-= 0.1 strange result

Question

Possible Duplicate:
Is double Multiplication Broken in .NET?

JavaScript code:

var n = 1; 
while ( n > 0 )
{
  n -= 0.1;
  document.body.innerHTML += n + "<br/>";
}

http://jsfiddle.net/UpUE6/4/

I expected:

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

But got this:

0.9
0.8
0.7000000000000001
0.6000000000000001
0.5000000000000001
0.40000000000000013
0.30000000000000016
0.20000000000000015
0.10000000000000014
1.3877787807814457e-16
-0.09999999999999987

Could anyone explain what's going on there?

Dupe of so many dupes [Strange loop on Java for 0.1 to 2](http://stackoverflow.com/questions/5400565/) or [Is double multiplication broken in .NET?](http://stackoverflow.com/questions/1420752/). See the [gory details](http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html). — R. Martinho Fernandes, Apr 13 '11 at 14:41

score 11 · Answer 1 · answered Apr 13 '11 at 14:53

From The Floating-Point Guide:

Why don’t my numbers, like 0.1 + 0.2 add up to a nice round 0.3, and instead I get a weird result like 0.30000000000000004?

Because internally, computers use a format (binary floating-point) that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.

When the code is compiled or interpreted, your “0.1” is already rounded to the nearest number in that format, which results in a small rounding error even before the calculation happens.

+1 This should have been the accepted answer (or alternatively mine). — Oswald, Apr 13 '11 at 14:58

score 7 · Accepted Answer · answered Apr 13 '11 at 14:40

7

You're seeing an artifact of the way computers do floating-point arithmetic. See, for example, http://en.wikipedia.org/wiki/Floating_point#Machine_precision

answered Apr 13 '11 at 14:40

Jonathan

7,536
4
30
44

score 6 · Answer 3 · answered Apr 13 '11 at 14:46

6

This will solve your problem

http://jsfiddle.net/AVPNx/

var n = 1;
while ( n > 0 ){
    n -= 0.1;
    result = n.toFixed(1);
    document.body.innerHTML += result + "<br/>";
}

answered Apr 13 '11 at 14:46

Vishwanath Dalvi

35,388
41
123
155

score 3 · Answer 4 · answered Apr 13 '11 at 14:41

3

You're dealing with a floating point number. Check out the toFixed and toPrecision methods.

answered Apr 13 '11 at 14:41

JamesB41

703
10
20

Oswald · Answer 5 · 2011-04-13T14:52:21.123

1

Not all numbers can be represented exactly, even if they have a simple representation in decimal notation. This is because JavaScript uses IEEE 754 to represent floating point values, and thus uses base 2 instead of base 10. This leads to you not actually subtracting 0.1 (even though your source code says so), but some value close to it.

edited Apr 13 '11 at 14:52

answered Apr 13 '11 at 14:41

Oswald

31,254
3
43
68

-= 0.1 strange result

5 Answers5