Simple subtraction bug in Javascript

Question

I was shocked to see such a basic subtraction bug in JS, I am sure most of you here would have experienced this issue, please help me with a work around here, All I am doing is subtracting a number from 100 which gives unexpected results, an example is stated below

100 - 99.1
// returns 0.9000000000000057

Am I doing something wrong here ? :S I am confused

You need to understand how floating point number are stored: http://floating-point-gui.de/ — codaddict, Mar 01 '11 at 10:14
@Tomalak, I am sorry to be a reason for that shock, I guess I dont comply to your standards of a programmer. I hope I will be on those standards soon :) — Umair Jabbar, Mar 01 '11 at 11:05

score 16 · Accepted Answer · answered Mar 01 '11 at 10:16

16

Floating point values are never accurate as you expect. You can use Number object to convert this to answer as you need.

Number(100).toFixed(2) - Number(99.1).toFixed(2)

answered Mar 01 '11 at 10:16

Sachin Shanbhag

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`Number(100 - 99.1).toFixed(2)` is terser, and you're not going to have increased error as 100 is already perfectly representable in base-2. – Lightness Races in Orbit Mar 01 '11 at 10:24
toFixed works for me, will start studying floating points now :) thanks you everyone – Umair Jabbar Mar 01 '11 at 10:58
1

"Floating point values are never accurate as you expect" - rather depends on one's expectation, doesn't it :) – AakashM Mar 01 '11 at 11:12
1

Try this: Number(100000).toFixed(2) - Number(99111.57).toFixed(2) – jonybekov Dec 08 '20 at 16:39

score 9 · Answer 2 · answered Mar 01 '11 at 10:58

9

You are working with floating point numbers, not integers. This is expected.

The reason is that you can't accurately represent numbers like 0.1 and 0.3 in binary. Just like you can't represent 1/3 accruately in decimal form.

answered Mar 01 '11 at 10:58

jrharshath

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Jacob Relkin · Answer 3 · 2011-03-01T10:20:26.737

In JavaScript, all of the numeric types are Number objects, which are represented as double-precision floating point numbers.

On another note, you can retrieve a string representation of these numbers to a specific decimal point by using Number's toFixed method, which accepts a number denoting how many decimal points you would like the returned string to represent based on the receiver.

An example:

var five = 5.00001;
console.log(five.toFixed(1)); //5.0

score 2 · Answer 4 · answered Mar 01 '11 at 10:15

2

This is how floating point numbers work, they are not exact. http://en.wikipedia.org/wiki/Floating_point might be useful reading.

answered Mar 01 '11 at 10:15

Tollef Fog Heen

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They *are* exact. But perhaps not exactly what might be expected. – AakashM Mar 01 '11 at 11:27

score 1 · Answer 5 · answered Sep 11 '12 at 17:42

At first prevent yourself from assigning the float value to variable then do it with direct expression to a variable. here is an example:

**var c = 80.35-20.35;
alert(c.toFixed(2));**

this will assign a 60.00 but other way to solve this will return 59.99999999.

score -1 · Answer 6 · answered Feb 17 '14 at 11:21

This is not a perfect answer but it works.

function format_float_bug(num)
{
   return parseFloat( num.toFixed(15) ); 
}

you can use as follows,

format_float_bug(4990 % 10);

because below number (49.89999999999999857891452848) first 15 decimal places are like 9999999

Simple subtraction bug in Javascript

6 Answers6

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