This introduction aims at making a gentle start to rpy2, either when coming from R to Python/rpy2, from Python to rpy2/R, or from elsewhere to Python/rpy2/R.

It is assumed here that the rpy2 package has been properly installed. In python, making a package or module available is achieved by importing it:

```
import rpy2.robjects as robjects
```

The object `r` in `rpy2.robjects` represents the running embedded
R process.

If familiar with R and the R console, `r` is a little like a
communication channel from Python to R.

In Python the [ operator is an alias for the ethod `__getitem__()`.

The `__getitem__()` method of `rpy2.robjects.r`,
evaluates a variable from the R console.

Example in R:

```
> pi
[1] 3.141593
```

With `rpy2`:

```
>>> pi = robjects.r['pi']
>>> pi[0]
3.14159265358979
```

Note

Under the hood, the variable pi is gotten by default from the
R *base* package, unless an other variable with the name pi was
created in R’s .globalEnv.

Whenever one wishes to be specific about where the symbol
should be looked for (which should be most of the time),
it possible to wrap R packages in Python namespace objects
(see *R packages*).

For more details on environments, see Section
*Environments*.

Also, note that *pi* is not a scalar but a vector of length 1

The `r` object is also callable, and the string passed to it evaluated
as R code.

This can be used to get variables, and provide an alternative to the method presented above.

Example in R:

```
> pi
[1] 3.141593
```

With `rpy2`:

```
>>> pi = robjects.r('pi')
>>> pi[0]
3.14159265358979
```

Warning

The result is an R vector. The Section
*R vectors* below will provide explanation
for the following behavior:

```
>>> piplus2 = robjects.r('pi') + 2
>>> piplus2.r_repr()
c(3.14159265358979, 2)
>>> pi0plus2 = robjects.r('pi')[0] + 2
>>> print(pi0plus2)
5.1415926535897931
```

The evaluation is performed in what is known to R users as the Global Environment, that is the place one starts at when starting the R console. Whenever the R code creates variables, those variables are “located” in that Global Environment by default.

Example:

```
robjects.r('''
f <- function(r, verbose=FALSE) {
if (verbose) {
cat("I am calling f().\n")
}
2 * pi * r
}
f(3)
''')
```

The expression above returns the value 18.85, but first creates an R function f. That function f is present in the R Global Environement, and can be accessed with the __getitem__ mechanism outlined above:

```
>>> r_f = robjects.globalenv['f']
>>> print(r_f.r_repr())
function (r, verbose = FALSE)
{
if (verbose) {
cat("I am calling f().\n")
}
2 * pi * r
}
```

Note

As shown earlier, an alternative way to get the function
is to get it from the `R` singleton

```
>>> r_f = robjects.r['f']
```

The function r_f is callable, and can be used like a regular Python function.

```
>>> res = r_f(3)
```

Jump to Section *Calling R functions* for more on calling
functions.

Against the first impression one may get from the title
of this section, simple and handy features of `rpy2` are
presented here.

An R object has a string representation that can be used directly into R code to be evaluated.

Simple example:

```
>>> letters = robjects.r['letters']
>>> rcode = 'paste(%s, collapse="-")' %(letters.r_repr())
>>> res = robjects.r(rcode)
>>> print(res)
"a-b-c-d-e-f-g-h-i-j-k-l-m-n-o-p-q-r-s-t-u-v-w-x-y-z"
```

In R, data are mostly represented by vectors, even when looking like scalars.

When looking closely at the R object pi used previously, we can observe that this is in fact a vector of length 1.

```
>>> len(robjects.r['pi'])
1
```

As such, the python method `add()` will result in a concatenation
(function c() in R), as this is the case for regular python lists.

Accessing the one value in that vector has to be stated explicitly:

```
>>> robjects.r['pi'][0]
3.1415926535897931
```

There is much that can be achieved with vectors, having them to behave
more like Python lists or R vectors.
A comprehensive description of the behavior of vectors is found in
`robjects.vector`.

Creating R vectors can be achieved simply:

```
>>> res = robjects.StrVector(['abc', 'def'])
>>> print(res.r_repr())
c("abc", "def")
>>> res = robjects.IntVector([1, 2, 3])
>>> print(res.r_repr())
1:3
>>> res = robjects.FloatVector([1.1, 2.2, 3.3])
>>> print(res.r_repr())
c(1.1, 2.2, 3.3)
```

R matrixes and arrays are just vectors with a dim attribute.

