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How do I subtract from numbers over a 122?

I have a bunch of user inputted integers assigned to a variable 'c,' and am trying to subtract from the values over 122. I've tried a load of different loops, but I usually get stuck with it not working or taking 90 away from all of them. So how exactly do I take 90 away from numbers over 122, anyway?
(this is for a caesar shift encryption program, 122 being lower-case 'z' in ASCII)
List<int> valerie = new List<int>();
for (int i = 32; i < 122; i++)
{
valerie.Add(i);
}
Console.WriteLine("E - Encrypt");
Console.WriteLine("D - Decrypt");
string choice = Console.ReadLine();
switch (choice.ToUpper())
{
case "E":
Console.WriteLine("Enter Caesar shift.");
string shift = Console.ReadLine();
int offset = int.Parse(shift);
Console.WriteLine("Enter phrase.");
string phrase = Console.ReadLine();
byte[] asciiBytes = Encoding.ASCII.GetBytes(phrase);
foreach(byte b in asciiBytes)
{
int a = Convert.ToInt32(b);
int c = a + offset;
Console.WriteLine(c);
char d = (char)c;
Console.WriteLine(d);
}

You have to use modular arithmetics: not just add a offset to each character, but take a remainder, so with a help of the Linq you can put it:
int offset = ...
String phrase = ...;
// Providing that the phrase constains 'A'..'z' ard/or 'a'..'z' only
String encoded = new String(phrase
.Select(ch => (Char) (ch <= 'Z' ?
(ch + offset) % 26 + 'A' : // note "% 26"
(ch + offset) % 26 + 'a')) // note "% 26"
.ToArray());

Either I misunderstood your question, or you just need to check your input...
//Version 1
int c = a;
if(a > 122)
c = c - offset;
//Version 2, more compact
int c = a > 122 ? a : a + offset;

Masking all characters of a string except for the last n characters

I want to know how can I replace a character of a string with condition of "except last number characters"?
Example:
string = "4111111111111111";
And I want to make it that
new_string = "XXXXXXXXXXXXX1111"
In this example I replace the character to "X" except the last 4 characters.
How can I possibly achieve this?

Would that suit you?
var input = "4111111111111111";
var length = input.Length;
var result = new String('X', length - 4) + input.Substring(length - 4);
Console.WriteLine(result);
// Ouput: XXXXXXXXXXXX1111

How about something like...
new_string = new String('X', YourString.Length - 4)
+ YourString.Substring(YourString.Length - 4);
create a new string based on the length of the current string -4 and just have it all "X"s. Then add on the last 4 characters of the original string

Here's a way to think through it. Call the last number characters to leave n:
How many characters will be replaced by X? The length of the string minus n.
How can we replace characters with other characters? You can't directly modify a string, but you can build a new one.
How to get the last n characters from the original string? There's a couple ways to do this, but the simplest is probably Substring, which allows us to grab part of a string by specifying the starting point and optionally the ending point.
So it would look something like this (where n is the number of characters to leave from the original, and str is the original string - string can't be the name of your variable because it's a reserved keyword):
// 2. Start with a blank string
var new_string = "";
// 1. Replace first Length - n characters with X
for (var i = 0; i < str.Length - n; i++)
new_string += "X";
// 3. Add in the last n characters from original string.
new_string += str.Substring(str.Length - n);

This might be a little Overkill for your ask. But here is a quick extension method that does this.
it defaults to using x as the masking Char but can be changed with an optional char
public static class Masking
{
public static string MaskAllButLast(this string input, int charsToDisplay, char maskingChar = 'x')
{
int charsToMask = input.Length - charsToDisplay;
return charsToMask > 0 ? $"{new string(maskingChar, charsToMask)}{input.Substring(charsToMask)}" : input;
}
}
Here a unit tests to prove it works
using Xunit;
namespace Tests
{
public class MaskingTest
{
[Theory]
[InlineData("ThisIsATest", 4, 'x', "xxxxxxxTest")]
[InlineData("Test", 4, null, "Test")]
[InlineData("ThisIsATest", 4, '*', "*******Test")]
[InlineData("Test", 16, 'x', "Test")]
[InlineData("Test", 0, 'y', "yyyy")]
public void Testing_Masking(string input, int charToDisplay, char maskingChar, string expected)
{
//Act
string actual = input.MaskAllButLast(charToDisplay, maskingChar);
//Assert
Assert.Equal(expected, actual);
}
}
}

StringBuilder sb = new StringBuilder();
Char[] stringChar = string.toCharArray();
for(int x = 0; x < stringChar.length-4; x++){
sb.append(stringChar[x]);
}
sb.append(string.substring(string.length()-4));
string = sb.toString();

I guess you could use Select with index
string input = "4111111111111111";
string new_string = new string(input.Select((c, i) => i < input.Length - 4 ? 'X' : c).ToArray());

Some of the other concise answers here did not account for strings less than n characters. Here's my take:
var length = input.Length;
input = length > 4 ? new String('*', length - 4) + input.Substring(length - 4) : input;

lui,
Please Try this one...
string dispString = DisplayString("4111111111111111", 4);
Create One function with pass original string and no of digit.
public string DisplayString(string strOriginal,int lastDigit)
{
string strResult = new String('X', strOriginal.Length - lastDigit) + strOriginal.Substring(strOriginal.Length - lastDigit);
return strResult;
}
May be help you....

