C＃代码问题：修订号和字母

Question

I'm trying to write a function that takes a revision number (int) and turns it into a revision name (string). The formula should produce outputs similar to this:

Number  Name
1       A
2       B
3       C
...     ...
25      Y
26      Z
27      AA
28      AB
29      AC
...     ...
51      AY
52      AZ
53      BA
54      BB
55      BC
...     ...

This seems simple, but I think it might involve recursion and I'm terrible at that. Any suggestions?

Answer 1

I think this is the same as working out the Excel column name from a column number:

private string GetExcelColumnName(int columnNumber)
{
    int dividend = columnNumber;
    string columnName = String.Empty;
    int modulo;

    while (dividend > 0)
    {
        modulo = (dividend - 1) % 26;
        columnName = Convert.ToChar(65 + modulo).ToString() + columnName;
        dividend = (int)((dividend - modulo) / 26);
    } 

    return columnName;
}

Answer 2

I think you basically need a transformation of a number in 10x numerical system to a number in 26x numerical system.

For example:

53 = 5*10^1 + 3*10^0 = [5][3]

53 = B*26^1 + A*26^0 = [B][A]

---

int value10 = 53;
int base10 = 10;

string value26 = "";
int base26 = 26;

int input = value10;
while (true)
{
    int mod = input / base26;
    if (mod > 0)
        value26 += Map26SymbolByValue10 (mod); // Will map 2 to 'B'
    else
        value26 += Map26SymbolByValue10 (input); // Will map 1 to 'A'

    int rest = input - mod * base26;
    if (input < base26) break;
    input = rest;
}

Answer 3

I really hope this isn't homework... (untested solution):

if(value == 1)
  return "A";
StringBuilder result = new StringBuilder();
value--;
while(value > 0)
{
  result.Insert(0, 'A' + (value % 26));
  value /= 26;
}

Recursive version based on tanascius' original answer (also untested):

string ConvertToChar(int value)
{
  char low = 'A' + (value - 1) % 26;
  if(value > 26)
    return ConvertToChar((value - 1) / 26 + 1) + low.ToString();
  else
    return low.ToString();
}

Answer 4

Tested solution:

private static string VersionName(int versionNum)
{
    StringBuilder sb = new StringBuilder();
    while (versionNum > 0)
    {
        versionNum--;
        sb.Insert(0, (char)('A' + (versionNum % 26)));
        versionNum /= 26;
    }
    return sb.ToString();
}

I wouldn't bother using recursion for this. Looping with a StringBuilder is more efficient than concatenating strings with each recursion, although you'd probably need a crazy number of revisions to notice the difference (4 letters is enough for over 400,000 revisions).

Answer 5

You can use the modulo operator and division to get your code.
Like 55 / 26 == 2 (that is B) and 55 % 26 = 3 (that is C). It works for two characters. When you have an unknown count of characters, you have to start looping:

[look at Aaron's solution, mine was wrong]