Calculate similarity between two strings in C#

This article explains how you can calculate similarity between two strings in C# in percentage or score from 0 to 1.

Basically, to compare two strings in C# we use the compare or equals method but when we want to match string similarity in percentage or in a score like from 0 to 1 then these methods are not helpful so you can use following manual methods to achieve this.

Here is logic to check the similarity between two strings, just copy & paste this into your code editor and run to test.

Main method :

   // Compares the two strings based on letter pair matches
    public static double CompareStrings(string str1, string str2)
    {
        List pairs1 = WordLetterPairs(str1.ToUpper());
        List pairs2 = WordLetterPairs(str2.ToUpper());

        int intersection = 0;
        int union = pairs1.Count + pairs2.Count;

        for (int i = 0; i < pairs1.Count; i++)
        {
            for (int j = 0; j < pairs2.Count; j++)
            {
                if (pairs1[i] == pairs2[j])
                {
                    intersection++;
                    pairs2.RemoveAt(j);//Must remove the match to prevent "AAAA" from appearing to match "AA" with 100% success
                    break;
                }
            }
        }

        return (2.0 * intersection * 100) / union; //returns in percentage
        //return (2.0 * intersection) / union; //returns in score from 0 to 1
    } 

Other methods :

These methods will be used in the above method to get word pairs & letter pairs from string

   // Gets all letter pairs for each
    private List WordLetterPairs(string str)
    {
        List AllPairs = new List();

        // Tokenize the string and put the tokens/words into an array
        string[] Words = Regex.Split(str, @"\s");

        // For each word
        for (int w = 0; w < Words.Length; w++)
        {
            if (!string.IsNullOrEmpty(Words[w]))
            {
                // Find the pairs of characters
                String[] PairsInWord = LetterPairs(Words[w]);

                for (int p = 0; p < PairsInWord.Length; p++)
                {
                    AllPairs.Add(PairsInWord[p]);
                }
            }
        }
        return AllPairs;
    }

    // Generates an array containing every two consecutive letters in the input string
    private string[] LetterPairs(string str)
    {
        int numPairs = str.Length - 1;
        string[] pairs = new string[numPairs];

        for (int i = 0; i < numPairs; i++)
        {
            pairs[i] = str.Substring(i, 2);
        }
        return pairs;
    }

How to use:

Just call method 'CompareStrings' and pass two strings as parameters like below & you will get a similarity percentage/score.

  CompareStrings("Good Morning", "Good Afternoon"); //Output: 40
  CompareStrings("Good Morning", "Good Evening"); //Output: 66.6666666666667
  CompareStrings("Good Morning", "Good Night"); //Output: 50
  CompareStrings("Good Morning", "Good Morning"); //Output: 100

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