Programming Riddle: How might you translate an Excel column name to a number?

Sounds like a standard reduce to me:

Python:

def excel2num(x): 
    return reduce(lambda s,a:s*26+ord(a)-ord('A')+1, x, 0)

C#:

int ExcelToNumber(string x) {
    return x.Aggregate(0, (s, c) => s * 26 + c - 'A' + 1 );
}

I wrote this ages ago for some Python script:

def index_to_int(index):
    s = 0
    pow = 1
    for letter in index[::-1]:
        d = int(letter,36) - 9
        s += pow * d
        pow *= 26
    # excel starts column numeration from 1
    return s

Read a column name from STDIN and print out its corresponding number:

perl -le '$x = $x * 26 - 64 + ord for <> =~ /./g; print $x'

Caveats: Assumes ASCII.

EDIT: Replaced " with ' so that your shell won't interpolate $x in the string.

Coincidentally I've solved the same problem using javascript

$(function() { //shorthand document.ready function
    var getNumber = function(x) {
        var result = 0;
        var multiplier = 1;
        for ( var i = x.length-1; i >= 0; i--)
        { 
            var value = ((x[i].charCodeAt(0) - "A".charCodeAt(0)) + 1);
            result = result + value * multiplier;
            multiplier = multiplier * 26;
        }
        return result;
    };
    
    $('#form').on('submit', function(e) { //use on if jQuery 1.7+
        e.preventDefault();  //prevent form from submitting
        var data = $("#number").val();
        $('#answer').text(getNumber(data));
    });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<form id="form">
<input type="text" id="number"></input>
    <button>submit</button>
</form>
<p id="answer"></p>

    var getNumber = function(x) {
        var result = 0;
        var multiplier = 1;
        for ( var i = x.length-1; i >= 0; i--)
        { 
            var value = ((x[i].charCodeAt(0) - "A".charCodeAt(0)) + 1);
            result = result + value * multiplier;
            multiplier = multiplier * 26;
        }
        return result;
    };

http://jsfiddle.net/M7Xty/1/

Hah - written it already in our code base - about 3 different times :(

%% @doc Convert an string to a decimal integer
%% @spec b26_to_i(string()) -> integer()

b26_to_i(List) when is_list(List) ->
    b26_to_i(string:to_lower(lists:reverse(List)),0,0).

%% private functions
b26_to_i([], _Power, Value) -> 
    Value;

b26_to_i([H|T],Power,Value)->
    NewValue = case (H > 96) andalso (H < 123) of
                   true ->
                       round((H - 96) * math:pow(26, Power));
                   _    ->
                       exit([H | T] ++ " is not a valid base 26 number")
               end,
    b26_to_i(T, Power + 1, NewValue + Value).

The riddle is that it isn't actually a Base26 representation of a number (we are lying to ourselves in our function name here) because there is no 0 in it.

The sequence is: A, B, C ... Z, AA, AB, AC

and not: A, B, C ...Z, BA, BB, BC

(the language is Erlang, mais oui).

You can do this in C like this:

unsigned int coltonum(char * string)
{
   unsigned result = 0;
   char ch;

   while(ch = *string++)
      result = result * 26 + ch - 'A' + 1;

  return result;
}

No error checking, only works for upper case strings, string must be null terminated.

Assuming column A = 1

int GetColumnNumber(string columnName)
{
  int sum = 0;
  int exponent = 0;
  for(int i = columnName.Length - 1; i>=0; i--)
  {
    sum += (columnName[i] - 'A' + 1) *  (GetPower(26, exponent));
    exponent++;
  }
  return sum;
}

int GetPower(int number, int exponent)
{
  int power = 1;
  for(int i=0; i<exponent; i++)
    power *= number;
  return power;
}

Caveat: both of these versions assume only uppercase letters A to Z. Anything else causes a miscalculation. It wouldn't be hard to add a bit of error checking and/or uppercasing to improve them.

Scala

def excel2Number(excel : String) : Int = 
  (0 /: excel) ((accum, ch) => accum * 26 + ch - 'A' + 1)

Haskell

excel2Number :: String -> Int
excel2Number = flip foldl 0 $ \accum ch -> accum * 26 + fromEnum ch - fromEnum 'A' + 1

Get the column number from its name

Java:

public int getColNum (String colName) {

    //remove any whitespace
    colName = colName.trim();

    StringBuffer buff = new StringBuffer(colName);

    //string to lower case, reverse then place in char array
    char chars[] = buff.reverse().toString().toLowerCase().toCharArray();

    int retVal=0, multiplier=0;

    for(int i = 0; i < chars.length;i++){
        //retrieve ascii value of character, subtract 96 so number corresponds to place in alphabet. ascii 'a' = 97 
        multiplier = (int)chars[i]-96;
        //mult the number by 26^(position in array)
        retVal += multiplier * Math.pow(26, i);
    }
    return retVal;
}

Get a column name from an int in Java(read more here):

public String getColName (int colNum) {

   String res = "";

   int quot = colNum;
   int rem;        
    /*1. Subtract one from number.
    *2. Save the mod 26 value.
   *3. Divide the number by 26, save result.
   *4. Convert the remainder to a letter.
   *5. Repeat until the number is zero.
   *6. Return that bitch...
   */
    while(quot > 0)
    {
        quot = quot - 1;
        rem = quot % 26;
        quot = quot / 26;

        //cast to a char and add to the beginning of the string
        //add 97 to convert to the correct ascii number
        res = (char)(rem+97) + res;            
    }   
    return res;
}

Does it help to think of the string as the reverse of the column number in base 26 with digits represented by A, B, ... Z?

