[R] strangely long floating point with write.table()

[Mike Miller]

Having just learned a few tricks from you guys, this might be the neatest 
way to show the issue:

> write.table(c(1-0.995, 2-1.995), row.names=F, col.names=F)
0.005
0.00499999999999989

options(digits) only works with write(), and not with write.table():

> options(digits=7)
> write(c(1-0.995, 2-1.995), file="/dev/stdout", sep="\n")
0.005
0.005

> options(digits=15)
> write(c(1-0.995, 2-1.995), file="/dev/stdout", sep="\n")
0.005
0.00499999999999989


It seems strangely inconsistent to me:

> options(digits=15)

> write(2-1.995, file="/dev/stdout")
0.00499999999999989

> write(2-1-0.995, file="/dev/stdout")
0.005

> write(2-0.995-1, file="/dev/stdout")
0.00499999999999989


So the order matters, but then why don't either of these give more digits?

> write(2-0.995, file="/dev/stdout")
1.005

> write(0.995-1, file="/dev/stdout")
-0.005

Thus,

2-0.995-1 is long, but
2-0.995   is short, and
   0.995-1 is short.

That doesn't seem right to me.


Now I see the problem better:  If I go to 17 digits, then I return to 
consistency:


> options(digits=15)
> write(1-0.995, file="/dev/stdout")
0.0050000000000000044
> write(2-1.995, file="/dev/stdout")
0.0049999999999998934
> write(2-1-0.995, file="/dev/stdout")
0.0050000000000000044
> write(2-0.995-1, file="/dev/stdout")
0.0049999999999998934
> write(2-0.995, file="/dev/stdout")
1.0049999999999999
> write(0.995-1, file="/dev/stdout")
-0.0050000000000000044

The problem was that some of the numbers round at 15 digits but not at 17 
digits, or something close to that.  I'm sure this has to do with the 
machine precision:

> .Machine$double.eps
[1] 2.2204460492503131e-16

I thing it is an undesirable feature that write.table() has to try to use 
so many digits -- all the way out to eps -- because it causes these kinds 
of inconsistencies.  Is there no way to get it to put out numbers in the 
obviously desirable way without having to write format statements?

Mike