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

user_244296 0
Viewed 61994

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;
}
optimization algorithm php
11 Answers

Number convert to Excel column letters:

/**
 * Number convert to Excel column letters
 * 
 * 1 = A
 * 2 = B
 * 3 = C
 * 27 = AA
 * 1234567789 = CYWOQRM
 * 
 * @link https://vector.cool/php-number-convert-to-excel-column-letters-2
 * 
 * @param int  $num       欄數
 * @param bool $uppercase 大小寫
 * @return void
 */
function num_to_letters($n)
{
    $n -= 1;
    for ($r = ""; $n >= 0; $n = intval($n / 26) - 1)
        $r = chr($n % 26 + 0x41) . $r;
    return $r;
}

ex:

echo num_to_letters(1);          // A
echo num_to_letters(2);          // B
echo num_to_letters(3);          // C
echo num_to_letters(27);         // AA
echo num_to_letters(1234567789); // CYWOQRM

Excel column letters convert to Number:

/**
 * Excel column letters convert to Number
 *
 * A = 1
 * B = 2
 * C = 3
 * AA = 27
 * CYWOQRM = 1234567789
 * 
 * @link https://vector.cool/php-number-convert-to-excel-column-letters-2
 * 
 * @param string $letters
 * @return mixed
 */
function letters_to_num($a)
{
    $l = strlen($a);
    $n = 0;
    for ($i = 0; $i < $l; $i++)
        $n = $n * 26 + ord($a[$i]) - 0x40;
    return $n;
}

ex:

echo letters_to_num('A');       // 1
echo letters_to_num('B');       // 2
echo letters_to_num('C');       // 3
echo letters_to_num('AA');      // 27
echo letters_to_num('CYWOQRM'); // 1234567789
user_67846620

I'd never use this in production because it's not readable, but for fun... Only does up to ZZ.

<?php
    $col = 55;
    print (($n = (int)(($col - 1) / 26)) ? chr($n + 64) : '') . chr((($col - 1) % 26) + 65);
?>
user_17546320

To anyone looking for a Javascript implementation of this, here is @ircmaxell's answer in Javascript..

function getNameFromNumber(num){
    let numeric = num%26;
    let letter = String.fromCharCode(65+numeric);
    let num2 = parseInt(num/26);
    if(num2 > 0) {
      return getNameFromNumber(num2 - 1)+letter;
    } else {
      return letter;
    }
}
user_71151220

Here's another impl I just wrote:

function excelColumnName($number) {
    $ordA = ord('A');
    $ordZ = ord('Z');
    $len = $ordZ - $ordA + 1;

    $result = '';
    while($number >= 0) {
        $result = chr($number % $len + $ordA) . $result;
        $number = intval($number/$len) - 1;
    }
    return $result;
}

Some test inputs:

>>> excelColumnName(0)
=> "A"
>>> excelColumnName(1)
=> "B"
>>> excelColumnName(25)
=> "Z"
>>> excelColumnName(26)
=> "AA"
>>> excelColumnName(27)
=> "AB"
>>> excelColumnName(28)
=> "AC"
>>> excelColumnName(52)
=> "BA"
>>> excelColumnName(51)
=> "AZ"
Mark_653870
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