C# Code Problem: Revision Numbers and Letters

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?

It looks like you want the same formula as Excel uses to take row numbers to row names. Have a search here for that - I remember not so long ago there was a question that sparked something of a heated debate... — AakashM, Dec 14 '09 at 21:23
`string x=abc[num%26]; x+=this(num-num%26)` or something like that.. (also note `abc=char [a...z]`) unless you want a non-elegant way :) — Earlz, Dec 14 '09 at 21:25
Yes I know, but just a start.. you'd need to subtract and divide and of course actually return something rather than making a recursive loop — Earlz, Dec 14 '09 at 21:30
See also: http://stackoverflow.com/questions/181596/how-to-convert-a-column-number-eg-127-into-an-excel-column-eg-aa — Graham, Dec 14 '09 at 21:52

score 6 · Accepted Answer · answered Dec 14 '09 at 21:35

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;
}

score 5 · Answer 2 · 2009-12-14T21:54:33.983

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;
}

Aaron · Answer 3 · 2009-12-14T21:43:00.757

2

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();
}

edited Dec 14 '09 at 21:43

answered Dec 14 '09 at 21:31

Aaron

9,123
5
40
38

@tanascius - I had already realized (and edited) that it was reversed -- but you still want to use modulo either way. – Aaron Dec 14 '09 at 21:35

Tara McGrew · Answer 4 · 2009-12-14T21:52:32.213

2

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).

edited Dec 14 '09 at 21:52

answered Dec 14 '09 at 21:43

Tara McGrew

1,987
19
28

It does. I tested it under VS2008 and got the same results as the questioner. What isn't working for you? – Tara McGrew Dec 14 '09 at 21:54
I apologise, I introduced a bug copying. – Roman Boiko Dec 14 '09 at 21:59
+1. I've used same code with uint and 0 value check for parameter. – DK. Dec 14 '09 at 22:30

tanascius · Answer 5 · 2009-12-14T21:34:20.740

1

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]

edited Dec 14 '09 at 21:34

answered Dec 14 '09 at 21:25

tanascius

53,078
22
114
136

What is GetChar() -- is that the function itself? If so, why bother with the loop at all? Better to just use the recursion... – Aaron Dec 14 '09 at 21:37

C# Code Problem: Revision Numbers and Letters

5 Answers5