# Re: [R] strange digit switching

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Tue 20 Jun 2006 - 04:21:29 EST

On Mon, 19 Jun 2006, Hans-Peter wrote:

> Hi,
>
> I execute the following code snippet.What I don't understand is, that in the
> first two cases the numbers shown don't correnspondant to the input (though
> I have set the options to show 22 digits). When I assign the third number
> which has one place, the display of the numbers is suddenly correct.
>
> Can anybody explain what happens here?

The problem is that you have set the options to show 22 digits, but the numbers are stored to fewer than 16 digits
> .Machine\$double.eps

[1] 2.220446049250313080847e-16

Everything displayed after the 16th digit is effectively random (the actual values probably depend on exactly when the numbers are loaded into the 80-bit floating point registers on the CPU).

The actual results will vary from system to system (on my Linux system the output of x is the same at all stages, and is different from what you report), but nothing beyond the sixteenth digit is at all meaningful.

-thomas

> (this is just a sample, my real problem lies in numbers that I import from a
> datafile and I don't know how to control them when this switching occurs)
>
> ###########################
> options( digits=22 )
> x <- matrix(NA, 3,1)
> x[1] <- 6.94788635771657950
> x # 6.947886357716580 # DIFFERENT ???
> # NA
> # NA
> x[1] == 6.94788635771657950 # but is the same
>
> x[2] <- 8.34284937565404050
> x # 6.947886357716580 # DIFFERENT ???
> # 8.342849375654041 # DIFFERENT ???
> # NA
> x[1] == 6.94788635771657950 # but is the same
>
> x[3] <- 0.19043842368566377
> x # 6.94788635771657950 # NOW OK!!!
> # 8.34284937565404050 # " "
> # 0.19043842368566377 # " "
> x[1] == 6.94788635771657950 # but is the same
>
>
> ###
> platform i386-pc-mingw32
> arch i386
> os mingw32
> system i386, mingw32
> status
> major 2
> minor 3.1
> year 2006
> month 06
> day 01
> svn rev 38247
> language R
> version.string Version 2.3.1 (2006-06-01)
>
>
> --
> Regards,
> Hans-Peter
>
> [[alternative HTML version deleted]]
>
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