Javascript rounding issue when summing values

Question

Possible Duplicate:
Is JavaScript's Math broken?

The user enters values in the first two text boxes and, as they type, Javascript (sorry, no jQuery, I'm not up to it yet) is used to calculate the precise sum and the sum rounded to 2 digits.

Why am I getting rounding error and what can I do to correct it?

Many thanks.

Hmmm....ParseFloat? Wrong data type?

What I would like to see if the precise answer as if it were added on a calculator. Is there a parseDecimal or other data type that I can use?

![enter image description here][1]

    function SumValues() {
        //debugger;
        var txtSubsContrRbtAmt = document.getElementById("<%=txtSubsContrRbtAmt.ClientID%>");
        var txtDeMinAmt = document.getElementById("<%=txtDeMinAmt.ClientID%>");
        var txtTotRbtAmt = document.getElementById("<%=txtTotRbtAmt.ClientID%>");
        var txtRndRbtAmt = document.getElementById("<%=txtRndRbtAmt.ClientID%>");

        var total = Add(txtSubsContrRbtAmt.value, txtDeMinAmt.value);

        txtTotRbtAmt.value = total;
        txtRndRbtAmt.value = RoundToTwoDecimalPlaces(total);
    }

    function Add() {
        var sum = 0;
        for (var i = 0, j = arguments.length; i < j; i++) {
            var currentValue;
            if (isNumber(arguments[i])) {
                currentValue = parseFloat(arguments[i]);
            }
            else {
                currentValue = 0;
            }

            sum += currentValue;
        }
        return sum;
    }

    function RoundToTwoDecimalPlaces(input) {

        return Math.round(input * 100) / 100
    }

    function IsNumeric(input) {
        return (input - 0) == input && input.length > 0;
    }

    function isNumber(n) {
        return !isNaN(parseFloat(n)) && isFinite(n);
    }

  [1]: https://i.stack.imgur.com/5Otrm.png

Update. I am evaluating something like this:

function AddWithPrecision(a, b, precision) {
        var x = Math.pow(10, precision || 2);
        return (Math.round(a * x) + Math.round(b * x)) / x;
    }

Answer 1

There is a golden rule for anyone writing software in the financial sector (or any software dealing with money): never use floats. Therefore most software dealing with money use only integers and represent decimal numbers as a data structure.

Here's one way of doing it:

(Note: this function adds two strings that looks like numbers)

(Additional note: No error checking is done to aid clarity. Also does not handle negative numbers)

function addNumberStrings (a,b) {
    a = a.split('.');
    b = b.split('.');
    var a_decimal = a[1] || '0';
    var b_decimal = b[1] || '0';
    diff = a_decimal.length - b_decimal.length;
    while (diff > 0) {
        b_decimal += '0';
        diff --;
    }
    while (diff < 0) {
        a_decimal += '0';
        diff ++;
    }
    var decimal_position = a_decimal.length;

    a = a[0] + a_decimal;
    b = b[0] + b_decimal;

    var result = (parseInt(a,10)+parseInt(b,10)) + '';

    if (result.length < decimal_position) {
        for (var x=result.length;x<decimal_position;x++) {
            result = '0'+result;
        }
        result = '0.'+result
    }
    else {
        p = result.length-decimal_position;
        result = result.substring(0,p)+'.'+result.substring(p);
    }
    return result;
}

*note: code is simplified, additional features left out as homework.

Answer 2

To fix your addition the way you want, I'd suggest counting the decimal places in each number somehow This method, for instance Then passing the max value to toFixed, and trimming any leftover zeroes.

function AddTwo(n1, n2) {
    var n3 = parseFloat(n1) + parseFloat(n2);
    var count1 = Decimals(n1, '.');
    var count2 = Decimals(n2, '.');
    var decimals = Math.max(count1, count2);
    var result = n3.toFixed(decimals)
    var resultDecimals = Decimals(result, '.');
    if (resultDecimals > 0) {
        return result.replace(/\.?0*$/,'');
    }
    else {
        return result;
    }
}

// Included for reference - I didn't write this
function Decimals(x, dec_sep)
{
    var tmp=new String();
    tmp=x;
    if (tmp.indexOf(dec_sep)>-1)
        return tmp.length-tmp.indexOf(dec_sep)-1;
    else
        return 0;
} 

Here's a JSFiddle of that