Fix #+CAPTION directive position.

[Worg.git] / FIXME / ob-doc-R-extended.org

#+OPTIONS:    H:3 num:nil toc:2 \n:nil @:t ::t |:t ^:{} -:t f:t *:t TeX:t LaTeX:t skip:t d:(HIDE) tags:not-in-toc
#+STARTUP:    align fold nodlcheck hidestars oddeven lognotestate 
#+SEQ_TODO:   TODO(t) INPROGRESS(i) WAITING(w@) | DONE(d) CANCELED(c@)
#+TAGS:       Write(w) Update(u) Fix(f) Check(c) noexport(n)
#+TITLE:    Org-mode and R: An Introduction
#+AUTHOR:    Erik Iverson
#+EMAIL:     eriki at ccbr dot umn dot edu
#+LANGUAGE:   en
#+STYLE:      <style type="text/css">#outline-container-introduction{ clear:both; }</style>
#+BABEL: :exports both

This is an introduction to using R ([[http://www.R-project.org]]) source
code within Emacs org-mode ([[http://orgmode.org]]). Org-mode files use
headlines to organize information. Each top-level headline in this
document starts with a single '*', like the "Introduction" headline
below. While this is not an introduction to using org-mode, you will
need to know one command to proceed: use the TAB key on a headline to
open it. TAB will cycle through the possible visibility states of the
information under the headline, and eventually TAB will collapse the
headline back to how you see it now. One last command to note: C-c C-o
opens links like those above in your web browser. I have tried to link
to the appropriate documentation for each feature I describe.

Since you are following along in org-mode, instead of reading this in
an exported format like HTML or PDF, you will need org-mode 7.01 or
greater to go interact with this tutorial. See [[http://orgmode.org/index.html#sec-3][here]] for instructions
on how to download the latest version of org-mode. To see what version
of org-mode you have installed, type M-x org-version, and hit
<ENTER>. The result will be in the minibuffer. If the version is
anything less than 7.01, you'll need to update to run the examples.
If you have an older version of org-mode, and just want to read about
the possibilities, you can continue on.

* Introduction

#+begin_html
  <div id="subtitle" style="float: center; text-align: center;">
  <p>
  Org-mode support for
  <a href="http://www.r-project.org/">R</a>
  </p>
  <p>
  <a href="http://www.r-project.org/">
  <img src="http://www.r-project.org/Rlogo.jpg"/>
  </a>
  </p>
  </div>
#+end_html

Emacs org-mode 7.01 has an exciting new feature that lets you submit
source code blocks within an org-mode document for evaluation. This
lets you do things like insert the results of R code into an Emacs
buffer, insert graphical and tabular material into a buffer, or pass
the results of R code to other programming languages such as Python. R
code and results can be included in your exported org-mode documents,
opening up several interesting possibilities for automatically
generating comprehensive documentation and advanced reports. You can
also extract the source code portions of an org-mode document for
further processing, through a process called tangling. This tutorial
will get you started using these org-mode features together with the R
programming language.

If you are unfamiliar with org-mode itself, you can learn a lot more
from the project's [[orgmode.org][website]]. There are many good tutorials available on
org-mode already. The [[http://orgmode.org/guide/index.html][compact guide]] is a great place to start. This
current document focuses on source code support. Note that while the
features being demonstrated in this document were being developed, the
project was known as org-babel. Thus, many of the variables and
function names reference 'org-babel' in their names. Org-babel is now
distributed with org-mode, so many of the previous configuration
hurdles are now avoided. Keep this in mind as you read old mailing
list posts and documentation. The authors of org-babel are Eric
Schulte and Dan Davison. They have worked very hard creating this
amazing system!

Although you may be viewing this tutorial in an exported format like
HTML or PDF, I wrote this tutorial in org-mode. You will benefit most
from it by following along in org-mode. Only then can you
interactively evaluate the examples to see org-mode in action. For
this reason, I suggest you download the [[http://repo.or.cz/w/Worg.git/blob_plain/HEAD:/org-contrib/babel/languages/ob-doc-R-extended.org][actual org mode file]] that this
document is based on, visit the file in Emacs, and follow along there.

