C# - Finding All Indices of a Substring [duplicate]

Question

This question already has answers here:

Finding all positions of substring in a larger string in C# (15 answers)

Closed 8 years ago.

EDIT: So it turns out what I had before was correct, I was simply counting the indexes wrong. Thank you for your input though.

Working on a method that finds all substring indices in a given string from the user. I am having issues with getting correct positions from userString.IndexOf. I know it's finding all occurrences of the substring, but the integer index is off by a substantial amount.

private static void getSubStringPositions(string userString, string userSubString)
{
    string subStringPositions = "";
    int position = 0;

    if (userString.IndexOf(userSubString, position) == -1)
    {
        Console.WriteLine("Your sub-string was not found in your string.\n");
        return;
    }
    while (position < userString.Length)
    {
        position += userString.IndexOf(userSubString, position);
        subStringPositions += Convert.ToString(position) + ", ";
    }

    Console.WriteLine("The occurernce(s) for your sub-string are: " + subStringPositions + "\n\n");
    return;
}

I think it may be an issue with position += userString.IndexOf(userSubString, position); but am not entirely sure how I would go about setting the new start position while maintaing an accurate record of the substring locations.

Answer 1

Remove the += in front of position

   position = userString.IndexOf(userSubString, position);

Also you should change your code to save the initial found position and set the position variable to search after the previous one

    // Initial check...
    position = userString.IndexOf(userSubString);
    if(position == -1)
    {
        Console.WriteLine("Your sub-string was not found in your string.\n");
        return;
    }
    // Save the found position and enter the loop
    subStringPositions = Convert.ToString(position) + ", ";

    while (position < userString.Length)
    {
        // Search restart from the character after the previous found substring
        position = userString.IndexOf(userSubString, position + 1);
        subStringPositions += Convert.ToString(position) + ", ";
    }

As a final note, if this search produces many hits it is better to change the string concatenation using a StringBuilder class instance

    StringBuilder subStringPositions = new StringBuilder();
    subStringPositions.Append(Convert.ToString(position) + ", ");

    while (position < userString.Length)
    {
        // Search restart from the character after the previous found substring
        position = userString.IndexOf(userSubString, position + 1);
        subStringPositions.Append(Convert.ToString(position) + ", ";
    }
    Console.WriteLine("The occurrence(s) for your sub-string are: " + 
                      subStringPositions.ToString() + "\n\n");

Answer 2

A concise way to find these indexes using LINQ:

public static IEnumerable<int> FindIndexes(string text, string query)
{
    return Enumerable.Range(0, text.Length - query.Length)
        .Where(i => query.Equals(text.Substring(i, query.Length));
}

FindIndexes("abcbcbc", "bcb") will find you the indexes 1 and 3.

Answer 3

You have another problem here. Let's say you call:

getSubStringPositions("abcabcabcabc", "abcabc");

You function will incorrectly report that the string occurs twice, when in fact the substring occurs 3 times, like so:

• abcabc.abcabc
• abc.abcabc.abc <-- your function jumps over this one
• abcabc.abcabc.