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<title>Introducing magrittr</title>
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<body>
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<h1 class="title toc-ignore">Introducing magrittr</h1>
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<h4 class="author">Stefan Milton Bache</h4>
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<h4 class="date">November, 2014</h4>
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<div id="abstract" class="section level1">
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<h1>Abstract</h1>
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<p>The <em>magrittr</em> (to be pronounced with a sophisticated french accent) package has two aims: decrease development time and improve readability and maintainability of code. Or even shortr: make your code smokin’ (puff puff)!</p>
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<p>To achieve its humble aims, <em>magrittr</em> (remember the accent) provides a new “pipe”-like operator, <code>%>%</code>, with which you may pipe a value forward into an expression or function call; something along the lines of <code>x %>% f</code>, rather than <code>f(x)</code>. This is not an unknown feature elsewhere; a prime example is the <code>|></code> operator used extensively in <code>F#</code> (to say the least) and indeed this – along with Unix pipes – served as a motivation for developing the magrittr package.</p>
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<p>This vignette describes the main features of <em>magrittr</em> and demonstrates some features which have been added since the initial release.</p>
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</div>
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<div id="introduction-and-basics" class="section level1">
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<h1>Introduction and basics</h1>
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<p>At first encounter, you may wonder whether an operator such as <code>%>%</code> can really be all that beneficial; but as you may notice, it semantically changes your code in a way that makes it more intuitive to both read and write.</p>
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<p>Consider the following example, in which the <code>mtcars</code> dataset shipped with R is munged a little:</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(magrittr)</span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>car_data <span class="ot"><-</span> </span>
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<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> mtcars <span class="sc">%>%</span></span>
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<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">subset</span>(hp <span class="sc">></span> <span class="dv">100</span>) <span class="sc">%>%</span></span>
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<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">aggregate</span>(. <span class="sc">~</span> cyl, <span class="at">data =</span> ., <span class="at">FUN =</span> . <span class="sc">%>%</span> mean <span class="sc">%>%</span> <span class="fu">round</span>(<span class="dv">2</span>)) <span class="sc">%>%</span></span>
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<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> <span class="fu">transform</span>(<span class="at">kpl =</span> mpg <span class="sc">%>%</span> <span class="fu">multiply_by</span>(<span class="fl">0.4251</span>)) <span class="sc">%>%</span></span>
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<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> print</span>
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<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a><span class="co">#> cyl mpg disp hp drat wt qsec vs am gear carb kpl</span></span>
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<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a><span class="co">#> 1 4 25.90 108.05 111.00 3.94 2.15 17.75 1.00 1.00 4.50 2.00 11.010090</span></span>
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<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a><span class="co">#> 2 6 19.74 183.31 122.29 3.59 3.12 17.98 0.57 0.43 3.86 3.43 8.391474</span></span>
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<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a><span class="co">#> 3 8 15.10 353.10 209.21 3.23 4.00 16.77 0.00 0.14 3.29 3.50 6.419010</span></span></code></pre></div>
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<p>We start with a value, here <code>mtcars</code> (a <code>data.frame</code>). From there, we extract a subset, aggregate the information based on the number of cylinders, and then transform the dataset by adding a variable for kilometers per liter as a supplement to miles per gallon. Finally we print the result before assigning it. Note how the code is arranged in the logical order of how you think about the task: data->transform->aggregate, which is also the same order as the code will execute. It’s like a recipe – easy to read, easy to follow!</p>
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<p>A horrific alternative would be to write:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>car_data <span class="ot"><-</span> </span>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">transform</span>(<span class="fu">aggregate</span>(. <span class="sc">~</span> cyl, </span>
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<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> <span class="at">data =</span> <span class="fu">subset</span>(mtcars, hp <span class="sc">></span> <span class="dv">100</span>), </span>
|
||
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> <span class="at">FUN =</span> <span class="cf">function</span>(x) <span class="fu">round</span>(<span class="fu">mean</span>(x), <span class="dv">2</span>)), </span>
|
||
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a> <span class="at">kpl =</span> mpg<span class="sc">*</span><span class="fl">0.4251</span>)</span></code></pre></div>
|
||
<p>There is a lot more clutter with parentheses, and the mental task of deciphering the code is more challenging—particularly if you did not write it yourself.</p>
|
||
<p>Note also how “building” a function on the fly for use in <code>aggregate</code> is very simple in <em>magrittr</em>: rather than an actual value as the left-hand side in the pipeline, just use the placeholder. This is also very useful in R’s <code>*apply</code> family of functions.</p>
|
||
<p>Granted, you may make the second example better, perhaps throw in a few temporary variables (which is often avoided to some degree when using <em>magrittr</em>), but one often sees cluttered lines like the ones presented.</p>
|
||
<p>And here is another selling point: suppose I want to quickly add another step somewhere in the process. This is very easy to do in the pipeline version, but a little more challenging in the “standard” example.</p>
