Hello,
I haven't followed this thread since its start but I think you now have
a case for FAQ 7.31. See inline below.
Em 15-03-2014 19:54, Mike Miller escreveu:
On Sat, 15 Mar 2014, peter dalgaard wrote:
I don't think so. I think some of your numbers differ sufficiently
from numbers with only a few digits to the right of the decimal that
write.table needs to write them with increased precision. You didn't
read them like that, didn't you? You did some calculations, and then
it _looked like_ the results have <= 6 digits after the decimal point?
These have different representations:
1-0.995
2-1.995
write(c(1-0.995, 2-1.995), file="data1.txt", sep="\n")
write.table(c(1-0.995, 2-1.995), file="data2.txt", row.names=F,
col.names=F)
$ head -2 data[12].txt
==> data1.txt <==
0.005
0.005
==> data2.txt <==
0.005
0.00499999999999989
The digits= setting has nothing to do with this, write.table alway
does its damndest to avoid loss of precision. This _is_ in
help(write.table):
In almost all cases the conversion of numeric quantities is
governed by the option ‘"scipen"’ (see ‘options’), but with the
internal equivalent of ‘digits = 15’. For finer control, use
‘format’ to make a character matrix/data frame, and call
‘write.table’ on that.
Yes! This was my mistake: write() not write.table() is controlled by
options(digits=7). Repeating the write() command using a different
number of digits:
options(digits=15)
getOption("digits")
[1] 15
write(c(1-0.995, 2-1.995), file="data1.txt", sep="\n")
$ cat data1.txt
0.005
0.00499999999999989
I don't know why it shows 17 digits and doesn't round to 15, but it is
showing that the numbers are different, for some reason.
Do you understand why there is a difference between 1-0.995 and 2-1.995
in their internal representations?
Try
> (1-0.995) - 0.005
[1] 4.336809e-18
> (2-1.995) - 0.005
[1] -1.066855e-16
Hope this helps,
Rui Barradas
Mike
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