Try this:
String maskedString = "...."+ (testString.substring(testString.length() - 4, testString.length()));

Late to the party but I also wanted to mask all but the last 'x' characters, but only mask numbers or letters so that any - ( ), other formatting, etc would still be shown. Here's my quick extension method that does this - hopefully it helps someone. I started with the example from Luke Hammer, then changed the guts to fit my needs.
public static string MaskOnlyChars(this string input, int charsToDisplay, char maskingChar = 'x')
{
StringBuilder sbOutput = new StringBuilder();
int intMaskCount = input.Length - charsToDisplay;
if (intMaskCount > 0) //only mask if string is longer than requested unmasked chars
{
for (var intloop = 0; intloop < input.Length; intloop++)
{
char charCurr = Char.Parse(input.Substring(intloop, 1));
byte[] charByte = Encoding.ASCII.GetBytes(charCurr.ToString());
int intCurrAscii = charByte[0];
if (intloop <= (intMaskCount - 1))
{
switch (intCurrAscii)
{
case int n when (n >= 48 && n <= 57):
//0-9
sbOutput.Append(maskingChar);
break;
case int n when (n >= 65 && n <= 90):
//A-Z
sbOutput.Append(maskingChar);
break;
case int n when (n >= 97 && n <= 122):
//a-z
sbOutput.Append(maskingChar);
break;
default:
//Leave other characters unmasked
sbOutput.Append(charCurr);
break;
}
}
else
{
//Characters at end to remain unmasked
sbOutput.Append(charCurr);
}
}
}
else
{
//if not enough characters to mask, show unaltered input
return input;
}
return sbOutput.ToString();
}

Incrementation of char

I found some question asking how to let char 'B' to return 'C' and then 'D' etc. The answers were quite complex and mostly just overkill.
Why not to use simply this:
char X='A';
X++;
EDIT: It goes from A to Z and what next?

If you just want to increment :
Char x = 'A';
Char y = (Char)(Convert.ToUInt16(x) + 1);
But, if you want an excel like column :
// (1 = A, 2 = B...27 = AA...703 = AAA...)
public static string GetColNameFromIndex(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;
}
// (A = 1, B = 2...AA = 27...AAA = 703...)
public static int GetColNumberFromName(string columnName)
{
char[] characters = columnName.ToUpperInvariant().ToCharArray();
int sum = 0;
for (int i = 0; i < characters.Length; i++)
{
sum *= 26;
sum += (characters[i] - 'A' + 1);
}
return sum;
}

If you're happy with the results that gives, then that's fine.
Usually when I've seen questions like that, they want to wrap from "Z" to "AA" or something like that that though - like Excel columns. Clearly just incrementing a char won't do that - it would go to '['.
Alternatively, even within a single character, the range of valid values may be non-contiguous - the obvious example being hex. If you increment '9' you get ':' instead of the 'a' or 'A' which you probably wanted. The desired order is rarely "whatever Unicode gives you".

Probably because it is also intended to go from Z to AA, ala Perl and PHP.

What is the algorithm to convert an Excel Column Letter into its Number?

I need an algorithm to convert an Excel Column letter to its proper number.
The language this will be written in is C#, but any would do or even pseudo code.
Please note I am going to put this in C# and I don't want to use the office dll.
For 'A' the expected result will be 1
For 'AH' = 34
For 'XFD' = 16384

public static int ExcelColumnNameToNumber(string columnName)
{
if (string.IsNullOrEmpty(columnName)) throw new ArgumentNullException("columnName");
columnName = columnName.ToUpperInvariant();
int sum = 0;
for (int i = 0; i < columnName.Length; i++)
{
sum *= 26;
sum += (columnName[i] - 'A' + 1);
}
return sum;
}

int result = colName.Select((c, i) =>
((c - 'A' + 1) * ((int)Math.Pow(26, colName.Length - i - 1)))).Sum();

int col = colName.ToCharArray().Select(c => c - 'A' + 1).
Reverse().Select((v, i) => v * (int)Math.Pow(26, i)).Sum();

