[R] Bug/Error in formatC? (Was: Why doesn't formatC( x, digits=2, format= "g")...)

Martin Maechler maechler at stat.math.ethz.ch
Tue Jun 10 15:05:05 CEST 2008

>>>>> "PD" == Peter Dunn <dunn at usq.edu.au>
>>>>>     on Tue, 10 Jun 2008 09:25:07 +1000 writes:

    PD> Hi all After posting what follows, Duncan Murdoch
    PD> suggested perhaps a bug in formatC, or an error on
    PD> documentation.  Any comments?

It could be called a bug in your platform's
implementation of the C-library internal sprintf() 
{to which R's sprintf() is an interface}.
Things work okay for me on three different Linux platforms and
on Solaris SPARC.

More details :

   formatC(x, digits=2, format="fg", flag = "#")

ends up calling

   sprintf("%#<n>.2g", x)

where the <n> is carefully determined from x,
in your example

  sprintf("%#3.2g", 0.005999)

For me, this correctly gives

> sprintf("%#3.2g", 0.005999)
[1] "0.0060"

and I assume that for your platform, it wrongly returns
"0.006" instead.

Can you confirm? 
What are the exact platform details?

Martin Maechler, ETH Zurich

    PD> In particular, bug, error or not, any ideas about how I
    PD> can consistently get two significant figures to print?

    PD> P.

    PD> ---------- Original Message ----------

    PD> Hi all

    PD> I am not a C programmer, but I am trying to understand
    PD> formatC to get consistent printing of reals to a given
    PD> number of significant digits.

    PD> Can someone please explain this to me?  These first
    PD> three give what I expect on reading ?formatC:

    >> formatC(0.0059999, digits=2,format="fg",flag="#")
    PD> [1] "0.0060"
    >> formatC(0.59999, digits=2,format="fg",flag="#")
    PD> [1] "0.60"
    >> formatC(5.9999, digits=2,format="fg",flag="#")
    PD> [1] "6.0"

    PD> This seems consistent with what I read (but perhaps do
    PD> not understand) in ?formatC, where I read this:

    PD>         digits the desired number of digits after the
    PD> decimal point (format = "f") or significant digits
    PD> (format = "g", = "e" or = "fg").

    PD> Since I am using format="fg" and digits=2, so I am
    PD> expecting two significant digits to always show, which I
    PD> have above. So I fail to understand this:

    >> formatC(0.000059999, digits=2,format="fg",flag="#")
    PD> [1] "0.00006"
    >> formatC(0.000059, digits=2, format="fg",flag="#")
    PD> [1] "0.000059"

    PD> I was expecting both of these to produce "0.000059".
    PD> But in the first case above, I get one significant digit
    PD> only.

    PD> I'm obviously misunderstanding something; can someone
    PD> enlighten me?  (No doubt, someone will point out a
    PD> nuance of the help files I didn't understand!)

    PD> Also, since the above obviously doesn't do what I hoped
    PD> (consistently printing two sig figs), could someone also
    PD> explain how I can consistently get two significant
    PD> figures in situation like above?

    PD> Thanks as always.

    PD> P.

    PD> -- Dr Peter Dunn | dunn <at> usq.edu.au Faculty of
    PD> Sciences, USQ; http://www.sci.usq.edu.au/staff/dunn
    PD> Aust. Centre for Sustainable Catchments:
    PD> www.usq.edu.au/acsc

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