[R] Why doesn't formatC( x, digits=2, format="g") doesn't always give 2 sig figs?

Peter Dunn dunn at usq.edu.au
Fri Jun 6 02:48:50 CEST 2008

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 

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.

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

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