For those following along in an exported version, such as HTML, 
in the actual org-mode file, source code blocks look like this:

#+begin_example
#+begin_src R 
  # some R code 
  square <- function(x) 
  {
    x * x
  }
    
  square(1:10)
#+end_src 
#+end_example

However, when they are /exported/ into documents like this they will
look like:

#+begin_src R :exports code
  # some R code 
  square <- function(x) 
  {
    x * x
  }
    
  square(1:10)
#+end_src 

It's something to be aware of when following along from an exported
version such as HTML, since I will be referencing source code block
arguments that you will not be able to see.  That is another very good
reason to follow along with the [[http://repo.or.cz/w/Worg.git/blob_plain/HEAD:/org-contrib/babel/languages/ob-doc-R-extended.org][raw org mode file]]. 

This tutorial was written in GNU Emacs 23.2 on Ubuntu 10.04, org-mode
version 7.01trans, pulled directly from the org-mode git repository.

Please send any questions or corrections to eriki at ccbr dot umn dot edu. 

** System Prerequisites for this tutorial 

First, we need to make sure our environment is setup correctly for the
examples to run.  This requires a bit more work under Windows than
others, see below.

Here is a list of software we need to run the examples:
1) org-mode 7.01 or greater, see [[http://orgmode.org]]
2) a working R installation, see [[http://www.R-project.org]]
3) The R examples use the ggplot2 and Hmisc packages from CRAN. Simply install from the
   R command line by issuing the command, 
   > install.packages(c("ggplot2", "Hmisc"))
   R must be in your PATH environment variable.  For Windows users, you will probably
   have to add this yourself. 

For LaTeX support, 
4) a working LaTeX installation, see [[http://latex-project.org]]. Windows users
   can use [[http://miktex.org/][MikTeX]].  
5) dvipng program (comes with MikTeX or texlive-full Ubuntu package)

6) Some extra LaTeX packages (comes with texlive-full Ubuntu package): 

   I found that on my Ubuntu 10.04 installation, I had to install the
   texlive-latex-extra and texlive-fonts-recommended packages to get
   the LaTeX documents that org-mode produces to compile. You can get
   both of these (plus dvipng) through the Ubuntu package
   texlive-full, so simply installing the `texlive-full` package may
   be the easiest option if you happen to be on Ubuntu.

   For Windows users who have installed MikTeX, I had to use the
   MikTeX package manager to install the following packages for LaTeX
   support to work: soul, marvosysm, wasysym, wasy, zhmetrics. Install
   these and you should be good to go.

For inline image support,
7) libpng, GNU/Linux users probably have this.  I found under Windows
   that I had to download
   http://downloads.sourceforge.net/gnuwin32/libpng-1.2.37-setup.exe
   and after running the installation program, *manually* copy the
   libpng12.dll and zlib1.dll files into my emacs-23.x\bin directory,
   and then restart emacs for inline image support to work. One easy
   way to test if png support is working is to simply open a png file
   within Emacs from dired.
  
* Setting up org-mode for source code evaluation 

Setting up org-mode to run source code is very simple. Since you are
reading the R tutorial, I will assume you want to specifically run R
source code blocks within org-mode. Since we use LaTeX later on in 
the tutorial, we'll also take the opportunity to set up org-mode
to evaluate LaTeX blocks. 

The absolute, bare minimum setup you need to perform is to run the
following Emacs lisp code. For a preview of what we're going to learn
with in this tutorial, simply hit C-c C-c anywhere in the following code
block! You will be asked in the minibuffer to confirm that you want
to evaluate the source code contained in the block. Confirm this, and
you'll be set up for the rest of the tutorial. You can also add the
lines between the #+begin\_src and #+end\_src lines to your Emacs
initialization file, so that they are always run when starting Emacs.

