math - Custom number system in C# -

this question has answer here:

• how convert column number (eg. 127) excel column (eg. aa) 44 answers

i have requirement custom number system in c# goes following:

a - 1 b - 2 ... z - 26 aa - 27 ab - 28 

i've made function converts arbitrary strings numbers this:

    private const int min = 'a';     private const int max = 'z';     private const int base = max - min + 1;      private static int getcharvalue(char c)     {         if (c < min || c > max)             throw new argumentoutofrangeexception(nameof(c), c, $"character needs between '{min}' , '{max}', '{c}'.");          return c - min + 1;     }      public static int getstringvalue(string s)     {         char[] chars = s.tochararray();         int[] values = new int[chars.length];         (var = 0; < chars.length; i++)         {             values[i] = getcharvalue(chars[i]);         }          int position = 1;         int value = 0;         (var = values.length - 1; >= 0; i--)         {             value += position * values[i];             position *= base;         }          return value;     } 

i've tested working aaa (not rigorously, skimming on output of printing them all). however, can't life of me figure out how write reverse function. in other words, need 1 return a, 26 return z , 27 return aa. "problem" number system has no 0, doesn't convert base. instance, if a 0, aa 0, it's not. how solve this?

you can generate this....

    public static ienumerable<string> generate()     {         long n = -1;         while (true) yield return tobase26(++n);     }      public static string tobase26(long i)     {         if (i == 0) return ""; i--;         return tobase26(i / 26) + (char)('a' + % 26);     }        public static void buildquery()     {         ienumerable<string> lstexcelcols = generate();         try         {              string s = lstexcelcols.elementatordefault(1) ;         }         catch (exception exc)         {          }       }