The easiest way to create such objects is to do it through R functions:

```
>>> v = robjects.FloatVector([1.1, 2.2, 3.3, 4.4, 5.5, 6.6])
>>> m = robjects.r['matrix'](v, nrow = 2)
>>> print(m)
[,1] [,2] [,3]
[1,] 1.1 3.3 5.5
[2,] 2.2 4.4 6.6
```

Calling R functions is disappointingly similar to calling Python functions:

```
>>> rsum = robjects.r['sum']
>>> rsum(robjects.IntVector([1,2,3]))[0]
6L
```

Keywords are also working:

```
>>> rsort = robjects.r['sort']
>>> res = rsort(robjects.IntVector([1,2,3]), decreasing=True)
>>> print(res.r_repr())
c(3L, 2L, 1L)
```

Note

By default, calling R functions return R objects.

More information on functions is in Section *Functions*.

R has a builtin help system that, just like the pydoc strings are used frequently in python during interactive sessions, is used very frequently by R programmmers. This help system is accessible from an R function, therefore accessible from rpy2.

```
>>> from rpy2.robjects.packages import importr
>>> utils = importr("utils")
>>> help_doc = utils.help("help")
>>> help_doc[0]
'/where/R/is/installed/library/utils/help/help'
```

Converting the object returned to a string produces the full help text on the topic:

```
>>> str(help_doc)
[...long output...]
```

Warning

The help message so produced is not a string returned to the console
but is directly printed by R to the standard output. The call to
`str()` only returns an empty string, and the reason for this is
somewhat involved for an introductory documentation.
This behaviour is rooted in **R** itself and in `rpy2` the
string representation of R objects is the string representation as
given by the **R** console,
which in that case takes a singular route.

For a Python friendly help to the R help system, consider the module
`rpy2.robjects.help`.

```
>>> help_where = utils.help_search("help")
```

As before with help, the result can be printed / converted to a string, giving a similar result to what is obtained from an R session.

Note

The data structure returned can otherwise be used to access the information returned in details.

```
>>> tuple(help_where)
(<StrVector - Python:0x1f9a968 / R:0x247f908>,
<StrVector - Python:0x1f9a990 / R:0x25079d0>,
<StrVector - Python:0x1f9a9b8 / R:0x247f928>,
<Matrix - Python:0x1f9a850 / R:0x1ec0390>)
>>> tuple(help_where[3].colnames)
('topic', 'title', 'Package', 'LibPath')
```

However, this is beyond the scope of an introduction, and one should
master the content of the module `robjects.vector` before anything else.

This section demonstrates some of the features of rpy2.

```
import rpy2.robjects as robjects
r = robjects.r
x = robjects.IntVector(range(10))
y = r.rnorm(10)
r.X11()
r.layout(r.matrix(robjects.IntVector([1,2,3,2]), nrow=2, ncol=2))
r.plot(r.runif(10), y, xlab="runif", ylab="foo/bar", col="red")
```

Setting dynamically the number of arguments in a function call can be done the usual way in python.

There are several ways to plot data in R, some of which are presented in this documentation:

The general setup is repeated here: .. literalinclude:: _static/demos/graphics.py

start-after: #– setup-begin end-before: #– setup-end

The setup specific to ggplot2 is: .. literalinclude:: _static/demos/graphics.py

start-after: #– setupggplot2-begin end-before: #– setupggplot2-end

```
pp = ggplot2.ggplot(mtcars) + \
ggplot2.aes_string(x='wt', y='mpg', col='factor(cyl)') + \
ggplot2.geom_point() + \
ggplot2.geom_smooth(ggplot2.aes_string(group = 'cyl'),
method = 'lm')
pp.plot()
```

More about plots and graphics in R, as well as more advanced
plots are presented in Section *Graphics*.

Warning

By default, the embedded R open an interactive plotting device,
that is a window in which the plot is located.
Processing interactive events on that devices, such as resizing or closing
the window must be explicitly required
(see Section *Processing interactive events*).

