Oliver Hanappi Oliver Hanappi - 14 days ago 5
C# Question

Comparing strings with tolerance

I'm looking for a way to compare a string with an array of strings. Doing an exact search is quite easy of course, but I want my program to tolerate spelling mistakes, missing parts of the string and so on.

Is there some kind of framework which can perform such a search? I'm having something in mind that the search algorithm will return a few results order by the percentage of match or something like this.

Answer

You could use the Levenshtein Distance algorithm.

"The Levenshtein distance between two strings is defined as the minimum number of edits needed to transform one string into the other, with the allowable edit operations being insertion, deletion, or substitution of a single character." - Wikipedia.com

This one is from dotnetperls.com:

using System;

/// <summary>
/// Contains approximate string matching
/// </summary>
static class LevenshteinDistance
{
    /// <summary>
    /// Compute the distance between two strings.
    /// </summary>
    public static int Compute(string s, string t)
    {
        int n = s.Length;
        int m = t.Length;
        int[,] d = new int[n + 1, m + 1];

        // Step 1
        if (n == 0)
        {
            return m;
        }

        if (m == 0)
        {
            return n;
        }

        // Step 2
        for (int i = 0; i <= n; d[i, 0] = i++)
        {
        }

        for (int j = 0; j <= m; d[0, j] = j++)
        {
        }

        // Step 3
        for (int i = 1; i <= n; i++)
        {
            //Step 4
            for (int j = 1; j <= m; j++)
            {
                // Step 5
                int cost = (t[j - 1] == s[i - 1]) ? 0 : 1;

                // Step 6
                d[i, j] = Math.Min(
                    Math.Min(d[i - 1, j] + 1, d[i, j - 1] + 1),
                    d[i - 1, j - 1] + cost);
            }
        }
        // Step 7
        return d[n, m];
    }
}

class Program
{
    static void Main()
    {
        Console.WriteLine(LevenshteinDistance.Compute("aunt", "ant"));
        Console.WriteLine(LevenshteinDistance.Compute("Sam", "Samantha"));
        Console.WriteLine(LevenshteinDistance.Compute("flomax", "volmax"));
    }
}

You may in fact prefer to use the Damerau-Levenshtein distance algorithm, which also allows characters to be transposed, which is a common human error in data entry. You'll find a C# implementation of it here.