String occurrence count

A substring may occur several times in a string. We can count the number of occurrences of one string in another string.

Counting occurrence of strings is useful for validity checks. It can help with checking that only one tag string is found in some HTML.

Example

This method uses IndexOf and a while-loop to count up the number of occurrences of a specified string in another string. It receives 2 string parameters.

using System;

class Program
{
    static void Main()
    {
        string value = "cat, frog, cat";
        // These should be 2 instances.
        Console.WriteLine(TextTool.CountStringOccurrences(value, "cat"));
        // There should be 0 instances.
        Console.WriteLine(TextTool.CountStringOccurrences(value, "DOG"));
    }
}

/// <summary>
/// Contains static text methods.
/// Put this in a separate class in your project.
/// </summary>
public static class TextTool
{
    /// <summary>
    /// Count occurrences of strings.
    /// </summary>
    public static int CountStringOccurrences(string text, string pattern)
    {
        // Loop through all instances of the string 'text'.
        int count = 0;
        int i = 0;
        while ((i = text.IndexOf(pattern, i)) != -1)
        {
            i += pattern.Length;
            count++;
        }
        return count;
    }
}
2
0

Notes, extensions

The code example is not an extension method. But you are free to adapt it with the this-keyword—place it in a static class.

We saw how to count occurrences of one, smaller string in another, larger string. We only loop once over the source, which reduces the cost of the method.