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<title>Software Carpentry: R for reproducible scientific analysis</title>
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<h1 class="title">R for reproducible scientific analysis</h1>
<h2 class="subtitle">Reference</h2>
<h2 id="introduction-to-r-and-rstudio"><a href="01-rstudio-intro.html">Introduction to R and RStudio</a></h2>
<ul>
<li>Use the escape key to cancel incomplete commands or running code (Ctrl+C) if you’re using R from the shell.</li>
<li>Basic arithmetic operations follow standard order of precedence:</li>
<li>Brackets: <code>(</code>, <code>)</code></li>
<li>Exponents: <code>^</code> or <code>**</code></li>
<li>Divide: <code>/</code></li>
<li>Multiply: <code>*</code></li>
<li>Add: <code>+</code></li>
<li>Subtract: <code>-</code></li>
<li>Scientific notation is available, e.g: <code>2e-3</code></li>
<li>Anything to the right of a <code>#</code> is a comment, R will ignore this!</li>
<li>Functions are denoted by <code>function_name()</code>. Expressions inside the brackets are evaluated before being passed to the function, and functions can be nested.</li>
<li>Mathematical functions: <code>exp</code>, <code>sin</code>, <code>log</code>, <code>log10</code>, <code>log2</code> etc.</li>
<li>Comparison operators: <code><</code>, <code><=</code>, <code>></code>, <code>>=</code>, <code>==</code>, <code>!=</code></li>
<li>Use <code>all.equal</code> to compare numbers!</li>
<li><code><-</code> is the assignment operator. Anything to the right is evaluate, then stored in a variable named to the left.</li>
<li><code>ls</code> lists all variables and functions you’ve created</li>
<li><code>rm</code> can be used to remove them</li>
<li>When assigning values to function arguments, you <em>must</em> use <code>=</code>.</li>
</ul>
<h2 id="project-management-with-rstudio"><a href="02-project-intro.html">Project management with RStudio</a></h2>
<ul>
<li>To create a new project, go to File -> New Project</li>
<li>Install the <code>packrat</code> package to create self-contained projects</li>
<li><code>install.packages</code> to install packages from CRAN</li>
<li><code>library</code> to load a package into R</li>
<li><code>packrat::status</code> to check whether all packages referenced in your scripts have been installed.</li>
</ul>
<h2 id="reading-data"><a href="03-reading-data.html">Reading data</a></h2>
<ul>
<li><code>read.table</code> to read in data in a regular structure</li>
<li><code>sep</code> argument to specify the separator
<ul>
<li>“,” for comma separated</li>
<li>“” for tab separated</li>
</ul></li>
<li>Other arguments:
<ul>
<li><code>header=TRUE</code> if there is a header row</li>
</ul></li>
</ul>
<h2 id="seeking-help"><a href="04-seeking-help.html">Seeking help</a></h2>
<ul>
<li><code>?</code> or <code>help()</code> to seek help for a function.</li>
<li><code>??</code> to search for a function.</li>
<li>Wrap special operators in quotes when searching for help: <code>help("+")</code>.</li>
<li><a href="http://cran.at.r-project.org/web/views">CRAN Task Views</a>.</li>
<li><a href="http://stackoverflow.com/">stackoverflow</a>.</li>
</ul>
<h2 id="data-structures"><a href="05-data-structures.html">Data structures</a></h2>
<p><strong>Basic data structures in R:</strong></p>
<ul>
<li>atomic <code>?vector</code> (can only contain one type)</li>
<li><code>?list</code> (containers for other objects)</li>
<li><code>?data.frame</code> two dimensional objects whose columns can contain different types of data</li>
<li><code>?matrix</code> two dimensional objects that can contain only one type of data.</li>
<li><code>?factor</code> vectors that contain predefined categorical data.</li>
<li><code>?array</code> multi-dimensional objects that can only contain one type of data</li>
</ul>
<p>Remember that matrices are really atomic vectors underneath the hood, and that data.frames are really lists underneath the hood (this explains some of the weirder behaviour of R).</p>
<p><strong>Data types:</strong></p>
<ul>
<li><code>?numeric</code> real (decimal) numbers</li>
<li><code>?integer</code> whole numbers only</li>
<li><code>?character</code> text</li>
<li><code>?complex</code> complex numbers</li>
<li><code>?logical</code> TRUE or FALSE values</li>
</ul>
<p><strong>Special types:</strong></p>
<ul>
<li><code>?NA</code> missing values</li>
<li><code>?NaN</code> “not a number” for undefined values (e.g. <code>0/0</code>).</li>
<li><code>?Inf</code>, <code>-Inf</code> infinity.</li>
<li><code>?NULL</code> a data structure that doesn’t exist</li>
</ul>
<p><code>NA</code> can occur in any atomic vector. <code>NaN</code>, and <code>Inf</code> can only occur in complex, integer or numeric type vectors. Atomic vectors are the building blocks for all other data structures. A <code>NULL</code> value will occur in place of an entire data structure (but can occur as list elements).</p>
<p><strong>Useful functions for querying data structures:</strong></p>
<ul>
<li><code>?str</code> structure, prints out a summary of the whole data structure</li>
<li><code>?typeof</code> tells you the type inside an atomic vector</li>
<li><code>?class</code> what is the data structure?</li>
<li><code>?head</code> print the first <code>n</code> elements (rows for two-dimensional objects)</li>
