PHP：使用递增的字符变量进行计数时的比较

Question

I am doing an Excel-like webapp where the columns are "numbered" like a,b,c,...,aa,ab,...,az,ba,...,zz,aaa,.... But if I write

$start = 'a';
$end = 'z';

while($start <= $end){
    echo $start++ . ", ";
}

(notice $start <= $end) it will go a up to yz, not just a-z because

echo 'aa' <= 'z'; // true, but
echo 'za' <= 'z'; // false.

Is there a function or a way to compare two variables taking in mind that

$a = "z"; echo ++$a; // aa
$a = "zz"; echo ++$a; // aaa

so that

$a = "z"; $b = $a; $b++; // $b = aa

AisLessThanB(a,b); // returns true

and so on? That means where

a < z < aa < az < zz < aaa < zzz < aaaa

is always true?

EDIT: Something like

$start = 'a';
$end = 'cv'; // 100 columns

should work since we work with loads of columns.

Thank you!

Answer 1

Try this:

// From https://stackoverflow.com/a/3580935/3088508
function getLetterIndexInAlphabet ($letter) {
    return ord($letter) - 96;
}

function convertStringToNumber ($inStr) {
    $letters = array_reverse(str_split($inStr));
    $outNum = 0;
    for ($i = count($letters) - 1; $i >= 0; $i--) {
        $outNum += getLetterIndexInAlphabet($letters[$i]) * (pow(26, $i));
    }
    return $outNum;
}

function AisLessThanB ($a, $b) {
    $aInt = convertStringToNumber($a);
    $bInt = convertStringToNumber($b);
    return ($aInt < $bInt);
}

convertStringToNumber is the most complicated function in here, so let's have a look at how it works on a few values:

• 'c' evaluates to 2 which is calculated by:
  1. 'c' = 3 (index in alphabet)
• 'ba' evaluates to 53 which is calculated by:
  1. 'b' = 52 = (2 (index in alphabet) x (26 (base value) ^ 1 (column number))).
  2. 'a' = 1 (index in alphabet)
• 'bca' evaluates to 1431 which is calculated by:
  1. 'b' = 1352 = (2 (index in alphabet) x (26 (base value) ^ 2 (column number))).
  2. 'c' = 78 = (3 (index in alphabet) x (26 (base value) ^ 1 (column number))).
  3. 'a' = 1 (index in alphabet)
• 'bdca' evaluates to 37935 which is calculated by:
  1. 'b' = 35152 = (2 (index in alphabet) x (26 (base value) ^ 3 (column number))).
  2. 'd' = 2704 = (4 (index in alphabet) x (26 (base value) ^ 2 (column number))).
  3. 'c' = 78 = (3 (index in alphabet) x (26 (base value) ^ 1 (column number))).
  4. 'a' = 1 (index in alphabet)

Here's some test cases:

echo "   a < z    = " . ((AisLessThanB(   'a',    'z')) ? 'true' : 'false') . "
";
echo "   z < aa   = " . ((AisLessThanB(   'z',   'aa')) ? 'true' : 'false') . "
";
echo "  aa < az   = " . ((AisLessThanB(  'aa',   'az')) ? 'true' : 'false') . "
";
echo "  az < zz   = " . ((AisLessThanB(  'az',   'zz')) ? 'true' : 'false') . "
";
echo "  zz < aaa  = " . ((AisLessThanB(  'zz',  'aaa')) ? 'true' : 'false') . "
";
echo " aaa < zzz  = " . ((AisLessThanB( 'aaa',  'zzz')) ? 'true' : 'false') . "
";
echo " zzz < aaaa = " . ((AisLessThanB( 'zzz', 'aaaa')) ? 'true' : 'false') . "
";
echo "
===================

";
echo "   z < a    = " . ((AisLessThanB(   'z',    'a')) ? 'true' : 'false') . "
";
echo "  aa < z    = " . ((AisLessThanB(  'aa',    'z')) ? 'true' : 'false') . "
";
echo "  az < aa   = " . ((AisLessThanB(  'az',   'aa')) ? 'true' : 'false') . "
";
echo "  zz < az   = " . ((AisLessThanB(  'zz',   'az')) ? 'true' : 'false') . "
";
echo " aaa < zz   = " . ((AisLessThanB( 'aaa',   'zz')) ? 'true' : 'false') . "
";
echo " zzz < aaa  = " . ((AisLessThanB( 'zzz',  'aaa')) ? 'true' : 'false') . "
";
echo "aaaa < zzz  = " . ((AisLessThanB('aaaa',  'zzz')) ? 'true' : 'false') . "
";

And they output:

   a < z    = true
   z < aa   = true
  aa < az   = true
  az < zz   = true
  zz < aaa  = true
 aaa < zzz  = true
 zzz < aaaa = true

===================

   z < a    = false
  aa < z    = false
  az < aa   = false
  zz < az   = false
 aaa < zz   = false
 zzz < aaa  = false
aaaa < zzz  = false

eval.in demo

Thanks for asking such an interesting question!