[R] Formatting numbers with a limited amount of digits consistently

[Gabor Grothendieck]

On 5/30/05, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> Gabor Grothendieck wrote:
> > On 5/30/05, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> >
> >>Henrik Andersson wrote:
> >>
> >>>I have tried to get signif, round and format to display numbers like
> >>>these consistently in a table, using e.g. signif(x,digits=3)
> >>>
> >>>17.01
> >>>18.15
> >>>
> >>>I want
> >>>
> >>>17.0
> >>>18.2
> >>>
> >>>Not
> >>>
> >>>17
> >>>18.2
> >>>
> >>>
> >>>Why is the last digit stripped off in the case when it is zero!
> >>
> >>signif() changes the value; you don't want that, you want to affect how
> >>a number is displayed.  Use format() or formatC() instead, for example
> >>
> >> > x <- c(17.01, 18.15)
> >> > format(x, digits=3)
> >>[1] "17.0" "18.1"
> >> > noquote(format(x, digits=3))
> >>[1] 17.0 18.1
> >>
> >
> >
> > That works in the above context but I don't think it works generally:
> >
> > R> f <- head(faithful)
> > R> f
> >   eruptions waiting
> > 1     3.600      79
> > 2     1.800      54
> > 3     3.333      74
> > 4     2.283      62
> > 5     4.533      85
> > 6     2.883      55
> >
> > R> format(f, digits = 3)
> >   eruptions waiting
> > 1      3.60      79
> > 2      1.80      54
> > 3      3.33      74
> > 4      2.28      62
> > 5      4.53      85
> > 6      2.88      55
> >
> > R> # this works in this case
> > R> noquote(prettyNum(round(f,1), nsmall = 1))
> >      eruptions waiting
> > [1,] 3.6       79.0
> > [2,] 1.8       54.0
> > [3,] 3.3       74.0
> > [4,] 2.3       62.0
> > [5,] 4.5       85.0
> > [6,] 2.9       55.0
> >
> > and even that does not work in the desired way (which presumably
> > is not to use exponent format) if you have some
> > large enough numbers like 1e6 which it will display using
> > the e notation rather than using ordinary notation.
> 
> formatC with format="f" seems to work for me, though it assumes you're
> specifying decimal places rather than significant digits.  It also wants
> a vector of numbers as input, not a dataframe.  So the following gives
> pretty flexible control over what a table will look like:
> 
>  > data.frame(eruptions = formatC(f$eruptions, digits=2, format='f'),
> +            waiting = formatC(f$waiting, digits=1, format='f'))
>    eruptions waiting
> 1 1000000.11    79.0
> 2       1.80    54.0
> 3       3.33    74.0
> 4       2.28    62.0
> 5       4.53    85.0
> 6       2.88    55.0
> 
> >
> > I have struggled with this myself and have generally been able
> > to come up with something for specific instances but I have generally
> > found it a pain to do a simple thing like format a table exactly as I want
> > without undue effort.  Maybe someone else has figured this out.
> 
> I think that formatting tables properly requires some thought, and R is
> no good at thinking.  You can easily recognize a badly formatted table,
> but it's very hard to write down rules that work in general
> circumstances.  It's also a matter of taste, so if I managed to write a
> function that matched my taste, you would find you wanted to make changes.
> 
> It's sort of like expecting plot(x, y) to always come up with the best
> possible plot of y versus x.  It's just not a reasonable expectation.
> It's better to provide tools (like abline() for plots or formatC() for
> tables) that allow you to tailor a plot or table to your particular needs.
> 

Thanks.  That seems to be the idiom I was missing.  One thing that would
be nice would be if formatC could handle data frames.