Loop through the characters from last to first. Multiply the value of each letter (A=1, Z=26) times 26**N, add to a running total. My string manipulation skill in C# is nonexistent, so here is some very mixed pseudo-code:
sum=0;
len=length(letters);
for(i=0;i<len;i++)
sum += ((letters[len-i-1])-'A'+1) * pow(26,i);

Here is a solution I wrote up in JavaScript if anyone is interested.
var letters = "abc".toUpperCase();
var sum = 0;
for(var i = 0; i < letters.length;i++)
{
sum *= 26;
sum += (letters.charCodeAt(i) - ("A".charCodeAt(0)-1));
}
alert(sum);

Could you perhaps treat it like a base 26 number, and then substitute letters for a base 26 number?
So in effect, your right most digit will always be a raw number between 1 and 26, and the remainder of the "number" (the left part) is the number of 26's collected? So A would represent one lot of 26, B would be 2, etc.
As an example:
B = 2 = Column 2
AB = 26 * 1(A) + 2 = Column 28
BB = 26 * 2(B) + 2 = Column 54
DA = 26 * 4(D) + 1 = Column 105
etc

Shorter version:
int col = "Ab".Aggregate(0, (a, c) => a * 26 + c & 31); // 28
To ignore non A-Za-z characters:
int col = " !$Af$3 ".Aggregate(0, (a, c) => (uint)((c | 32) - 'a') > 25 ? a : a * 26 + (c & 31)); // 32

Here is a basic c++ answer for those who are intrested in c++ implemention.
int titleToNumber(string given) {
int power=0;
int res=0;
for(int i=given.length()-1;i>=0;i--)
{
char c=given[i];
res+=pow(26,power)*(c-'A'+1);
power++;
}
return res;
}

in Excel VBA you could use the .Range Method to get the number, like so:
Dim rng as Range
Dim vSearchCol as variant 'your input column
Set rng.Thisworkbook.worksheets("mySheet").Range(vSearchCol & "1:" & vSearchCol & "1")
Then use .column property:
debug.print rng.column
if you need full code see below:
Function ColumnbyName(vInput As Variant, Optional bByName As Boolean = True) As Variant
Dim Rng As Range
If bByName Then
If Not VBA.IsNumeric(vInput) Then
Set Rng = ThisWorkbook.Worksheets("mytab").Range(vInput & "1:" & vInput & "1")
ColumnbyName = Rng.Column
Else
MsgBox "Please enter valid non Numeric column or change paramter bByName to False!"
End If
Else
If VBA.IsNumeric(vInput) Then
ColumnbyName = VBA.Chr(64 + CInt(vInput))
Else
MsgBox "Please enter valid Numeric column or change paramter bByName to True!"
End If
End If
End Function

I guess this essentially works out pretty much the same as some of the other answers, but it may make a little more clear what's going on with the alpha equivalent of a numeric digit. It's not quite a base 26 system because there is no 0 placeholder. That is, the 26th column would be 'A0' or something instead of Z in base 26. And it's not base 27 because the 'alpha-gits' don't represent powers of 27. Man, it really makes you appreciate what a mess arithmetic must have been before the Babylonians invented the zero!
UInt32 sum = 0, gitVal = 1;
foreach (char alphagit in ColumnName.ToUpperInvariant().ToCharArray().Reverse())
{
sum += gitVal * (UInt32)(alphagit - 'A' + 1)
gitVal *= 26;
}
Like some others, I reversed the character array so I don't need to know anything about exponents.

For this purpose I use only one line:
int ColumnNumber = Application.Range[MyColumnName + "1"].Column;

How to convert a column number (e.g. 127) into an Excel column (e.g. AA)

How do you convert a numerical number to an Excel column name in C# without using automation getting the value directly from Excel.
Excel 2007 has a possible range of 1 to 16384, which is the number of columns that it supports. The resulting values should be in the form of excel column names, e.g. A, AA, AAA etc.

Here's how I do it:
private string GetExcelColumnName(int columnNumber)
{
string columnName = "";
while (columnNumber > 0)
{
int modulo = (columnNumber - 1) % 26;
columnName = Convert.ToChar('A' + modulo) + columnName;
columnNumber = (columnNumber - modulo) / 26;
}
return columnName;
}

If anyone needs to do this in Excel without VBA, here is a way:
=SUBSTITUTE(ADDRESS(1;colNum;4);"1";"")
where colNum is the column number
And in VBA:
Function GetColumnName(colNum As Integer) As String
Dim d As Integer
Dim m As Integer
Dim name As String
d = colNum
name = ""
Do While (d > 0)
m = (d - 1) Mod 26
name = Chr(65 + m) + name
d = Int((d - m) / 26)
Loop
GetColumnName = name
End Function

