Formatting numbers with a limited amount of digits consistently

On Mon, 2005-05-30 at 23:53 -0400, Gabor Grothendieck wrote:

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.

Guys, perhaps I am missing something here, but there seems to be some
confusion as to how the numbers are stored internally, versus how the
output is displayed and the meaning of "significant digits", which is
what I believe Henrik's original query was about.

By default, R's printed output uses the settings from options("digits")
and options("scipen") to define output based upon the number of
significant digits, which is of course not the same as the number of
decimal places. Hence the variance in the output that Henrik gets and
why the trailing zero is dropped.

The use of signif() does not help here because it is still based upon
the number of significant digits, where the trailing zero still gets
dropped.

The use of the above are "inexact" when it comes to creating formatted
output for a table with a consistent number of decimal places to align
columns of numbers.

format() is still problematic here because it too uses the number of
significant digits, defaulting to options("digits").