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Posted 3 Jan 2015

Levenshtein Algorithm in Visual Basic

, 3 Jan 2015
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How to calculate the distance between two strings according to Levenshtein algorithm

Introduction

Levenshtein's distance measures the minimum number of character edits required to change a word into another. In this tip, we'll see a simple implementation of the Levenshtein algorithm in Visual Basic. It will be useful in several situations, when managing - for example - large amount of text, and we are in need of fast and massive modifications in our data, to spot and correct typos, or to match strings in terms of similarity, and not simply in terms of equal/not equal. Levenshtein's algorithm allows to compute a score regarding string similarity. We'll see how in a moment.

Standard Levenshtein Algorithm

Here follows the standard Levenshtein implementation in VB.NET, according to the algorithm as shown at Wikipedia.

Public Function Levenshtein(ByVal s As String, ByVal t As String) As Integer
  Dim n As Integer = s.Length
  Dim m As Integer = t.Length
  Dim d(n + 1, m + 1) As Integer

  If n = 0 Then
      Return m
  End If
  If m = 0 Then
      Return n
  End If

  Dim i As Integer
  Dim j As Integer

  For i = 0 To n
      d(i, 0) = i
  Next
  For j = 0 To m
      d(0, j) = j
  Next
  For i = 1 To n
      For j = 1 To m
          Dim cost As Integer
          If t(j - 1) = s(i - 1) Then
             cost = 0
          Else
             cost = 1
          End If
          d(i, j) = Math.Min(Math.Min(d(i - 1, j) + 1, d(i, j - 1) + 1), d(i - 1, j - 1) + cost)
      Next
  Next
  Return d(n, m)
End Function

Examples

Using the above function(s) is trivial: it's sufficient to call it by passing two strings, plus the optional boolean flag for case-sensitivity check. Some examples could be:

MsgBox(Levenshtein("Inwards", "inwards").ToString)       ' Returns 1
MsgBox(Levenshtein("Inwards", "inwards").ToString)       ' Returns 1
MsgBox(Levenshtein("towards", "towards").ToString)       ' Returns 0
MsgBox(Levenshtein("dinner", "breakfast").ToString)      ' Returns 9
MsgBox(Levenshtein("breakfast", "braekfast").ToString)   ' Returns 2

MsgBox(Levenshtein("efficient", "sufficient").ToString)  ' Returns 2
MsgBox(Levenshtein("grandma", "anathema").ToString)      ' Returns 5
MsgBox(Levenshtein("aunt", "ant").ToString)              ' Returns 1

The results (1, 0, 0, 9, 2, 2, 5, 1) are the Levenshtein's distances between given strings, i.e., a score regarding strings similarity. The lower the score, the nearer are the two entities. A value of zero means, obviously, a total convergence of the two. We could use a function like this (with predetermined conditions to be satisfied, like "the score must not exceed 3", for example) to correct typos (as in the "breakfast - braekfast" example), or to search for differences in hypothetical data (like "new York - New York"), and so on.

Bibliography

History

  • 2015-01-06 Added standard algorithm, revised text, revised code
  • 2015-01-03 First release for CodeProject

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

Emiliano Musso
Software Developer
Italy Italy
Working in IT since 2003 as Software Developer for Essetre Srl, a company in Northern Italy.
I was awarded in 2014, 2015 and 2016 with Microsoft MVP, for Visual Studio and Development Technologies expertise. My technology interests and main skills are in .NET Framework, Visual Basic, Visual C# and SQL Server, but i'm proficient in PHP and MySQL also.

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Comments and Discussions

 
GeneralMy vote of 1 Pin
Etter Frédéric5-Jan-15 22:43
memberEtter Frédéric5-Jan-15 22:43 
BugWrong code - Sorry Pin
Etter Frédéric5-Jan-15 22:42
memberEtter Frédéric5-Jan-15 22:42 
GeneralRe: Wrong code - Sorry Pin
Emiliano Musso5-Jan-15 23:21
memberEmiliano Musso5-Jan-15 23:21 
GeneralRe: Wrong code - Sorry Pin
Etter Frédéric6-Jan-15 3:46
memberEtter Frédéric6-Jan-15 3:46 
GeneralRe: Wrong code - Sorry Pin
Emiliano Musso6-Jan-15 7:02
memberEmiliano Musso6-Jan-15 7:02 
GeneralMy vote of 1 Pin
Etter Frédéric5-Jan-15 22:36
memberEtter Frédéric5-Jan-15 22:36 

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