You might need conversion both ways, e.g from Excel column adress like AAZ to integer and from any integer to Excel. The two methods below will do just that. Assumes 1 based indexing, first element in your "arrays" are element number 1.
No limits on size here, so you can use adresses like ERROR and that would be column number 2613824 ...
public static string ColumnAdress(int col)
{
if (col <= 26) {
return Convert.ToChar(col + 64).ToString();
}
int div = col / 26;
int mod = col % 26;
if (mod == 0) {mod = 26;div--;}
return ColumnAdress(div) + ColumnAdress(mod);
}
public static int ColumnNumber(string colAdress)
{
int[] digits = new int[colAdress.Length];
for (int i = 0; i < colAdress.Length; ++i)
{
digits[i] = Convert.ToInt32(colAdress[i]) - 64;
}
int mul=1;int res=0;
for (int pos = digits.Length - 1; pos >= 0; --pos)
{
res += digits[pos] * mul;
mul *= 26;
}
return res;
}

Sorry, this is Python instead of C#, but at least the results are correct:
def ColIdxToXlName(idx):
if idx < 1:
raise ValueError("Index is too small")
result = ""
while True:
if idx > 26:
idx, r = divmod(idx - 1, 26)
result = chr(r + ord('A')) + result
else:
return chr(idx + ord('A') - 1) + result
for i in xrange(1, 1024):
print "%4d : %s" % (i, ColIdxToXlName(i))

I discovered an error in my first post, so I decided to sit down and do the the math. What I found is that the number system used to identify Excel columns is not a base 26 system, as another person posted. Consider the following in base 10. You can also do this with the letters of the alphabet.
Space:.........................S1, S2, S3 : S1, S2, S3
....................................0, 00, 000 :.. A, AA, AAA
....................................1, 01, 001 :.. B, AB, AAB
.................................... …, …, … :.. …, …, …
....................................9, 99, 999 :.. Z, ZZ, ZZZ
Total states in space: 10, 100, 1000 : 26, 676, 17576
Total States:...............1110................18278
Excel numbers columns in the individual alphabetical spaces using base 26. You can see that in general, the state space progression is a, a^2, a^3, … for some base a, and the total number of states is a + a^2 + a^3 + … .
Suppose you want to find the total number of states A in the first N spaces. The formula for doing so is A = (a)(a^N - 1 )/(a-1). This is important because we need to find the space N that corresponds to our index K. If I want to find out where K lies in the number system I need to replace A with K and solve for N. The solution is N = log{base a} (A (a-1)/a +1). If I use the example of a = 10 and K = 192, I know that N = 2.23804… . This tells me that K lies at the beginning of the third space since it is a little greater than two.
The next step is to find exactly how far in the current space we are. To find this, subtract from K the A generated using the floor of N. In this example, the floor of N is two. So, A = (10)(10^2 – 1)/(10-1) = 110, as is expected when you combine the states of the first two spaces. This needs to be subtracted from K because these first 110 states would have already been accounted for in the first two spaces. This leaves us with 82 states. So, in this number system, the representation of 192 in base 10 is 082.
The C# code using a base index of zero is
private string ExcelColumnIndexToName(int Index)
{
string range = string.Empty;
if (Index < 0 ) return range;
int a = 26;
int x = (int)Math.Floor(Math.Log((Index) * (a - 1) / a + 1, a));
Index -= (int)(Math.Pow(a, x) - 1) * a / (a - 1);
for (int i = x+1; Index + i > 0; i--)
{
range = ((char)(65 + Index % a)).ToString() + range;
Index /= a;
}
return range;
}
//Old Post
A zero-based solution in C#.
private string ExcelColumnIndexToName(int Index)
{
string range = "";
if (Index < 0 ) return range;
for(int i=1;Index + i > 0;i=0)
{
range = ((char)(65 + Index % 26)).ToString() + range;
Index /= 26;
}
if (range.Length > 1) range = ((char)((int)range[0] - 1)).ToString() + range.Substring(1);
return range;
}

This answer is in javaScript:
function getCharFromNumber(columnNumber){
var dividend = columnNumber;
var columnName = "";
var modulo;
while (dividend > 0)
{
modulo = (dividend - 1) % 26;
columnName = String.fromCharCode(65 + modulo).toString() + columnName;
dividend = parseInt((dividend - modulo) / 26);
}
return columnName;
}

Easy with recursion.
public static string GetStandardExcelColumnName(int columnNumberOneBased)
{
int baseValue = Convert.ToInt32('A');
int columnNumberZeroBased = columnNumberOneBased - 1;
string ret = "";
if (columnNumberOneBased > 26)
{
ret = GetStandardExcelColumnName(columnNumberZeroBased / 26) ;
}
return ret + Convert.ToChar(baseValue + (columnNumberZeroBased % 26) );
}