So go ahead, hit C-c C-c with point in the following code block. 

#+begin_src emacs-lisp :results silent
  (org-babel-do-load-languages
   'org-babel-load-languages
   '((R . t)
     (latex . t)))
#+end_src

If you received any type of error message, please make sure that you
have the proper version of org-mode installed by typing M-x
org-version <Enter>. You should have at least 7.01. If you still are
running org-mode version 6.xx or before, please visit the project web
site for instructions on downloading the latest version.

If you didn't get any errors, org-mode is now setup to run the R
examples that follow.

Note to Windows users. Make sure the directory containing the R
executable is added to your PATH variable for you to run these
examples.

** Prompting for confirmation before evaluating code 
There is one more variable I set in my Emacs initialization file
relating to evaluating source code in org-mode. By default, org-mode
will ask you to confirm each and every time you evaluate a source code
block. If you ran the above source code block with C-c C-c, you will
have noticed that behavior. I turn this feature off with the
following line. If you choose, simply hit C-c C-c to evaluate it for
this session, or put it in your Emacs initialization file. Then, you
won't be asked before org-mode evaluates source code blocks. You may
view this as a security risk. Always look over the code you're going
to evaluate before submitting it. 

#+begin_src emacs-lisp :results silent :exports code
  (setq org-confirm-babel-evaluate nil)
#+end_src

** Other supported languages

Besides R, which we just set up with the above source code block, see
[[http://orgmode.org/manual/Languages.html#Languages][here]] for a list of languages that org-mode currently supports. You can
then add more languages to your personal setup if you desire, by
modifying the variable we defined above to include more languages.

* Org-mode source code blocks 
** Exporting pretty-printed source code blocks

Before I show you how to evaluate code in org-mode, let's start off
with looking at a what a typical org-mode code block looks like. We
just saw a couple examples above of Emacs lisp source code blocks. In
what follows, we will be working with very simple R functions to show
off the capabilities of org-mode.

The following is a simple R code block in org-mode. You can edit the
code in its own buffer by typing C-c ' (that's a single quote), or
just by editing the code within the org-mode buffer. The nice thing
about opening the code in its own buffer with C-c ', is that the
buffer is then in ESS mode. All the ESS key bindings, interaction
with the inferior R process, and syntax highlighting work as expected.

So here is an example of a source code block. The defining feature is
the #+begin\_src and #+end\_src lines, with the language definition,
"R", on the first line. 

Try opening this code block by putting point anywhere inside of it,
and hitting C-c ' (that's a single quote). This will open a new
buffer, with the contents of the source code block. You can then edit
this buffer just like any other R file, as it is in R-mode from
ESS. When finished editing, hit C-c ' again, and you'll see any
changes you made reflected in this org-mode buffer. You can control
how this new buffer is displayed by setting the org-src-window-setup
variable in Emacs.

#+begin_src R :exports code
  square <- function(x) 
  {
    x * x
  }
    
  square(1:10)
#+end_src 

So now we have this code block defined. Why would we want to do
something like that with org-mode? Mostly so that when we export an
org-mode document to a more human-readable format, org-mode recognizes
those lines as syntax, and highlights them appropriately in the HTML
or LaTeX output. The lines will be syntax highlighted just like they
would be in an R code buffer in Emacs.

Try this for yourself. With point anywhere in this subtree, for
example, put it here [ ], hit C-c C-e 1 b (that's the number 'one').
This subtree should be exported to an HTML file and displayed in your
web browser. Notice how the source code is syntax highlighted. 

Note: for syntax highlighting in exported HTML to work, htmlize.el
must be in your load-path. The easiest way to make that happen if you
haven't already is to run the following Emacs lisp code, *after*
changing the "/path/to" portion to reflect your local setup. I have
the following in my Emacs init file. 

