[R] 0.1 + 0.2 != 0.3 revisited

[Christian Hoffmann]

Hi,

IEEE says that real numbers are normalized (a few below 10^(-16) may be 
not [gradual underflow]), so that they look like 0.1ddd2^ex. Then only 
ddd and ex are kept:
0.1 = 0.00011.. 2^0 = 0.11001100.. 2^(-3) -> (11001100.., -3)
0.2 = 0.0011..  2^0 = 0.11001100.. 2^(-2) -> (11001100.., -2)
0.3 = 0.010011..2^0 = 0.10011001.. 2^(-1) -> (10011001.., -1)

Duncan Murdoch wrote:

> On Fri, 6 Feb 2004 12:55:05 -0000, "Simon Fear"
> <Simon.Fear at synequanon.com> wrote :
> 
> 
>>Prompted by Peter Dalgard's recent elegant "intbin" function,
>>I have been playing with the extension to converting reals to binary
>>representation. The decimal part can be done like this:
>>
>>decbase <- function(x, n=52, base=2) {
>> if(n) {
>>   x <- x*base
>>   paste(trunc(x), decbase(x%%1, n-1, base), sep="")
>> }
>>}
>>
>>n=52 default because that's the number of bits in the significand of
>>a 64-bit float.
> 
> 
> Remember that IEEE double formats are complicated, they're not fixed
> point formats.
> 
> Both 0.1 and 0.2 are less than 1, so the n=52 count is wrong.  I think
> 0.1 would be stored as (1 + 0.6)*2^(-4) and 0.2 would be stored as (1
> + 0.6)*2^(-3), whereas 0.3 would be stored as (1 + 0.2)*2^(-2).  You
> should expect 56 decimal (binary?) place accuracy on 0.1, 55 place
> accuracy on 0.2, and 54 place accuracy on 0.3.  It's not surprising
> weird things happen!

I don *not* think so: all mantissas here have *52 binary* places!

> Duncan Murdoch
Christian Hoffmann
-- 
Dr.sc.math.Christian W. Hoffmann, 
http://www.wsl.ch/staff/christian.hoffmann
Mathematics + Statistical Computing   e-mail: hoffmacw at wsl.ch
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