Formatting numbers with a limited amount of digits consistently
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