Algorithm to get the excel-like column name of a number

Question

I'm working on a script that generate some Excel documents and I need to convert a number into its column name equivalent. For example:

1 => A 2 => B 27 => AA 28 => AB 14558 => UMX 

I have already written an algorithm to do so, but I'd like to know whether are simpler or faster ways to do it:

function numberToColumnName($number){     $abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";     $abc_len = strlen($abc);      $result_len = 1; // how much characters the column's name will have     $pow = 0;     while( ( $pow += pow($abc_len, $result_len) ) < $number ){         $result_len++;     }      $result = "";     $next = false;     // add each character to the result...     for($i = 1; $i<=$result_len; $i++){         $index = ($number % $abc_len) - 1; // calculate the module          // sometimes the index should be decreased by 1         if( $next || $next = false ){             $index--;         }          // this is the point that will be calculated in the next iteration         $number = floor($number / strlen($abc));          // if the index is negative, convert it to positive         if( $next = ($index < 0) ) {             $index = $abc_len + $index;         }          $result = $abc[$index].$result; // concatenate the letter     }     return $result; } 

Do you know a better way to do it? Maybe something to keep it simpler? or a performance improvement?

Edit

ircmaxell's implementation works pretty fine. But, I'm going to add this nice short one:

function num2alpha($n) {     for($r = ""; $n >= 0; $n = intval($n / 26) - 1)         $r = chr($n%26 + 0x41) . $r;     return $r; } 

Answer 1

Here's a nice simple recursive function (Based on zero indexed numbers, meaning 0 == A, 1 == B, etc)...

function getNameFromNumber($num) {     $numeric = $num % 26;     $letter = chr(65 + $numeric);     $num2 = intval($num / 26);     if ($num2 > 0) {         return getNameFromNumber($num2 - 1) . $letter;     } else {         return $letter;     } } 

And if you want it one indexed (1 == A, etc):

function getNameFromNumber($num) {     $numeric = ($num - 1) % 26;     $letter = chr(65 + $numeric);     $num2 = intval(($num - 1) / 26);     if ($num2 > 0) {         return getNameFromNumber($num2) . $letter;     } else {         return $letter;     } } 

Tested with numbers from 0 to 10000...

Answer 2

Using PhpSpreadsheet (PHPExcel is deprecated)

// result = 'A' \PhpOffice\PhpSpreadsheet\Cell\Coordinate::stringFromColumnIndex(1); 

Note index 0 results in 'Z'

https://phpspreadsheet.readthedocs.io/en/develop/

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The correct answer (if you use PHPExcel Library) is:

// result = 'A' $columnLetter = PHPExcel_Cell::stringFromColumnIndex(0); // ZERO-based!  

and backwards:

// result = 1 $colIndex = PHPExcel_Cell::columnIndexFromString('A');