Javascript floating point number confusion

Question

I came across an operation with some confusion.

var a = 0.1;
var b = 0.2;
var c = 0.3;

console.log(a); // 0.1
console.log(b); // 0.2
console.log(c); // 0.3

But,

consolo.log(a+b+c) // 0.6000000000000001.

While

console.log(a+(b+c)) // 0.6

I understand that Javascript use binary floating point, thus can't accurate represent 0.1, 0.2, 0.3 But what does the bracket around (b+c). Is there any conversion or round up here?

Many thanks,

Answer 1

How a JavaScript Number is Defined

JavaScript number is represented in IEEE754 which is double precision binary floating point (binary64), it is in scientific notation and using 2 as base. There are 64 bits in a number, and they are split into 3 parts (from high to low bits):

• The first bit is for sign: 0 - positive; 1 - negative
• The next 11 bits are exponent part
• The last 52 bits are for mantissa / fraction

So, a float number is calculated as: (-1) ^ sign * (2 ^ exponent) * significand

Note: as the exponent part of a scientific notation could be either positive or negative, the actual exponent value for a binary64 number should be calculated by subtracting exponent bias (which is the middle value 1023) from the 11 bit exponent value.

The standard also defines the significand value to be between [1, 2). As the first number of significand part is always 1, so it is implied and not presented in the above figure. So, basically the significand part has 53 bits precision actually, and the red part in above figure is just the mantissa or fraction part.

0.1, 0.2 and 0.3 in binary64 Format

Based on the standard, it's not hard to find 0.1, 0.2 and 0.3 in binary64 format (you can calculate either manually or by this tool http://bartaz.github.io/ieee754-visualization/):

0.1

0 01111111011 1001100110011001100110011001100110011001100110011010

and in scientific notation, it is

1.1001100110011001100110011001100110011001100110011010 * 2e-4

Note: the significand part is in binary format, and the following numbers are in same format

0.2

0  01111111100 1001100110011001100110011001100110011001100110011010

and in scientific notation, it is

1.1001100110011001100110011001100110011001100110011010 * 2e-3

0.3

0 01111111101 0011001100110011001100110011001100110011001100110011

and in scientific notation, it is

1.0011001100110011001100110011001100110011001100110011 * 2e-2

Steps to Add Up 2 binary64 Numbers

Step 1 - Align the exponents

• Shift the significand for the number which has smaller exponent
• Shift the significand right
• Increase the exponent by 1 for every significand shift until both exponents are same
• After shift, the significand should be round up.

Step 2 - Add up the significand

• if the added up significand is not satisfying [1,2) requirement, shift it into that range and change the exponent

• After shift, the significand should be round up.

0.1 + 0.2 + 0.3 == 0.6000000000000001

As above explained, 0.1 has exponent -4 and 0.2 has exponent -3, so need to do exponent alignment first:

Shift 0.1 from

1.1001100110011001100110011001100110011001100110011010 * 2e-4

to

0.1100110011001100110011001100110011001100110011001101 * 2e-3

Then add the significand

0.1100110011001100110011001100110011001100110011001101

with

1.1001100110011001100110011001100110011001100110011010

we get added up significand value:

10.0110011001100110011001100110011001100110011001100111

But it is not in range [1,2) so need right shift it (with round up) to:

1.0011001100110011001100110011001100110011001100110100 (* 2e-2)

then add it to

0.3 (1.0011001100110011001100110011001100110011001100110011 * 2e-2) 

we get:

10.0110011001100110011001100110011001100110011001100111 * 2e-2

Again, we need shift and round up it, and finally get the value:

1.0011001100110011001100110011001100110011001100110100 * 2e-1

it is exactly the value of 0.6000000000000001 (decimal)

With same workflow, you get calculate 0.1 + (0.2 + 0.3)

Tools

This web page http://bartaz.github.io/ieee754-visualization/ helps you quickly convert a decimal number to binary64 format, you can use it to verify the calculation steps.

If you are processing a single precision binary float number, you would refer to this tool: http://www.h-schmidt.net/FloatConverter/IEEE754.html

Answer 2

The general problem is described in Is floating point math broken?.

In the remainder I will just look at the difference between the two computations.

From my comment:

Well, in the first case you are doing (0.1 + 0.2) + 0.3 = 0.3 + 0.3 and in the second case you do 0.1 + (0.2 + 0.3) = 0.1 + 0.5. I guess the rounding error in the first case larger than in the second case.

Lets have a closer look at the actual values in this computation:

var a = 0.1;
var b = 0.2;
var c = 0.3;

console.log('          a:', a.toPrecision(21));
console.log('          b:', b.toPrecision(21));
console.log('          c:', c.toPrecision(21));

console.log('      a + b:', (a + b).toPrecision(21));
console.log('      b + c:', (b + c).toPrecision(21));

console.log('  a + b + c:', (a + b + c).toPrecision(21));
console.log('a + (b + c):', (a + (b + c)).toPrecision(21));

The output is

          a: 0.100000000000000005551
          b: 0.200000000000000011102
          c: 0.299999999999999988898

      a + b: 0.300000000000000044409
      b + c: 0.500000000000000000000

  a + b + c: 0.600000000000000088818
a + (b + c): 0.599999999999999977796 

So, it's clear that both computations have rounding errors, but the errors are different because you are performing the additions in a different order. It just happens that a + b + c produces a larger error.

---

The console seems to round the number to the 16th decimal:

> (a + b + c).toPrecision(16)
"0.6000000000000001"
> (a + (b + c)).toPrecision(16)
"0.6000000000000000"

That's why the second computation will simply output 0.6. If the console would round to the 17th decimal, things would look different:

> (a + b + c).toPrecision(17)
"0.60000000000000009"
> (a + (b + c)).toPrecision(17)
"0.59999999999999998"

Answer 3

That's not problem of JavaScript, you would get similar surprises in other languages too.

Please read this: What Every Programmer Should Know About Floating-Point Arithmetic