R help - Jun 2008 - Why doesn't formatC( x, digits=2, format="g") doesn't always give 2 sig figs?

Hi all

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

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

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


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


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


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

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


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

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

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

Thanks as always.

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

This email (including any attached files) is confidentia...{{dropped:15}}

On 06-Jun-08 00:48:50, Peter Dunn wrote:
> Hi all
> I am not a C programmer, but I am trying to understand  formatC  to 
> get consistent printing of reals to a given number of significant 
> digits.
> Can someone please explain this to me?  These first three give what 
> I expect on reading ?formatC:
> 
>> formatC(0.0059999, digits=2,format="fg",flag="#")
> [1] "0.0060"
>> formatC(0.59999, digits=2,format="fg",flag="#")
> [1] "0.60"
>> formatC(5.9999, digits=2,format="fg",flag="#")
> [1] "6.0"
> 
> This seems consistent with what I read (but perhaps do not 
> understand) in ?formatC, where I read this:
> 
>       digits   the desired number of digits after the decimal point
>       (format = "f") or significant digits (format =
"g", = "e" or
>        = "fg"). 
> 
> Since I am using  format="fg" and digits=2, so I am expecting two
> significant digits to always show, which I have above. So I fail to 
> understand this:
> 
>> formatC(0.000059999, digits=2,format="fg",flag="#")
> [1] "0.00006"
>> formatC(0.000059, digits=2, format="fg",flag="#")
> [1] "0.000059"
> 
> I was expecting both of these to produce "0.000059".  But in the 
> first case above, I get one significant digit only.  
> 
> I'm obviously misunderstanding something; can someone enlighten me?  
> (No doubt, someone will point out a nuance of the help files I 
> didn't understand!) 
I think that what is happening is that 0.000059999 rounds (to 2
significant digits) to 0.000060 in the first instance, and the
"0", not being "significant" (it adds no information to
0.00006),
is dropped.
> Also, since the above obviously doesn't do what I hoped 
> (consistently printing two sig figs), could someone also explain 
> how I can consistently get two significant figures in situation 
> like above?
I don't know! I thought using the "width" parameter might do it,
but it does not affect the number of decimal places printed.
> Thanks as always.
> P.
Sorry I coujldn't be more helpful!
Ted.

--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 06-Jun-08                                       Time: 02:18:00
------------------------------ XFMail ------------------------------

On 05/06/2008 8:48 PM, Peter Dunn wrote:
> Hi all
> 
> I am not a C programmer, but I am trying to understand  formatC  to 
> get consistent printing of reals to a given number of significant 
> digits.
> 
> Can someone please explain this to me?  These first three give what 
> I expect on reading ?formatC:
> 
> 
>> formatC(0.0059999, digits=2,format="fg",flag="#")
> [1] "0.0060"
>> formatC(0.59999, digits=2,format="fg",flag="#")
> [1] "0.60"
>> formatC(5.9999, digits=2,format="fg",flag="#")
> [1] "6.0"
> 
> 
> This seems consistent with what I read (but perhaps do not 
> understand) in ?formatC, where I read this:
> 
> 
> 	digits   the desired number of digits after the decimal point
> 	(format = "f") or significant digits (format = "g", =
"e" or
> 	 = "fg"). 
> 
> 
> Since I am using  format="fg" and digits=2, so I am expecting two
> significant digits to always show, which I have above. So I fail to 
> understand this:
> 
> 
>> formatC(0.000059999, digits=2,format="fg",flag="#")
> [1] "0.00006"
>> formatC(0.000059, digits=2, format="fg",flag="#")
> [1] "0.000059"
> 
> 
> I was expecting both of these to produce "0.000059".  But in the 
> first case above, I get one significant digit only.  
> 
> I'm obviously misunderstanding something; can someone enlighten me?  
> (No doubt, someone will point out a nuance of the help files I 
> didn't understand!) 
> 
> Also, since the above obviously doesn't do what I hoped 
> (consistently printing two sig figs), could someone also explain 
> how I can consistently get two significant figures in situation 
> like above?
I'll let someone else comment on whether this is a bug in formatC; it 
looks like a bug or a documentation error to me.

You can get 2 sig figs in your situation like this:

sprintf("%.6f",0.000059999)

but obviously this doesn't help with numbers on a different scale.  For 
those, I think you're going to have to put together something like

  x <- 0.0000599999
  sprintf("%.*f", -trunc(log10(x))+2, x)

(which only works for numbers less than 1).

Duncan Murdoch