Question

Given a sting, how can I skip the first x characters and then insert a value for every y characters?

For example:

“Lorem ipsum dolor sit amet,” 

when skipping the first 10 caracters and then indsert “[here]” for every 3 caracters becomes:

“Lorem ipsu[here]m d[here]olo[here]r s[here]it [here]ame[here]t,”

What is the most efficient, fastest way of doing this in C#?

My current function looks like this but isn’t doing the skipping part, I know how to implement the skipping part but the technique used does not seem to be optimal:

public static string InsertHere(string source)
    {
        if (string.IsNullOrWhiteSpace(source))
        {
            return string.Empty;
        }

        int count = 0;
        var sb = new StringBuilder();
        foreach (char c in source)
        {
            count++;
            sb.Append(c);
            if (count == 10)
            {
                count = 0;
                sb.Append(@"[here]");
            }
        }
        return sb.ToString();
    }

Answer 1

You’d have to profile them to see what’s best, but here’s my effort, I’ve gone for a string reader into a buffer approach.

    public static string InsertStringRepeatedly(string source, int skip, int insertEvery, string toInsert)
    {
        var sb = new StringBuilder();
        using (var sr = new StringReader(source))
        {
            var buffer = new char[Math.Max(skip, insertEvery)];
            var read = sr.Read(buffer, 0, skip);
            sb.Append(buffer, 0, read);
            while (sr.Peek() > 0)
            {
                sb.Append(toInsert);
                read = sr.Read(buffer, 0, insertEvery);
                sb.Append(buffer, 0, read);
            }
        }
        return sb.ToString();
    }

Edit:

Fixed for edge cases source.Length < 10 or not whole multiple of 10 + 3*x. Also use just one buffer now.

Usage example:

    private static void Main(string[] args)
    {
        var result = InsertStringRepeatedly("Lorem ipsum dolor sit amet,", 10, 3, "[Here]");
        Console.Write("\"");
        Console.Write(result);
        Console.WriteLine("\""); //quotes to show we dealt with edge cases correctly
        Console.ReadLine();
    }