#+begin_src emacs-lisp :results silent :exports code
 (add-to-list 'load-path "/path/to/org-mode/contrib/lisp")
#+end_src

** Evaluating the code block using org-mode

As I mentioned, defining the above code block would be useful if
we wanted to export the org-mode document and have the R code in the
resulting, say, HTML file, syntax highlighted. The feature that
org-mode now adds in version 7.01 is letting us actually submit the
code block to R to compute results for either display or further
computation.

It is worth pointing out here that org-mode works with many
languages, and they can all be intertwined in a single org-mode
document. So you might get results from submitting an R function, and
then pass those results to a Python or shell script through an
org-table. Org-mode then becomes a meta-programming tool. We only
concentrate on R code here, however.

We did see above in the setup section that we have Emacs lisp code in
this same org-mode file. To be clear, you can mix many languages in
the same file, which can be very useful when writing documentation,
for instance.

Next, let's actually submit some R code.

*** Obtaining the return value of an R code block

We will now see how to submit a code block. Just as in the
Introduction with Emacs lisp code, simply hit C-c C-c anywhere in the
code block to submit it to R. If you didn't set the confirmation
variable to nil as I described above, you'll have to confirm that you
want to evaluate the following R code. So go ahead, evaluate the
following R code block with C-c C-c and see what happens. 

#+begin_src R
  square <- function(x) {
    x * x
  }
  
  square(1:10)
#+end_src 

If you've submitted the code block using C-c C-c, and everything went
well, you should have noticed that your buffer was modified. Org-mode
has inserted a results section underneath the code block, and above
this text. These results are from running the R code block, and
recording the last value. This is just like how R returns the last
value of a function as its return value. Notice how the results have
been inserted as an org-table. This can be very useful. However,
what if we wanted to see the standard R output? You will see how to do
that in the next section.

You can also try changing the source code block, and re-running it.
For example, try changing the call to the square function to 1:12,
then hit C-c C-c again. The results have updated to the new value!

*** Obtaining all code block output 

We just saw how the last value after evaluating our code is put into
an org-mode table by default. That is potentially very useful, but
what if we just want to see the R output as it would appear printed in
the R console? Well, just as R function have arguments, org-mode
source blocks have arguments. One of the arguments controls how the
output is displayed, the :results argument. It is set to 'value' by
default, but we can change it to 'output' to see the usual R
output. Notice the syntax for setting source code block arguments
below.

#+begin_src R :results output
  square <- function(x) {
    x * x
  }

  square(1:10)
#+end_src 


Now we see the typical R notation for printing a vector. Note in the
following example that setting `:results output` captures *all*
function output, not just the return value. We capture things printed
to the screen with the `cat` function for example, or the printing of
the variable `x`.

#+begin_src R :results output
  x <- 1:10
  x
  square <- function(x) {
    cat("This is the square function.\n")
    x * x
  }
  
  square(1:10)
#+end_src 

Try changing the :results argument to `value` (which is the same as
omitting it completely), and re-run the above code block. You should
see the same org-table output as we saw above.
*** More information on org-mode source block headers

See [[http://orgmode.org/manual/Header-arguments.html#Header-arguments]]
for more information on source code block header arguments, including
the various ways they can be set in an org-mode document: per block,
per file, or system-wide.

*** Inline code evaluation 
Much like the Sweave \Sexpr command, we can evaluate small blocks of
inline code using the

#+begin_example
SRC_R[optional header arguments]{R source code} 
#+end_example

syntax.  So, in org-mode I will type

#+begin_example
SRC_R[:exports results]{round(pi, 2)}
#+end_example 

and you will see src_R[:exports results]{round(pi, 2)} in the exported
output.  You'll see examples of how to use the :exports code block
header in a few sections. 
* Passing data between code blocks

One of the biggest limitations to using code blocks like above is that
a new R session is started up `behind the scenes` when we evaluate
each code block. So, if we define a function in one code block, and
want to use it another code block later on, we are out of luck. This
limitation can be overcome by using R session-based evaluation, which
sends the R code to a running ESS process.