The R code is:

```
ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
group <- gl(2, 10, 20, labels = c("Ctl","Trt"))
weight <- c(ctl, trt)
anova(lm.D9 <- lm(weight ~ group))
summary(lm.D90 <- lm(weight ~ group - 1))# omitting intercept
```

One way to achieve the same with `rpy2.robjects` is

```
from rpy2.robjects import FloatVector
from rpy2.robjects.packages import importr
stats = importr('stats')
base = importr('base')
ctl = FloatVector([4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14])
trt = FloatVector([4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69])
group = base.gl(2, 10, 20, labels = ["Ctl","Trt"])
weight = ctl + trt
robjects.globalenv["weight"] = weight
robjects.globalenv["group"] = group
lm_D9 = stats.lm("weight ~ group")
print(stats.anova(lm_D9))
# omitting the intercept
lm_D90 = stats.lm("weight ~ group - 1")
print(base.summary(lm_D90))
```

This way to perform a linear fit it matching precisely the way in R presented
above, but there are other ways (see Section *Formulae*
for storing the variables directly in the lookup environment of the formula).

Q: Now how to extract data from the resulting objects ?

A: Well, it all depends on the object. R is very much designed for interactive sessions, and users often inspect what a function is returning in order to know how to extract information.

When taking the results from the code above, one could go like:

```
>>> print(lm_D9.rclass)
[1] "lm"
```

Here the resulting object is a list structure, as either inspecting the data structure or reading the R man pages for lm would tell us. Checking its element names is then trivial:

```
>>> print(lm_D9.names)
[1] "coefficients" "residuals" "effects" "rank"
[5] "fitted.values" "assign" "qr" "df.residual"
[9] "contrasts" "xlevels" "call" "terms"
[13] "model"
```

And so is extracting a particular element:

```
>>> print(lm_D9.rx2('coefficients'))
(Intercept) groupTrt
5.032 -0.371
```

or

```
>>> print(lm_D9.rx('coefficients'))
$coefficients
(Intercept) groupTrt
5.032 -0.371
```

More about extracting elements from vectors is available
at *Extracting items*.

The R code is

```
m <- matrix(rnorm(100), ncol=5)
pca <- princomp(m)
plot(pca, main="Eigen values")
biplot(pca, main="biplot")
```

The `rpy2.robjects` code can be as close to the
R code as possible:

```
import rpy2.robjects as robjects
r = robjects.r
m = r.matrix(r.rnorm(100), ncol=5)
pca = r.princomp(m)
r.plot(pca, main="Eigen values")
r.biplot(pca, main="biplot")
```

However, the same example can be made a little tidier (with respect to being specific about R functions used)

```
from rpy2.robjects.packages import importr
base = importr('base')
stats = importr('stats')
graphics = importr('graphics')
m = base.matrix(stats.rnorm(100), ncol = 5)
pca = stats.princomp(m)
graphics.plot(pca, main = "Eigen values")
stats.biplot(pca, main = "biplot")
```

```
from rpy2.robjects import NA_Real
from rpy2.rlike.container import TaggedList
from rpy2.robjects.packages import importr
base = importr('base')
# create a numerical matrix of size 100x10 filled with NAs
m = base.matrix(NA_Real, nrow=100, ncol=10)
# fill the matrix
for row_i in xrange(1, 100+1):
for col_i in xrange(1, 10+1):
m.rx[TaggedList((row_i, ), (col_i, ))] = row_i + col_i * 100
```

```
None
```

```
"""
short demo.
"""
from rpy2.robjects.packages import importr
graphics = importr('graphics')
grdevices = importr('grDevices')
base = importr('base')
stats = importr('stats')
import array
x = array.array('i', range(10))
y = stats.rnorm(10)
grdevices.X11()
graphics.par(mfrow = array.array('i', [2,2]))
graphics.plot(x, y, ylab = "foo/bar", col = "red")
kwargs = {'ylab':"foo/bar", 'type':"b", 'col':"blue", 'log':"x"}
graphics.plot(x, y, **kwargs)
m = base.matrix(stats.rnorm(100), ncol=5)
pca = stats.princomp(m)
graphics.plot(pca, main="Eigen values")
stats.biplot(pca, main="biplot")
```