I'm surprised all of the solutions so far contain either iteration or recursion.
Here's my solution that runs in constant time (no loops). This solution works for all possible Excel columns and checks that the input can be turned into an Excel column. Possible columns are in the range [A, XFD] or [1, 16384]. (This is dependent on your version of Excel)
private static string Turn(uint col)
{
if (col < 1 || col > 16384) //Excel columns are one-based (one = 'A')
throw new ArgumentException("col must be >= 1 and <= 16384");
if (col <= 26) //one character
return ((char)(col + 'A' - 1)).ToString();
else if (col <= 702) //two characters
{
char firstChar = (char)((int)((col - 1) / 26) + 'A' - 1);
char secondChar = (char)(col % 26 + 'A' - 1);
if (secondChar == '#') //Excel is one-based, but modulo operations are zero-based
secondChar = 'Z'; //convert one-based to zero-based
return string.Format("{0}{1}", firstChar, secondChar);
}
else //three characters
{
char firstChar = (char)((int)((col - 1) / 702) + 'A' - 1);
char secondChar = (char)((col - 1) / 26 % 26 + 'A' - 1);
char thirdChar = (char)(col % 26 + 'A' - 1);
if (thirdChar == '#') //Excel is one-based, but modulo operations are zero-based
thirdChar = 'Z'; //convert one-based to zero-based
return string.Format("{0}{1}{2}", firstChar, secondChar, thirdChar);
}
}

Same implementation in Java
public String getExcelColumnName (int columnNumber)
{
int dividend = columnNumber;
int i;
String columnName = "";
int modulo;
while (dividend > 0)
{
modulo = (dividend - 1) % 26;
i = 65 + modulo;
columnName = new Character((char)i).toString() + columnName;
dividend = (int)((dividend - modulo) / 26);
}
return columnName;
}

int nCol = 127;
string sChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
string sCol = "";
while (nCol >= 26)
{
int nChar = nCol % 26;
nCol = (nCol - nChar) / 26;
// You could do some trick with using nChar as offset from 'A', but I am lazy to do it right now.
sCol = sChars[nChar] + sCol;
}
sCol = sChars[nCol] + sCol;
Update: Peter's comment is right. That's what I get for writing code in the browser. :-) My solution was not compiling, it was missing the left-most letter and it was building the string in reverse order - all now fixed.
Bugs aside, the algorithm is basically converting a number from base 10 to base 26.
Update 2: Joel Coehoorn is right - the code above will return AB for 27. If it was real base 26 number, AA would be equal to A and the next number after Z would be BA.
int nCol = 127;
string sChars = "0ABCDEFGHIJKLMNOPQRSTUVWXYZ";
string sCol = "";
while (nCol > 26)
{
int nChar = nCol % 26;
if (nChar == 0)
nChar = 26;
nCol = (nCol - nChar) / 26;
sCol = sChars[nChar] + sCol;
}
if (nCol != 0)
sCol = sChars[nCol] + sCol;

..And converted to php:
function GetExcelColumnName($columnNumber) {
$columnName = '';
while ($columnNumber > 0) {
$modulo = ($columnNumber - 1) % 26;
$columnName = chr(65 + $modulo) . $columnName;
$columnNumber = (int)(($columnNumber - $modulo) / 26);
}
return $columnName;
}

Just throwing in a simple two-line C# implementation using recursion, because all the answers here seem far more complicated than necessary.
/// <summary>
/// Gets the column letter(s) corresponding to the given column number.
/// </summary>
/// <param name="column">The one-based column index. Must be greater than zero.</param>
/// <returns>The desired column letter, or an empty string if the column number was invalid.</returns>
public static string GetColumnLetter(int column) {
if (column < 1) return String.Empty;
return GetColumnLetter((column - 1) / 26) + (char)('A' + (column - 1) % 26);
}

