On 05/06/2008 8:48 PM, Peter Dunn wrote:
Hi allI 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
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