Convert list to number range string

Question

This question is pretty much the opposite of this question: Does C# have built-in support for parsing page-number strings?

So given

1,3,5,6,7,8,9,10         


        

Answer 1

This should work pretty well, not tested for all cases though.

        string s = "1,2,3,4,5,7,8,9,10,11,12,13";
        string[] ints = s.Split(',');
        StringBuilder result = new StringBuilder();

        int num, last = -1;
        bool dash = false;

        for (int ii = 0; ii < ints.Length; ii++)
        {
            num = Int32.Parse(ints[ii]);

            if (num - last > 1)
            {
                if (dash)
                {
                    result.Append(last);
                    dash = false;
                }
                if (result.Length > 0)
                {
                    result.Append(",");
                }
                result.Append(num);                    
            }
            else
            {
                if (dash == false)
                {
                    result.Append("-");
                    dash = true;
                }
            }

            last = num;

            if (dash && ii == ints.Length - 1)
            {
                result.Append(num);
            }
        }

        Console.WriteLine(result);