Although there are already a bunch of valid answers1, none get into the theory behind it.
Excel column names are bijective base-26 representations of their number. This is quite different than an ordinary base 26 (there is no leading zero), and I really recommend reading the Wikipedia entry to grasp the differences. For example, the decimal value 702 (decomposed in 26*26 + 26) is represented in "ordinary" base 26 by 110 (i.e. 1x26^2 + 1x26^1 + 0x26^0) and in bijective base-26 by ZZ (i.e. 26x26^1 + 26x26^0).
Differences aside, bijective numeration is a positional notation, and as such we can perform conversions using an iterative (or recursive) algorithm which on each iteration finds the digit of the next position (similarly to an ordinary base conversion algorithm).
The general formula to get the digit at the last position (the one indexed 0) of the bijective base-k representation of a decimal number m is (f being the ceiling function minus 1):
m - (f(m / k) * k)
The digit at the next position (i.e. the one indexed 1) is found by applying the same formula to the result of f(m / k). We know that for the last digit (i.e. the one with the highest index) f(m / k) is 0.
This forms the basis for an iteration that finds each successive digit in bijective base-k of a decimal number. In pseudo-code it would look like this (digit() maps a decimal integer to its representation in the bijective base -- e.g. digit(1) would return A in bijective base-26):
fun conv(m)
q = f(m / k)
a = m - (q * k)
if (q == 0)
return digit(a)
else
return conv(q) + digit(a);
So we can translate this to C#2 to get a generic3 "conversion to bijective base-k" ToBijective() routine:
class BijectiveNumeration {
private int baseK;
private Func<int, char> getDigit;
public BijectiveNumeration(int baseK, Func<int, char> getDigit) {
this.baseK = baseK;
this.getDigit = getDigit;
}
public string ToBijective(double decimalValue) {
double q = f(decimalValue / baseK);
double a = decimalValue - (q * baseK);
return ((q > 0) ? ToBijective(q) : "") + getDigit((int)a);
}
private static double f(double i) {
return (Math.Ceiling(i) - 1);
}
}
Now for conversion to bijective base-26 (our "Excel column name" use case):
static void Main(string[] args)
{
BijectiveNumeration bijBase26 = new BijectiveNumeration(
26,
(value) => Convert.ToChar('A' + (value - 1))
);
Console.WriteLine(bijBase26.ToBijective(1)); // prints "A"
Console.WriteLine(bijBase26.ToBijective(26)); // prints "Z"
Console.WriteLine(bijBase26.ToBijective(27)); // prints "AA"
Console.WriteLine(bijBase26.ToBijective(702)); // prints "ZZ"
Console.WriteLine(bijBase26.ToBijective(16384)); // prints "XFD"
}
Excel's maximum column index is 16384 / XFD, but this code will convert any positive number.
As an added bonus, we can now easily convert to any bijective base. For example for bijective base-10:
static void Main(string[] args)
{
BijectiveNumeration bijBase10 = new BijectiveNumeration(
10,
(value) => value < 10 ? Convert.ToChar('0'+value) : 'A'
);
Console.WriteLine(bijBase10.ToBijective(1)); // prints "1"
Console.WriteLine(bijBase10.ToBijective(10)); // prints "A"
Console.WriteLine(bijBase10.ToBijective(123)); // prints "123"
Console.WriteLine(bijBase10.ToBijective(20)); // prints "1A"
Console.WriteLine(bijBase10.ToBijective(100)); // prints "9A"
Console.WriteLine(bijBase10.ToBijective(101)); // prints "A1"
Console.WriteLine(bijBase10.ToBijective(2010)); // prints "19AA"
}
1 This generic answer can eventually be reduced to the other, correct, specific answers, but I find it hard to fully grasp the logic of the solutions without the formal theory behind bijective numeration in general. It also proves its correctness nicely. Additionally, several similar questions link back to this one, some being language-agnostic or more generic. That's why I thought the addition of this answer was warranted, and that this question was a good place to put it.
2 C# disclaimer: I implemented an example in C# because this is what is asked here, but I have never learned nor used the language. I have verified it does compile and run, but please adapt it to fit the language best practices / general conventions, if necessary.
3 This example only aims to be correct and understandable ; it could and should be optimized would performance matter (e.g. with tail-recursion -- but that seems to require trampolining in C#), and made safer (e.g. by validating parameters).

I wanted to throw in my static class I use, for interoping between col index and col Label. I use a modified accepted answer for my ColumnLabel Method
public static class Extensions
{
public static string ColumnLabel(this int col)
{
var dividend = col;
var columnLabel = string.Empty;
int modulo;
while (dividend > 0)
{
modulo = (dividend - 1) % 26;
columnLabel = Convert.ToChar(65 + modulo).ToString() + columnLabel;
dividend = (int)((dividend - modulo) / 26);
}
return columnLabel;
}
public static int ColumnIndex(this string colLabel)
{
// "AD" (1 * 26^1) + (4 * 26^0) ...
var colIndex = 0;
for(int ind = 0, pow = colLabel.Count()-1; ind < colLabel.Count(); ++ind, --pow)
{
var cVal = Convert.ToInt32(colLabel[ind]) - 64; //col A is index 1
colIndex += cVal * ((int)Math.Pow(26, pow));
}
return colIndex;
}
}
Use this like...
30.ColumnLabel(); // "AD"
"AD".ColumnIndex(); // 30

private String getColumn(int c) {
String s = "";
do {
s = (char)('A' + (c % 26)) + s;
c /= 26;
} while (c-- > 0);
return s;
}
Its not exactly base 26, there is no 0 in the system. If there was, 'Z' would be followed by 'BA' not by 'AA'.

