Length];
}

The Searching

In the search process, for each word in the wordlist, the Levenshtein-distance is computed, and with this distance, a score. This score represents how good the strings match. The input argument fuzzyness determines how much the strings can differ.

public static List<string> Search(
    string word,
    List<string> wordList,
    double fuzzyness)
{
    List<string> foundWords = new List<string>();

    foreach (string s in wordList)
    {
        // Calculate the Levenshtein-distance:
        int levenshteinDistance =
            LevenshteinDistance(word, s);

        // Length of the longer string:
        int length = Math.Max(word.Length, s.Length);

        // Calculate the score:
        double score = 1.0 - (double)levenshteinDistance / length;

        // Match?
        if (score > fuzzyness)
            foundWords.Add(s);
    }
    return foundWords;
}

LINQ-variant

This piece of code could be written in LINQ too.

public static List<string> Search(
    string word,
    List<string> wordList,
    double fuzzyness)
{
    // Tests have prove that the !LINQ-variant is about 3 times
    // faster!
    List<string> foundWords =
        (
            from s in wordList
            let levenshteinDistance = LevenshteinDistance(word, s)
            let length = Math.Max(s.Length, word.Length)
            let score = 1.0 - (double)levenshteinDistance / length
            where score > fuzzyness
            select s
        ).ToList();

    return foundWords;
}

History

  • 2009 June 1st: Initial release.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Günther M. FOIDL
Software Developer (Senior) Foidl Günther
Austria Austria
Engineer in combustion engine development.
Programming languages: C#, FORTRAN 95, Matlab
 
FIS-overall worldcup winner in Speedski (Downhill) 2008/09 and 2009/10.

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Comments and Discussions

 
Questionqwestion Pinmemberscheee17-Jan-13 1:42 
Hi!
I can't understand why we need a "score" parametr.
I thought that we schould compare the "levenshteinDistance" with number of mistales.
Could you explain the differense?
SuggestionParallelize may help but it is quite limited PinmemberMember 95751485-Nov-12 18:29 
GeneralMy vote of 5 PinmemberPetr Abdulin18-Apr-12 19:36 
GeneralMy vote of 5 PinmemberArlen Navasartian9-Apr-12 8:26 
GeneralMy vote of 5 Pinmembermanoj kumar choubey26-Feb-12 21:32 
GeneralWorks Great ! Pinmemberdamon8810-Dec-09 19:27 
GeneralRe: Works Great ! PinmemberGünther M. FOIDL29-Dec-09 13:33 
GeneralAbout dictionary PinmemberWin32nipuh21-Oct-09 21:54 
GeneralRe: About dictionary PinmemberGünther M. FOIDL22-Oct-09 10:55 
Generalgreat explanation PinmemberWin32nipuh8-Jun-09 20:29 
GeneralRe: great explanation PinmemberGünther M. FOIDL9-Jun-09 14:31 
GeneralRe: great explanation Pinmembercoduresearch2-Sep-09 7:45 
GeneralRe: great explanation PinmemberGünther M. FOIDL10-Sep-09 6:22 
GeneralRe: great explanation PinmemberNicolas Dorier21-Oct-09 8:28 
GeneralSearching large text Pinmemberdaniel.zolnjan4-Jun-09 5:56 
GeneralRe: Searching large text PinmemberGünther M. FOIDL4-Jun-09 9:25 
GeneralI would also have used extension methods to make my code more readable PinmemberMoshe Plotkin1-Jun-09 14:06 
GeneralRe: I would also have used extension methods to make my code more readable PinmemberGünther M. FOIDL1-Jun-09 21:46 
GeneralLinq does not seam to be slower [modified] PinmemberMoshe Plotkin1-Jun-09 13:55 
GeneralRe: Linq does not seam to be slower PinmemberGünther M. FOIDL1-Jun-09 21:44 
GeneralRe: Linq does not seam to be slower PinmemberMoshe Plotkin2-Jun-09 2:15 
GeneralRe: Linq does not seam to be slower PinmemberGünther M. FOIDL2-Jun-09 3:14 
GeneralRe: Linq does not seam to be slower Pinmemberriskka10-Jun-09 2:28 
Questionmore optimized algorithm? PinmemberUnruled Boy1-Jun-09 3:11 
AnswerRe: more optimized algorithm? PinmemberGünther M. FOIDL1-Jun-09 3:22 
GeneralRe: more optimized algorithm? Pinmembergstolarov10-Jun-09 12:18 
SuggestionRe: more optimized algorithm? PinprofessionalPsycho_Coder22-May-14 19:10 

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