R: formatting numbers for output

I often produce tables of numbers with a lot of significant digits after the decimal point. It's confusing to look at 20 sig [fd]igs, especially in a large table of results, so I tend to format my output to make it more concise and easier to read.

The function 'format' is pretty good for this. Note that the 'format' call returns a character vector (which is fine if you're only going to write the number to file or the console).

Here's a simple example just using a randomly generated number:

> num <- rnorm(1, mean=10)
> num
[1] 10.24339

We call format with digits=4 (show 4 significant digits) and nsmall=4 (display at least 4 digits after the decimal - for real/complex numbers in non-scientific format):

> format(num, digits=4, nsmall=4)
[1] "10.2434"

You can see that the format command rounds the numbers. This uses the IEC 60559 standard - 'go to the even digit'. So 0.5 is rounded to 0 and 1.5 is rounded to 2...

Of course, if you're used to sprintf style commands then you can also use the sprintf function for this:

> sprintf("%.4f", num)
[1] "10.2434"