if you just want it for a cell formula without code, here's a formula for it:
IF(COLUMN()>=26,CHAR(ROUND(COLUMN()/26,1)+64)&CHAR(MOD(COLUMN(),26)+64),CHAR(COLUMN()+64))

In Delphi (Pascal):
function GetExcelColumnName(columnNumber: integer): string;
var
dividend, modulo: integer;
begin
Result := '';
dividend := columnNumber;
while dividend > 0 do begin
modulo := (dividend - 1) mod 26;
Result := Chr(65 + modulo) + Result;
dividend := (dividend - modulo) div 26;
end;
end;

A little late to the game, but here's the code I use (in C#):
private static readonly string _Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
public static int ColumnNameParse(string value)
{
// assumes value.Length is [1,3]
// assumes value is uppercase
var digits = value.PadLeft(3).Select(x => _Alphabet.IndexOf(x));
return digits.Aggregate(0, (current, index) => (current * 26) + (index + 1));
}

In perl, for an input of 1 (A), 27 (AA), etc.
sub excel_colname {
my ($idx) = #_; # one-based column number
--$idx; # zero-based column index
my $name = "";
while ($idx >= 0) {
$name .= chr(ord("A") + ($idx % 26));
$idx = int($idx / 26) - 1;
}
return scalar reverse $name;
}

Though I am late to the game, Graham's answer is far from being optimal. Particularly, you don't have to use the modulo, call ToString() and apply (int) cast. Considering that in most cases in C# world you would start numbering from 0, here is my revision:
public static string GetColumnName(int index) // zero-based
{
const byte BASE = 'Z' - 'A' + 1;
string name = String.Empty;
do
{
name = Convert.ToChar('A' + index % BASE) + name;
index = index / BASE - 1;
}
while (index >= 0);
return name;
}

More than 30 solutions already, but here's my one-line C# solution...
public string IntToExcelColumn(int i)
{
return ((i<16926? "" : ((char)((((i/26)-1)%26)+65)).ToString()) + (i<2730? "" : ((char)((((i/26)-1)%26)+65)).ToString()) + (i<26? "" : ((char)((((i/26)-1)%26)+65)).ToString()) + ((char)((i%26)+65)));
}

After looking at all the supplied Versions here, I decided to do one myself, using recursion.
Here is my vb.net Version:
Function CL(ByVal x As Integer) As String
If x >= 1 And x <= 26 Then
CL = Chr(x + 64)
Else
CL = CL((x - x Mod 26) / 26) & Chr((x Mod 26) + 1 + 64)
End If
End Function

Refining the original solution (in C#):
public static class ExcelHelper
{
private static Dictionary<UInt16, String> l_DictionaryOfColumns;
public static ExcelHelper() {
l_DictionaryOfColumns = new Dictionary<ushort, string>(256);
}
public static String GetExcelColumnName(UInt16 l_Column)
{
UInt16 l_ColumnCopy = l_Column;
String l_Chars = "0ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String l_rVal = "";
UInt16 l_Char;
if (l_DictionaryOfColumns.ContainsKey(l_Column) == true)
{
l_rVal = l_DictionaryOfColumns[l_Column];
}
else
{
while (l_ColumnCopy > 26)
{
l_Char = l_ColumnCopy % 26;
if (l_Char == 0)
l_Char = 26;
l_ColumnCopy = (l_ColumnCopy - l_Char) / 26;
l_rVal = l_Chars[l_Char] + l_rVal;
}
if (l_ColumnCopy != 0)
l_rVal = l_Chars[l_ColumnCopy] + l_rVal;
l_DictionaryOfColumns.ContainsKey(l_Column) = l_rVal;
}
return l_rVal;
}
}

Here is an Actionscript version:
private var columnNumbers:Array = ['A', 'B', 'C', 'D', 'E', 'F' , 'G', 'H', 'I', 'J', 'K' ,'L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'];
private function getExcelColumnName(columnNumber:int) : String{
var dividend:int = columnNumber;
var columnName:String = "";
var modulo:int;
while (dividend > 0)
{
modulo = (dividend - 1) % 26;
columnName = columnNumbers[modulo] + columnName;
dividend = int((dividend - modulo) / 26);
}
return columnName;
}

