JavaScript multiplication arithmetic

Question

I tried this expression in both my FireFox and Chrome console:

17.99 * 100;

Expected Result: 1799

Actual Result: 1798.9999999999998

I also tried:

parseInt(17.99 * 100);

Expected Result: 1799

Actual Result: 1798

Why is this happening, and how do I get the expected result?

score 1 · Accepted Answer · answered Feb 21 '14 at 17:03

Floating point arithmetic isn't an exact science. The reason is that in memory, your precision is stored in binary (all powers of two). If it can't be represented by an exact power of two you can get some lost precision.

Your number, 1798.9999999999998 had enough lost precision that it didn't round up in the multiplication.

http://en.wikipedia.org/wiki/IEEE_floating_point

Galileo_Galilei · Answer 2 · 2014-02-21T17:20:35.113

0

Try this:

Math.round(17.99*100)

As the previous answer explained, multiplying a float number is not exact science; what you can do is to expect a result in a certain precision range. Take a look at Number.prototype.toPrecision().

(17.99*100).toPrecision(4)

edited Feb 21 '14 at 17:20

answered Feb 21 '14 at 17:05

Galileo_Galilei

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wont work for all floating point operations – 999k Feb 21 '14 at 17:07
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@555k you should give an example as to when and why this wont work. – Drew Dahlman Feb 21 '14 at 17:09
@DrewDahlman Try Math.round(17.656*100) – 999k Feb 21 '14 at 17:13
@555k in Firefox scratchpad Math.round(17.656*100) gave 1766 which is correct – jing3142 Feb 21 '14 at 17:22
Yeah this works correctly in chrome as well @555k – Drew Dahlman Feb 21 '14 at 17:43
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Try Math.round(17.999*100). 1800 is not correct. 1799.9 would be correct – Clint Powell Feb 21 '14 at 18:13

JavaScript multiplication arithmetic

2 Answers2