** R session-based evaluation 

Often in R, we will define functions or objects in one code block
and want to use these objects in subsequent code blocks. However,
each time we submit a code block using C-c C-c, org-mode is firing up
an R session, submitting the code, obtaining the return values, and
closing down R. So, by default, our R objects aren't persistent!
That's an important point. Fortunately, there is an easy way to tell
org-mode to submit our code blocks to a running R process in Emacs,
just like we do with R files in ESS.

You simply use the :session argument to the org-mode source block.   

#+begin_src R :session :results output
  square <- function(x) {
    x * x
  }
  x <- 1:10 
#+end_src 

So, the above code block defines our function (square) and object (x).
Now we want to apply call our square function with the x
object. Without :session, we could not do this.

#+begin_src R
  square(x)
#+end_src

Running the above code block will result in an error, since a new R
session was started, and our objects were not available. Now try the
same code block, but with the :session argument, as below.

#+begin_src R :session :results output
  square(x)
#+end_src

The results we expect are now inserted, since we submitted this code
block to the same R session where the square function was defined.

** Code blocks using different languages

Even though this tutorial covers the R language, one of org-mode's
main strengths is its ability to act as a meta programming language,
using results from a program written in one language as input to a
program in another language.

See
[[http://orgmode.org/worg/org-contrib/babel/intro.php#meta-programming-language]]
for an example of this. To keep things as focused on R as possible, I
chose not to include an example like the one found in the link in this
tutorial.

* Inserting R graphical output 

Here is a really cool feature of evaluating source code in
org-mode. We can insert images generated by R code blocks inline in
our Emacs buffer! To enable this functionality, we need to evaluate a
bit of Emacs lisp code. If this feature is something you want every
time you use org-mode, consider placing the code in your Emacs
initialization file. Either way, evaluate it with C-c C-c.

#+begin_src emacs-lisp :results silent :exports code
  (add-hook 'org-babel-after-execute-hook 'org-display-inline-images)   
  (add-hook 'org-mode-hook 'org-display-inline-images)   
#+end_src

The following R code generates some graphical output. There are
several things to notice.

1) :results output is specified. This is because the figure is
   generated using the ggplot2 package in R, which is based on
   something called `grid` graphics. Grid graphics need to be
   explicitly printed when called within a function for their output
   to be displayed. See, for example, [[http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-do-lattice_002ftrellis-graphics-not-work_003f][R FAQ 7.22]]. When :results value
   (the default) is active, Org-mode is generating an R function
   wrapper. The upshot is: when generating grid-based graphical output
   within org-mode, you need to either use :results output, wrap the
   graphical function in a print call, or use the :session
   argument. See this mailing list [[http://www.mail-archive.com/emacs-orgmode@gnu.org/msg25944.html][post]] for more explanation if you'd
   like.

2) We use a new source code block argument, :file. This argument will
   capture graphical output from the source block and generate a file
   with the given name. Then, the results section becomes an org-mode
   link to the newly created file. In the example below, the file
   generated is called diamonds.png.

   Finally, If you have defined the Emacs lisp code for inline-image
   support above, an overlay of the file will be inserted inline in
   the actual org-mode document! Run the following source code block
   to see how it works.

#+begin_src R :results output :file diamonds.png
  library(ggplot2)
  data(diamonds)
  dsmall <-diamonds[sample(nrow(diamonds), 100), ] 
  qplot(carat, price, data = dsmall)
#+end_src
  
This opens up many opportunities for doing interesting things with R
within your org-mode documents!

* Inserting LaTeX output

We have just seen how to include graphical output in our org-mode
buffer. We can also do something similar with LaTeX output generated
by R. Of course, this requires at least a working LaTeX
installation. You will also need to install the dvipng program (dvipng
package in Ubuntu, for instance). See the System Requirements section
for other prerequisites.

** A simple example 

Let's work on a very simple example, displaying a LaTeX description
in our org-mode buffer, using the official LaTeX logo. We will use R
to generate the code that will display the official logo. There's
obviously no reason to do this except for demonstration purposes.