This is basically a number in base 26, with the difference that the number doesn't use 0-9 and then letters but only letters.

Here's a CFML one:

<cffunction name="ColToNum" returntype="Numeric">
    <cfargument name="Input" type="String" />
    <cfset var Total = 0 />
    <cfset var Pos = 0 />

    <cfloop index="Pos" from="1" to="#Len(Arguments.Input)#">
        <cfset Total += 26^(Pos-1) * ( Asc( UCase( Mid(Arguments.Input,Pos,1) ) ) - 64 ) />
    </cfloop>

    <cfreturn Total />
</cffunction>

<cfoutput>
    #ColToNum('AABCCE')#
</cfoutput>

And because I'm in an odd mood, here's a CFScript version:

function ColToNum ( Input )
{
    var Total = 0;

    for ( var Pos = 1 ; Pos <= Len(Arguments.Input) ; Pos++ )
    {
        Total += 26^(Pos-1) * ( Asc( UCase( Mid(Arguments.Input,Pos,1) ) ) - 64 );
    }

    return Total;
}

WriteOutput( ColToNum('AABCCE') );

Common Lisp:

(defun excel->number (string)
  "Converts an Excel column name to a column number."
  (reduce (lambda (a b) (+ (* a 26) b))
          string
          :key (lambda (x) (- (char-int x) 64))))

edit: the inverse operation:

(defun number->excel (number &optional acc)
  "Converts a column number to Excel column name."
  (if (zerop number)
      (concatenate 'string acc)
      (multiple-value-bind (rest current) (floor number 26)
        (if (zerop current)
            (number->excel (- rest 1) (cons #\Z acc))
            (number->excel rest (cons (code-char (+ current 64)) acc))))))

Slightly related, the better challenge is the other way around: given the column number, find the column label as string.

Qt version as what I implemented for KOffice:

QString columnLabel( unsigned column )
{
  QString str;
  unsigned digits = 1;
  unsigned offset = 0;

  column--;
  for( unsigned limit = 26; column >= limit+offset; limit *= 26, digits++ )
    offset += limit;

  for( unsigned c = column - offset; digits; --digits, c/=26 )
    str.prepend( QChar( 'A' + (c%26) ) );

  return str;
}

another [more cryptic] erlang example:

col2int(String) -> col2int(0,String).
col2int(X,[A|L]) when A >= 65, A =< 90 ->
col2int(26 * X + A - 65 + 1, L);
col2int(X,[]) -> X.

and inverse function:

int2col(Y) when Y > 0 -> int2col(Y,[]).
int2col(0,L) -> L;
int2col(Y,L) when Y rem 26 == 0 -> 
   int2col(Y div 26 - 1,[(26+65-1)|L]);
int2col(Y,L) ->
   P = Y rem 26,
   int2col((Y - P) div 26,[P + 65-1|L]).

Delphi:

// convert EXcel column name to column number 1..256
// case-sensitive; returns 0 for illegal column name
function cmColmAlfaToNumb( const qSRC : string ) : integer;
var II : integer;
begin
   result := 0;
   for II := 1 to length(qSRC) do begin
      if (qSRC[II]<'A')or(qSRC[II]>'Z') then begin
         result := 0;
         exit;
      end;
      result := result*26+ord(qSRC[II])-ord('A')+1;
   end;
   if result>256 then result := 0;
end;

-Al.

def ExcelColumnToNumber(ColumnName):
    ColNum = 0
    for i in range(0, len(ColumnName)):
        # Easier once formula determined: 'PositionValue * Base^Position'
        # i.e. AA=(1*26^1)+(1*26^0)   or  792=(7*10^2)+(9*10^1)+(2*10^0)
        ColNum += (int(ColumnName[i],36) -9) * (pow(26, len(ColumnName)-i-1))
    return ColNum

p.s. My first Python script!

This version is purely functional and permits alternative 'code' sequences, for example if you wanted to only uses the letters 'A' to 'C'. In Scala, with a suggestion from dcsobral.

def columnNumber(name: String) = {
    val code = 'A' to 'Z'

    name.foldLeft(0) { (sum, letter) =>
        (sum * code.length) + (code.indexOf(letter) + 1)
    }
}