I have a large amount of numeric values y in javascript. I want to group them by rounding them down to the nearest multiple of x and convert the result to a string.

How do I get around the annoying floating point precision?

For example:

0.2 + 0.4 = 0.6000000000000001

Two things I have tried:

>>> y = 1.23456789 >>> x = 0.2 >>> parseInt(Math.round(Math.floor(y/x))) * x; 1.2000000000000002

and:

>>> y = 1.23456789 >>> x = 0.2 >>> y - (y % x) 1.2000000000000002
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5 Answers

From this post: How to deal with floating point number precision in JavaScript?

You have a few options:

  • Use a special datatype for decimals, like decimal.js
  • Format your result to some fixed number of significant digits, like this: (Math.floor(y/x) * x).toFixed(2)
  • Convert all your numbers to integers
5

You could do something like this:

> +(Math.floor(y/x)*x).toFixed(15); 1.2

Edit: It would be better to use big.js.

big.js

A small, fast, easy-to-use library for arbitrary-precision decimal arithmetic.

>> bigX = new Big(x) >> bigY = new Big(y) >> bigY.div(bigX).round().times(bigX).toNumber() // => 1.2
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> var x = 0.1 > var y = 0.2 > var cf = 10 > x * y 0.020000000000000004 > (x * cf) * (y * cf) / (cf * cf) 0.02

Quick solution:

var _cf = (function() { function _shift(x) { var parts = x.toString().split('.'); return (parts.length < 2) ? 1 : Math.pow(10, parts[1].length); } return function() { return Array.prototype.reduce.call(arguments, function (prev, next) { return prev === undefined || next === undefined ? undefined : Math.max(prev, _shift (next)); }, -Infinity); }; })(); Math.a = function () { var f = _cf.apply(null, arguments); if(f === undefined) return undefined; function cb(x, y, i, o) { return x + f * y; } return Array.prototype.reduce.call(arguments, cb, 0) / f; }; Math.s = function (l,r) { var f = _cf(l,r); return (l * f - r * f) / f; }; Math.m = function () { var f = _cf.apply(null, arguments); function cb(x, y, i, o) { return (x*f) * (y*f) / (f * f); } return Array.prototype.reduce.call(arguments, cb, 1); }; Math.d = function (l,r) { var f = _cf(l,r); return (l * f) / (r * f); }; > Math.m(0.1, 0.2) 0.02

You can check the full explanation here.

2

Check out this link.. It helped me a lot.

The toPrecision(no_of_digits_required) function returns a string so don't forget to use the parseFloat() function to convert to decimal point of required precision.

3

Tackling this task, I'd first find the number of decimal places in x, then round y accordingly. I'd use:

y.toFixed(x.toString().split(".")[1].length);

It should convert x to a string, split it over the decimal point, find the length of the right part, and then y.toFixed(length) should round y based on that length.

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