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

[Duncan Murdoch]

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.

Duncan Murdoch