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?
Mike
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