First we must define an R source block that generates some LaTeX code
that displays the logo. That's fairly straightforward. Notice we have
given the source code block a name, so that we can call it later. We
use the #+srcname syntax to do this. Note that you *don't* have to
run the following code block, it will be run automatically by the next
one.

#+srcname: R-latex
#+begin_src R :results silent :exports code
  lf <- function() {
      "\\LaTeX"
  }
  
  lf()
#+end_src

Next, we define a new source block using the "latex" language, instead
of "R", as we have been using. If we use a :file argument with a LaTeX
source code block, org-mode will generate a file of the resulting dvi
file that LaTeX produces, and display it. This is just like
generating graphical output from R using a :file argument, so there is
nothing new there.

However, note we have a new argument, :noweb. What does that mean? In
short, it let's us use syntax like <<CodeBlock()>> to insert the
results of running a code block named CodeBlock into another source
code block. So, in our example, we're running the R-latex code block
defined above, and inserting the results, which need to be valid LaTeX
code, into our latex code block. For this example, we of course didn't
need to write an R function to generate such simple LaTeX output, but
it can be much more complicated, as our next example shows. In short,
our R code block is helping to write the LaTeX code block for us.

Noweb was not invented for org-mode, it's been around for a while, and
is used in Sweave, for example. See [[http://en.wikipedia.org/wiki/Noweb][its Wikipedia page]]. The :noweb
argument is set to 'no' be default, because the <<X>> syntax is
actually valid in some languages that org-mode supports.

Run the following code block. The "R-latex" R code block will be run,
generating the string \\LaTeX, which is then substituted into this
LaTeX code block, and then turned into the LaTeX logo by the latex
program. Don't worry about the complicated header arguments, those
will be explained in more detail in the next section. 

#+begin_src latex :noweb yes :file (if (and (boundp 'htmlp) htmlp) "latex-logo-html.png" "latex-logo.png") :buffer (if (and (boundp 'htmlp) htmlp) "no" t)
<<R-latex()>>~is a high-quality typesetting system; it includes
features designed for the production of technical and scientific
documentation. <<R-latex()>>~is the de facto standard for the
communication and publication of scientific
documents. <<R-latex()>>~is available as free software.
#+end_src

** A more complicated example, exporting LaTeX in buffer, to HTML, and to PDF

Now let's try something a little more complex, using an R function
that generates a full LaTeX table. This particular example depends on
having the R package Hmisc installed. If you don't have it installed,
start up R and then do: > install.packages("Hmisc")

What follows is an R source block that generates some LaTeX code
representing a table.  We want to be able to insert a =png= image of
the table in the buffer when run with C-c C-c, using the colors of our
current Emacs buffer.

A few sections from now, I'll touch on the exporting features of
org-mode.  Org can generate HTML and PDF versions of documents like
this one. 

Back to our example, for HTML export, we also want to generate a
=png=. However, we want the background to be transparent, not whatever
color our Emacs buffer happened to be.

For LaTeX output, we don't need a =png= file at all, we would of
course prefer to simply insert the auto-generated LaTeX code in the
exported LaTeX document, and then compile to PDF.

The following should accomplish all three goals.  

We tell the R code block to output latex code using the syntax
/:results output latex/.  Also, only export the code.  If we export
both, then the LaTeX results would get exported twice when we export
to PDF, once from each code block.  It would actually be exported
twice when we export to HTML, but in that case, since the results are
wrapped in #+BEGIN\_LATEX/#+END\_LATEX lines, and are therefore not
included in the HTML export.

In the LaTeX code block, a file will be generated for in-buffer
evaluation and HTML export, but we don't want it produced for LaTeX
export, otherwise the image /and/ the actual table will be included in
the PDF.  

The final /buffer/ argument controls the color selection through the
=org-format-latex-options= variable. Essentially, if buffer is set to
'yes', your Emacs buffer colors will be used as arguments to the
=dvipng= program used to produce the image, assuming you don't change
that values of the elements to something other than 'default' in
=org-format-latex-options=. If buffer is 'no', then the html* elements
of that variable will be used.

