Recipe 2.4 Finding All Occurrences of a Character Within a String

Problem

You need a way of searching a string for multiple occurrences of a specific character.

Solution

Use IndexOf in a loop to determine how many occurrences of a character exist, as well as identify their location within the string:

using System;
using System.Collections;

public static int[] FindAllOccurrences(char matchChar, string source)
{
    return (FindAllOccurrences(matchChar, source, -1, false));
}

public static int[] FindAllOccurrences(char matchChar, string source, 
                                           int maxMatches)
{
    return (FindAllOccurrences(matchChar, source, maxMatches, false));
}

public static int[] FindAllOccurrences(char matchChar, string source, 
                                           bool caseSensitivity)
{
    return (FindAllOccurrences(matchChar, source, -1, caseSensitivity));
}
        
public static int[] FindAllOccurrences(char matchChar, string source, 
                                           int maxMatches, bool caseSensitivity)
{
    ArrayList occurrences = new ArrayList( );
    int foundPos = -1;   // -1 represents not found
    int numberFound = 0;
    int startPos = 0;
    char tempMatchChar = matchChar;
    string tempSource = source;

    if (!caseSensitivity)
    {
        tempMatchChar = char.ToUpper(matchChar);
        tempSource = source.ToUpper( );
    }

    do
    {
        foundPos = tempSource.IndexOf(matchChar, startPos);        
        if (foundPos > -1)
        {
            startPos = foundPos + 1;
            numberFound++;

            if (maxMatches > -1 && numberFound > maxMatches)
            {
                break;
            }
            else
            {
                occurrences.Add(foundPos);
            }
        }
    }while (foundPos > -1);

    return ((int[])occurrences.ToArray(typeof(int)));
}

Discussion

The FindAllOccurrences method is overloaded to allow the last two parameters (maxMatches and caseSensitivity) to be set to a default value if the developer chooses not to pass in one or both of these parameters. The maxMatches parameter defaults to -1, indicating that all matches are to be found. The caseSensitivity parameter defaults to false to allow for a case-insensitive search.

The FindAllOccurrences method starts out by determining whether case sensitivity is turned on. If false was passed in to the caseSensitivity parameter, both matchChar and source are set to all uppercase. This prevents a case-sensitive search.

The main loop in this method is a simple do loop that terminates when foundPos returns -1, meaning that no more matchChar characters can be found in the source string. We use a do loop so that the IndexOf operation would be executed at least one time before the check in the while clause is performed to determine whether there are any more character matches to be found in the source string.

Once a match is found by the IndexOf method, the numberFound variable is incremented by one to indicate that another match was found, and startPos is moved past the previously found match to indicate where the next IndexOf operation should start. The startPos is increased to the starting position of the last match found plus one. The +1 is needed so that we do not keep matching the same character that was previously matched. An infinite loop would occur in the code if at least one match was found in the source string.

Finally, a check is made to determine whether we are done searching for matchChar characters. If the maxMatches parameter is set to -1, the code keeps searching until it arrives at the end of the source string. Any other number indicates the maximum number of matchChar characters to search for. The maxMatches parameter limits the number of matches that can be made in the source string. If this check indicates that we are able to keep this match, it is stored in the occurrences ArrayList.