[R] rounding down with as.integer

[Mike Miller]

On Thu, 1 Jan 2015, Duncan Murdoch wrote:

> On 01/01/2015 1:21 PM, Mike Miller wrote:
>
>> I understand that it's all about the problem of representing digital 
>> numbers in binary, but I still find some of the results a little 
>> surprising, like that list of numbers from the table() output.  For 
>> another example:
>>
>>> 1000+3 - 1000*(1+3/1000)
>> [1] 1.136868e-13
>>
>>> 3 - 1000*(0+3/1000)
>> [1] 0
>>
>>> 2000+3 - 1000*(2+3/1000)
>> [1] 0
>>
>> See what I mean?  So there is something special about the numbers 
>> around 1000.
>
> I think it's really that there is something special about the numbers 
> near 1, and you're multiplying that by 1000.
>
> Numbers from 1 to just below 2 are stored as their fractional part, with 
> 52 bit precision.  Some intermediate calculations will store them with 
> 64 bit precision.  52 bits gives about 15 or 16 decimal places.


This is how big those errors are:

> 512*.Machine$double.eps
[1] 1.136868e-13

Under other conditions you also were seeing errors of twice that, or 
1024*.Machine$double.eps.  It might not be a coincidence that the largest 
number giving me an error was 1023.

> 2^-43
[1] 1.136868e-13

> .Machine$double.eps
[1] 2.220446e-16

> 2^-52
[1] 2.220446e-16

I guess the 52 comes from the IEEE floating point spec...

http://en.wikipedia.org/wiki/Double-precision_floating-point_format#IEEE_754_double-precision_binary_floating-point_format:_binary64

...but why are we seeing errors so much bigger than the machine precision? 
Why does it change at 2?

It doesn't really matter to my work, but it is a curious thing, so I would 
be interested to learn about it.

Mike