#+srcname:Hmisc-latex
#+begin_src R :results output :exports code
  set.seed(1) 
  library(Hmisc)
    
  df <- data.frame(age = rnorm(100, 10),
                   gender = sample(c("Male", "Female"), 100, replace = TRUE),
                   study.drug = sample(c("Active", "Placebo"), 100, replace = TRUE))

  label(df$study.drug) <- "Treatment" 
  label(df$age) <- "Age at randomization" 
  label(df$gender) <- "Gender" 
    
  latex(summary(study.drug ~ age + gender, data = df,
                method = "reverse", overall = TRUE, test = TRUE), 
        long = TRUE,  file = "", round = 2, exclude1 = FALSE, npct = "both",
        where="!htbp")
#+end_src

#+begin_src latex :noweb yes :file (if (and (boundp 'latexp) latexp) nil (if (and (boundp 'htmlp) htmlp) "hmisc-html.png" "hmisc.png")) :buffer (if (and (boundp 'htmlp) htmlp) "no" t)
<<Hmisc-latex()>>
#+end_src 

* Putting it all together, a notebook interface to R

Combining the techniques shown above: submitting code blocks,
capturing output for further manipulation, and inserting graphical and
tabular material, we essentially have a basic notebook-style
interface for R.

This is potentially useful for countless tasks such as: a laboratory
notebook, time series analysis of diet/exercise habits, tracking your
favorite baseball team over the course of a season, or any reporting
task you can think of. Since org-mode is a general-purpose authoring
tool, with very strong exporting capabilities, almost anything is
possible.

For instance, I use org-mode to generate HTML for an R blog that I
run. Several posters to the org-mode mailing list have mentioned
writing their entire graduate theses in org-mode, and even books.

I look at this workflow as an alternative to the excellent [[http://www.stat.uni-muenchen.de/~leisch/Sweave/][Sweave]]
package that cuts out the need for learning LaTeX to produce
high-quality documents. Org-mode is doing all the exporting for you,
including LaTeX if you'd like. Getting LaTeX and HTML output
essentially "for free" should not be underestimated!

On some level, all these activities assume that you are a comfortable
org-mode user, and that you will be writing code, conducting analyses,
and possibly exporting results through the familiar Emacs and org-mode
user interface. Through the exporting functionality, org-mode offers
many useful and easy-to-use options to share /results/ of your efforts
with others, but what about the code itself? 

Most people you have to share code with aren't going to want an
org-mode file full of source code!

* Tangling code 

With many projects, you will have to share /code/ with other
programmers, who are most likely not going to be programming in
org-mode. Therefore, sharing an org-mode file full of code is not an
option.

Or, consider development of an R package. The package building
process obviously operates on .R files, each full of R functions.
However, that's not what we have in a document like this one.

It is in situations like these where /tangling/ can be used. 

The process of tangling an org-mode document essentially extracts the
code contained in org-mode source code blocks, and places it in a file
of the appropriate type. How do we do this? We use the :tangle
source code block header argument to direct org-mode what to do. Then,
we call the tangle function on the file to extract the source code!

Read on to learn how to perform each of these steps. 

** Instructing org-mode how to tangle with header arguments 
Let's take a look at a few examples. Each example contains an R
comment, so that you can see in the resulting .R file where it came
from.

This first example will not extract any code from the source block.
It is the default behavior. 

#+begin_src R :tangle no :exports code
# tangle was not specified
x <- 1:10
print(x) 
#+end_src


This will place the code in source code block in
org-mode-R-tutorial.R, since we don't specify a filename for the .R
file.

#+begin_src R :tangle yes :exports code
# tangle was specified, but no file given
x <- 1:10
print(x)
#+end_src

This will place the tangled code in Rcode.R, since we specify that name. 

