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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.javascript
Subject: Re: Bignumb addition function, suggestion improvements.
Date: Sat, 14 Mar 2015 13:41:30 +0000
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jonas.thornvall@gmail.com writes:

> I just wrote a big numb addition function this morning, it is a bit
> silly and naive, not pure string not pure integer. I think it is
> correct, but i may have missed leading last digit.

Yes, you do miss the leading digit in some cases.  And it's only for
positive numbers.

> Suggestions to write it shorter and more clear, but keeping the naive
> approach appreciated.
>
> <SCRIPT LANGUAGE="Javascript">
>
> function naiveAdd(strOne,strTwo) {
> if (strOne.length>=strTwo.length) {length=strOne.length;} else {length=strTwo.length;}
> oneS = strOne.split("").reverse().join("");
> twoS = strTwo.split("").reverse().join("");

I would avoid this reversing operation.  It's not needed since you can
get the charCodeAt any index value you like.

> out="";
> remainder=0;

Use var.

> for (i=0;i<length;i++){
>          one=oneS.charAt(i);
>          two=twoS.charAt(i);
>          one=parseInt(one);
>          two=parseInt(two);
>          if (isNaN(one)) one = 0;
>          if (isNaN(two)) two = 0;
>          sum=one+two+remainder;
>          if (sum>=10) { sum=sum-10; remainder=1;} else {remainder=0;}
>          out=sum+out;
>     }

There's a bit of duplication here and a bug: you forget that the carry
(the usual name for your 'remainder') can be set at the end of the loop.

> document.write("Sum = ",out,"<BR>");
> }
>
> strOne="9003";
> strTwo="13039";
> naiveAdd(strOne,strTwo);
>
> </SCRIPT>

String.prototype.digitValueAt = function (pos) {
    var d = this.charCodeAt(pos);
    return isNaN(d) ? 0 : d - 48;
}

function naiveAddX(strOne, strTwo) {
    var i1 = strOne.length, i2 = strTwo.length, result = "", carry = 0;
    while (i1 > 0 || i2 > 0 || carry > 0) {
        var dsum = strOne.digitValueAt(--i1) + strTwo.digitValueAt(--i2) + carry;
        if (dsum >= 10) {
            dsum -= 10;
            carry = 1;
        }
        else carry = 0;
        result = dsum + result;
    }
    return result;
}

You could also build a result array and create the string on one go.
That's probably faster.

-- 
Ben.