JavaScript Solution
/**
* Calculate the column letter abbreviation from a 1 based index
* #param {Number} value
* #returns {string}
*/
getColumnFromIndex = function (value) {
var base = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('');
var remainder, result = "";
do {
remainder = value % 26;
result = base[(remainder || 26) - 1] + result;
value = Math.floor(value / 26);
} while (value > 0);
return result;
};

These my codes to convert specific number (index start from 1) to Excel Column.
public static string NumberToExcelColumn(uint number)
{
uint originalNumber = number;
uint numChars = 1;
while (Math.Pow(26, numChars) < number)
{
numChars++;
if (Math.Pow(26, numChars) + 26 >= number)
{
break;
}
}
string toRet = "";
uint lastValue = 0;
do
{
number -= lastValue;
double powerVal = Math.Pow(26, numChars - 1);
byte thisCharIdx = (byte)Math.Truncate((columnNumber - 1) / powerVal);
lastValue = (int)powerVal * thisCharIdx;
if (numChars - 2 >= 0)
{
double powerVal_next = Math.Pow(26, numChars - 2);
byte thisCharIdx_next = (byte)Math.Truncate((columnNumber - lastValue - 1) / powerVal_next);
int lastValue_next = (int)Math.Pow(26, numChars - 2) * thisCharIdx_next;
if (thisCharIdx_next == 0 && lastValue_next == 0 && powerVal_next == 26)
{
thisCharIdx--;
lastValue = (int)powerVal * thisCharIdx;
}
}
toRet += (char)((byte)'A' + thisCharIdx + ((numChars > 1) ? -1 : 0));
numChars--;
} while (numChars > 0);
return toRet;
}
My Unit Test:
[TestMethod]
public void Test()
{
Assert.AreEqual("A", NumberToExcelColumn(1));
Assert.AreEqual("Z", NumberToExcelColumn(26));
Assert.AreEqual("AA", NumberToExcelColumn(27));
Assert.AreEqual("AO", NumberToExcelColumn(41));
Assert.AreEqual("AZ", NumberToExcelColumn(52));
Assert.AreEqual("BA", NumberToExcelColumn(53));
Assert.AreEqual("ZZ", NumberToExcelColumn(702));
Assert.AreEqual("AAA", NumberToExcelColumn(703));
Assert.AreEqual("ABC", NumberToExcelColumn(731));
Assert.AreEqual("ACQ", NumberToExcelColumn(771));
Assert.AreEqual("AYZ", NumberToExcelColumn(1352));
Assert.AreEqual("AZA", NumberToExcelColumn(1353));
Assert.AreEqual("AZB", NumberToExcelColumn(1354));
Assert.AreEqual("BAA", NumberToExcelColumn(1379));
Assert.AreEqual("CNU", NumberToExcelColumn(2413));
Assert.AreEqual("GCM", NumberToExcelColumn(4823));
Assert.AreEqual("MSR", NumberToExcelColumn(9300));
Assert.AreEqual("OMB", NumberToExcelColumn(10480));
Assert.AreEqual("ULV", NumberToExcelColumn(14530));
Assert.AreEqual("XFD", NumberToExcelColumn(16384));
}

Sorry, this is Python instead of C#, but at least the results are correct:
def excel_column_number_to_name(column_number):
output = ""
index = column_number-1
while index >= 0:
character = chr((index%26)+ord('A'))
output = output + character
index = index/26 - 1
return output[::-1]
for i in xrange(1, 1024):
print "%4d : %s" % (i, excel_column_number_to_name(i))
Passed these test cases:
Column Number: 494286 => ABCDZ
Column Number: 27 => AA
Column Number: 52 => AZ

For what it is worth, here is Graham's code in Powershell:
function ConvertTo-ExcelColumnID {
param (
[parameter(Position = 0,
HelpMessage = "A 1-based index to convert to an excel column ID. e.g. 2 => 'B', 29 => 'AC'",
Mandatory = $true)]
[int]$index
);
[string]$result = '';
if ($index -le 0 ) {
return $result;
}
while ($index -gt 0) {
[int]$modulo = ($index - 1) % 26;
$character = [char]($modulo + [int][char]'A');
$result = $character + $result;
[int]$index = ($index - $modulo) / 26;
}
return $result;
}

Another VBA way
Public Function GetColumnName(TargetCell As Range) As String
GetColumnName = Split(CStr(TargetCell.Cells(1, 1).Address), "$")(1)
End Function

Here's my super late implementation in PHP. This one's recursive. I wrote it just before I found this post. I wanted to see if others had solved this problem already...
public function GetColumn($intNumber, $strCol = null) {
if ($intNumber > 0) {
$intRem = ($intNumber - 1) % 26;
$strCol = $this->GetColumn(intval(($intNumber - $intRem) / 26), sprintf('%s%s', chr(65 + $intRem), $strCol));
}
return $strCol;
}