#+begin_src R :tangle Rcode.R :exports code
# tangle was specified, and a file name given (Rcode.R)
x <- 1:10
print(x)
#+end_src

Note that we will have multiple source code blocks in an org-mode
file, and they might have different types. For example, we might have
R and Python code in the same document, but different source blocks. 

This is no problem, as the tangling mechanism will generate
appropriate files of each type, containing only the code of that type.

Finally, you can specify the :tangle argument as a buffer-wide
setting, so that you don't have to specify it for every source code
block.

This opens up exciting possibilities like having a *single* org-mode
file that includes:
- all code for an R package
- all documentation for the package
- unit tests for the package
- material to generate slides for presentations, through org-beamer
- notes taken during package development 
- links to emails with bug reports, feature requests, etc. 
- a Makefile to build the package and documentation

** Tangling the document 

Now that we have seen how to instruct org-mode how to produce source
code files from our org-mode document, how do we actually tangle the
document?

We simply have to call the org-babel-tangle function, bound by default
to C-c C-v C-t. 

Org-mode confirms in the minibuffer how many code blocks have been
tangled, and inspecting the file system should show that your source
code files have been created. There exists a hook function that will
run any post-processing programs you have defined, for example, a
compiler, `R CMD build`, or running `make` with a Makefile, possibly
itself generated from the org-mode document!

* Exporting documents containing code and results

Org-mode provides a rich set of functions and customizations for
exporting documents into more human-readable forms, and for users who
are not Emacs or org-mode users. The most common methods are
generating PDF documents through LaTeX, and HTML output. Source code
will be syntax highlighted, in HTML.  There are various options for
PDF, including using the listings package.

With org-mode source blocks, you can choose to export the source code,
the results of evaluating the source code, neither, or both.
The :exports header argument controls this. See the [[http://orgmode.org/manual/Exporting-code-blocks.html#Exporting-code-blocks][documentation]] for
further examples. 

As an example, type C-c C-e b to see an HTML version of this document.

Some fairly sophisticated processes, including complete report
generation using R graphics and tables, can be achieved through this
facility.

Using org-mode in this manner is essentially an alternative to Sweave,
with the advantages of:
- do not need to learn LaTeX or other markup language
- any future org-mode export engines will be available to you
- writing code in org-mode gives you access to a hyper-commenting
  system, with features such as TODO items, in-document linking, tags, 
  and code folding.

If you're an advanced LaTeX user, you probably don't view point 1
above as an advantage. :) 

Whether or not you use all the features that org-mode provides, you
can use the system for literate programming and reproducible research,
on projects large and small.

* Where to go from here? 

We have seen how to submit R code for evaluation in org-mode. There
are many good reasons to do this, including tying results to source
code, code folding, exporting of code and results into many common
formats, improving documentation, and the innumerable features that
org-mode provides, and will continue to provide in the future. 

As with all new processes, it can be a challenge to start working with
source code this way. As a current org-mode user, I think the
benefits are clear.

As for what to do next, try looking at the [[http://orgmode.org/worg/org-contrib/babel/uses.php][results]] of some of those
who use org-mode with org-babel to accomplish interesting things. You
can look at current documentation for R support [[http://orgmode.org/worg/org-contrib/babel/languages/ob-doc-R.php][here]].

For an exercise in using org-mode with source code, you can write your
Emacs initialization file in org-mode! These [[http://orgmode.org/worg/org-contrib/babel/intro.php#sec-8_2_1][instructions]] are slightly
out of date, but they give you a general idea of how to proceed.
Essentially, your master Emacs init file will simply tangle an
org-mode file full Emacs lisp code blocks, and then load the resulting
file. My Emacs init file is around 1000 lines long, so organizing it
in a hierarchy with embedded tags and links is very useful to me.

In short, there are many possibilities using these techniques! In many
ways, I have only scratched the surface of the capabilities of
org-mode in this tutorial. As always, the [[http://orgmode.org/manual/index.html#Top][official manual]] will be the
source of the most up-to-date information and features of this great
tool.