<li><code>?tail</code> print the last <code>n</code> elements (rows for two-dimensional objects)</li>
<li><code>?rownames</code>, <code>?colnames</code>, <code>?dimnames</code> retrieve or modify the row names and column names of an object.</li>
<li><code>?names</code> retrieve or modify the names of an atomic vector or list (or columns of a data.frame).</li>
<li><code>?length</code> get the number of elements in an atomic vector</li>
<li><code>?nrow</code>, <code>?ncol</code>, <code>?dim</code> get the dimensions of a n-dimensional object (Won’t work on atomic vectors or lists).</li>
</ul>
<h2 id="data-subsetting"><a href="06-data-subsetting.html">Data subsetting</a></h2>
<ul>
<li>Elements can be accessed by:</li>
<li>Index</li>
<li>Name</li>
<li><code>:</code> to generate a sequence of numbers to extract slices</li>
<li><code>[</code> single square brackets:</li>
<li><em>extract</em> single elements or <em>subset</em>: - vectors</li>
<li><em>extract</em> single elements of a list</li>
<li><em>extract</em> columns from a data.frame</li>
<li><code>[</code> with two arguments to:</li>
<li><em>extract</em> rows and/or columns of
<ul>
<li>matrices</li>
<li>data.frames</li>
</ul></li>
<li><code>[[</code> double square brackets to subset lists</li>
<li><code>$</code> to access columns or list elements by name</li>
<li>negative indices skip elements</li>
</ul>
<h2 id="writing-data"><a href="07-writing-data.html">Writing data</a></h2>
<ul>
<li><code>write.table</code> to write out objects in regular format</li>
<li>set <code>quote=FALSE</code> so that text isn’t wrapped in <code>"</code> marks</li>
</ul>
<h2 id="vectorisation"><a href="08-vectorisation.html">Vectorisation</a></h2>
<ul>
<li>Most functions and operations apply to each element of a vector</li>
<li><code>*</code> applies element-wise to matrices</li>
<li><code>%*%</code> for true matrix multiplication</li>
<li><code>any()</code> will return <code>TRUE</code> if any element of a vector is <code>TRUE</code></li>
<li><code>all()</code> will return <code>TRUE</code> if <em>all</em> elements of a vector are <code>TRUE</code></li>
</ul>
<h2 id="control-flow"><a href="09-control-flow.html">Control flow</a></h2>
<ul>
<li>Use <code>if</code> condition to start a conditional statement, <code>else if</code> condition to provide additional tests, and <code>else</code> to provide a default</li>
<li>The bodies of the branches of conditional statements must be indented.</li>
<li>Use <code>==</code> to test for equality.</li>
<li><code>X && Y</code> is only true if both X and Y are <code>TRUE</code>.</li>
<li><code>X || Y</code> is true if either X or Y, or both, are <code>TRUE</code>.</li>
<li>Zero is considered <code>FALSE</code>; all other numbers are considered <code>TRUE</code></li>
<li>Nest loops to operate on multi-dimensional data.</li>
</ul>
<h2 id="functions"><a href="10-functions.html">Functions</a></h2>
<ul>
<li>Put code whose parameters change frequently in a function, then call it with different parameter values to customize its behavior.</li>
<li>The last line of a function is returned, or you can use <code>return</code> explictly</li>
<li>Any code written in the body of the function is isolated to the function when it is called.</li>
<li>Document Why, then What, then lastly How (if the code isn’t self explanatory)</li>
</ul>
<h2 id="split-apply-combine"><a href="11-plyr.html">Split-apply-combine</a></h2>
<ul>
<li>Use the <code>xxply</code> family of functions to apply functions to groups within some data.</li>
<li>the first letter, <code>a</code>rray , <code>d</code>ata.frame or <code>l</code>ist corresponds to the input data</li>
<li>the second letter denotes the output data structure</li>
<li>Anonymous functions (those not assigned a name) are used inside the <code>plyr</code> family of functions on groups within data.</li>
</ul>
<h2 id="ggplot2"><a href="12-ggplot2.html">GGplot2</a></h2>
<ul>
<li>figures can be created with the grammar of graphics:</li>
<li><code>ggplot</code> to create the base figure</li>
<li><code>aes</code>thetics specify the data axes, shape, color, and data size</li>
<li><code>geom</code>etry functions specify the type of plot, e.g. <code>point</code>, <code>line</code>, <code>density</code>, <code>box</code></li>
<li><code>geom</code>etry functions also add statistical transforms, e.g. <code>geom_smooth</code></li>
<li><code>scale</code> functions change the mapping from data to aesthetics</li>
<li><code>facet</code> functions stratify the figure into panels</li>
<li><code>aes</code>thetics apply to individual layers, or can be set for the whole plot inside <code>ggplot</code>.</li>
<li><code>theme</code> functions change the overall look of the plot</li>
<li>order of layers matters!</li>
<li><code>ggsave</code> to save a figure.</li>
</ul>
<h2 id="defensive-programming">Defensive Programming</h2>
<ul>
<li>Program defensively, i.e., assume that errors are going to arise, and write code to detect them when they do.</li>
<li>Write tests before writing code in order to help determine exactly what that code is supposed to do.</li>
<li>Know what code is supposed to do before trying to debug it.</li>
<li>Make it fail every time.</li>
<li>Make it fail fast.</li>
<li>Change one thing at a time, and for a reason.</li>
<li>Keep track of what you’ve done.</li>
<li>Be humble</li>
</ul>
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