The Art of R Programming

The speedup is breathtaking.

You can see that writing certain functions in C can be worth the effort.

This is especially true for functions that involve iteration, as R’s own itera-

tion constructs, such asfor(), are slow.

15.2 Using R from Python........................................................

Python is an elegant and powerful language, but it lacks built-in facilities for

statistical and data manipulation, two areas in which R excels. This section

demonstrates how to call R from Python, using RPy, one of the most popular

interfaces between the two languages.

15.2.1 Installing RPy...................................................

RPy is a Python module that allows access to R from Python. For extra effi-

ciency, it can be used in conjunction with NumPy.

You can build the module from the source, available fromhttp://rpy

.sourceforge.net, or download a prebuilt version. If you are running Ubuntu,

simply type this:

sudo apt-get install python-rpy

To load RPy from Python (whether in Python interactive mode or from

code), execute the following:

from rpy import*

This will load a variabler, which is a Python class instance.

15.2.2 RPy Syntax.....................................................

Running R from Python is in principle quite simple. Here is an example of a

command you might run from the>>>Python prompt:

>>> r.hist(r.rnorm(100))

This will call the R functionrnorm()to produce 100 standard normal

variates and then input those values into R’s histogram function,hist().

As you can see, R names are prefixed byr., reflecting the fact that

Python wrappers for R functions are members of the class instancer.

The preceding code will, if not refined, produce ugly output, with your

(possibly voluminous!) data appearing as the graph title and thex-axis label.

You can avoid this by supplying a title and label, as in this example:

>>> r.hist(r.rnorm(100),main='',xlab='')

RPy syntax is sometimes less simple than these examples would lead you

to believe. The problem is that R and Python syntax may clash. For instance,

330 Chapter 15