Count with A, B, C, D instead of 0, 1, 2, 3, ... with JavaScript

Question

This is probably an unusual request, but for my script I need a function that increments by letter instead of number. For example:

This is a numeric example:

var i = 0;
while(condition){
 window.write('We are at '+i);
 ++i;
}

Essentially, I want to count with letters, like Microsoft Excel does, instead of numbers. So instead of printing "We are at 0", "We are at 1", "We are at 2", etc., I need to print "We are at A", "We are at B", "We are at C", etc.

To mimic Excel (the only example I can think of), after reaching index 25 (Z), we could move on to 'AA', 'AB', 'AC', etc.

So it would work great like so:

var i = 0;
while(condition){
 window.write('We are at '+toLetter(i));
 ++i;
}

Even better if somebody can write a function that then converts a letter back into a digit, i.e. toNumber('A') = 0 or toNumber('DC') = 107 (I think).

Thanks!

Answer 1

Here's a simple recursive function to convert the numbers to letters.

It's one-based, so 1 is A, 26 is Z, 27 is AA.

function toLetters(num) {
    "use strict";
    var mod = num % 26,
        pow = num / 26 | 0,
        out = mod ? String.fromCharCode(64 + mod) : (--pow, 'Z');
    return pow ? toLetters(pow) + out : out;
}

Here's a matching function to convert the strings back to numbers:

function fromLetters(str) {
    "use strict";
    var out = 0, len = str.length, pos = len;
    while (--pos > -1) {
        out += (str.charCodeAt(pos) - 64) * Math.pow(26, len - 1 - pos);
    }
    return out;
}

A test: http://jsfiddle.net/St6c9/

Answer 2

Something like this you mean?

function num2chars(num, upper){
 num2chars.letters = num2chars.letters || 'abcdefghijklmnopqrstuvwxyz'.split('');
 var ret = repeat(num2chars.letters[num%26],Math.floor(num/26));

 function repeat(chr,n){
  if (n<1) {return chr;}
  return new Array(n+1).join(chr);
 }

 return upper ? ret.toUpperCase() : ret;
}
//usage
while(i<104){
 console.log(num2chars((i+=1),true));
}
//=> A..Z, AA..ZZ, AAA..ZZZ