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

[Mike Miller]

What I'm using:

R version 3.0.1 (2013-05-16) -- "Good Sport"
Copyright (C) 2013 The R Foundation for Statistical Computing
Platform: x86_64-unknown-linux-gnu (64-bit)


According to some docs, options(digits) controls numerical precision in 
output of write.table().  I'm using the default value for digits:

> getOption("digits")
[1] 7

I have a bunch of numbers in a data frame that are only a few digits to 
the right of the decimal:


> data[c(94,120), c(18,20,21)]
       V18   V20      V21
94  0.008 0.008 0.000064
120 0.023 0.023 0.000529


I write the data to a file:

> write.table(data, file="data.txt", sep="\t", row.names=F, col.names=F)


Then I look at those same values and this is what I see:

$ gawk -F'\t' 'NR==94 || NR==120 {print $18,$20,$21}' data.txt
0.00800000000000001 0.00800000000000001 6.40000000000001e-05
0.0229999999999999 0.0229999999999999 0.000528999999999996


This is the weird thing:  Only those two records get the long, annoyingly 
"precise" 17-digit numbers.  Other records look like this:

0.052 1.052 1.106704
0.178 0.178 0.031684

I understand that binary representations won't reflect decimal 
representations precisely, etc., but why do I get this junk for only two 
records out of 197 records?  Records that contain only integral values 
can't get it wrong, but the other 30 of 32 records with decimals look fine 
-- see below.

Also, if precision should be to 7 digits, why am I getting 17 digits for 
exactly two of the records?  Why does this happen for all three numbers in 
those two records?

If you think this is a bug that I should report elsewhere, let me know.

Thanks.

Mike



$ gawk -F'\t' '{print $18,$20,$21}' data.txt | grep -F .
0.944 0.944 0.891136
0.885 1.885 3.553225
0.052 1.052 1.106704
0.178 0.178 0.031684
1.996 1.996 3.984016
0.86 1.86 3.4596
0.765 1.765 3.115225
0.986 1.986 3.944196
0.998 0.998 0.996004
0.998 0.998 0.996004
0.956 0.956 0.913936
0.99 1.99 3.9601
0.00800000000000001 0.00800000000000001 6.40000000000001e-05
0.99 0.99 0.9801
0.0229999999999999 0.0229999999999999 0.000528999999999996
0.938 0.938 0.879844
0.034 1.034 1.069156
0.86 1.86 3.4596
0.911 1.911 3.651921
0.971 0.971 0.942841
0.994 0.994 0.988036
0.418 0.418 0.174724
0.805 1.805 3.258025
0.996 1.996 3.984016
0.998 1.998 3.992004
0.623 1.623 2.634129
0.998 0.998 0.996004
1.628 1.628 2.650384
0.981 0.981 0.962361
0.998 0.998 0.996004
1.676 1.676 2.808976
0.986 1.986 3.944196