|
||
<p>The combined example shows a few neat features of the pipe (which it is not):</p>
|
||
<ol style="list-style-type: decimal">
|
||
<li>By default the left-hand side (LHS) will be <em>piped in</em> as the first argument of the function appearing on the right-hand side (RHS). This is the case in the <code>subset</code> and <code>transform</code> expressions.</li>
|
||
<li><code>%>%</code> may be used in a nested fashion, e.g. it may appear in expressions within arguments. This is illustrated in the <code>mpg</code> to <code>kpl</code> conversion.</li>
|
||
<li>When the LHS is needed at a position other than the first, one can use the dot,<code>'.'</code>, as placeholder. This is shown in the <code>aggregate</code> expression.</li>
|
||
<li>The dot in e.g. a formula is <em>not</em> confused with a placeholder, which is utilized in the <code>aggregate</code> expression.</li>
|
||
<li>Whenever only <em>one</em> argument (the LHS) is needed, one can omit the empty parentheses. This is shown in the call to <code>print</code> (which also returns its argument). Here, <code>LHS %>% print()</code>, or even <code>LHS %>% print(.)</code> would also work.</li>
|
||
<li>A pipeline with a dot (<code>.</code>) as the LHS will create a unary function. This is used to define the aggregator function.</li>
|
||
</ol>
|
||
<p>One feature, which was not demonstrated above is piping into <em>anonymous functions</em>, or <em>lambdas</em>. This is possible using standard function definitions, e.g.:</p>
|
||
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>car_data <span class="sc">%>%</span></span>
|
||
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>(<span class="cf">function</span>(x) {</span>
|
||
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> (<span class="fu">nrow</span>(x) <span class="sc">></span> <span class="dv">2</span>) </span>
|
||
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">rbind</span>(<span class="fu">head</span>(x, <span class="dv">1</span>), <span class="fu">tail</span>(x, <span class="dv">1</span>))</span>
|
||
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span> x</span>
|
||
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>})</span></code></pre></div>
|
||
<p>However, <em>magrittr</em> also allows a short-hand notation:</p>
|
||
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>car_data <span class="sc">%>%</span></span>
|
||
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>{ </span>
|
||
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> (<span class="fu">nrow</span>(.) <span class="sc">></span> <span class="dv">0</span>)</span>
|
||
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">rbind</span>(<span class="fu">head</span>(., <span class="dv">1</span>), <span class="fu">tail</span>(., <span class="dv">1</span>))</span>
|
||
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span> .</span>
|
||
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a>}</span>
|
||
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="co">#> cyl mpg disp hp drat wt qsec vs am gear carb kpl</span></span>
|
||
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a><span class="co">#> 1 4 25.9 108.05 111.00 3.94 2.15 17.75 1 1.00 4.50 2.0 11.01009</span></span>
|
||
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a><span class="co">#> 3 8 15.1 353.10 209.21 3.23 4.00 16.77 0 0.14 3.29 3.5 6.41901</span></span></code></pre></div>
|
||
<p>Since all right-hand sides are really “body expressions” of unary functions, this is only the natural extension of the simple right-hand side expressions. Of course, longer and more complex functions can be made using this approach.</p>
|
||
<p>In the first example, the anonymous function is enclosed in parentheses. Whenever you want to use a function- or call-generating statement as right-hand side, parentheses are used to evaluate the right-hand side before piping takes place.</p>
|
||
<p>Another, less useful example is:</p>
|
||
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="dv">1</span><span class="sc">:</span><span class="dv">10</span> <span class="sc">%>%</span> (<span class="fu">substitute</span>(<span class="fu">f</span>(), <span class="fu">list</span>(<span class="at">f =</span> sum)))</span>
|
||
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="co">#> [1] 55</span></span></code></pre></div>
|
||
</div>
|
||
<div id="additional-pipe-operators" class="section level1">
|
||
<h1>Additional pipe operators</h1>
|
||
<p><em>magrittr</em> also provides three related pipe operators. These are not as common as <code>%>%</code> but they become useful in special cases.</p>
|
||
<p>The “tee” pipe, <code>%T>%</code> works like <code>%>%</code>, except it returns the left-hand side value, and not the result of the right-hand side operation. This is useful when a step in a pipeline is used for its side-effect (printing, plotting, logging, etc.). As an example (where the actual plot is omitted here):</p>
|
||
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="fu">rnorm</span>(<span class="dv">200</span>) <span class="sc">%>%</span></span>
|
||
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="fu">matrix</span>(<span class="at">ncol =</span> <span class="dv">2</span>) <span class="sc">%T>%</span></span>
|
||
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>plot <span class="sc">%>%</span> <span class="co"># plot usually does not return anything. </span></span>
|
||
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>colSums</span>
|
||
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a><span class="co">#> [1] -4.018676 -27.018219</span></span></code></pre></div>
|
||
<p>The “exposition” pipe, <code>%$%</code> exposes the names within the left-hand side object to the right-hand side expression. Essentially, it is a short-hand for using the <code>with</code> functions (and the same left-hand side objects are accepted). This operator is handy when functions do not themselves have a data argument, as for example <code>lm</code> and <code>aggregate</code> do. Here are a few examples as illustration:</p>
|
||
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>iris <span class="sc">%>%</span></span>
|
||
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">subset</span>(Sepal.Length <span class="sc">></span> <span class="fu">mean</span>(Sepal.Length)) <span class="sc">%$%</span></span>
|
||
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">cor</span>(Sepal.Length, Sepal.Width)</span>
|
||
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a> </span>
|
||
<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a><span class="fu">data.frame</span>(<span class="at">z =</span> <span class="fu">rnorm</span>(<span class="dv">100</span>)) <span class="sc">%$%</span> </span>
|
||
<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">ts.plot</span>(z)</span></code></pre></div>
|
||
<p>Finally, the “assignment” pipe <code>%<>%</code> can be used as the first pipe in a chain. The effect will be that the result of the pipeline is assigned to the left-hand side object, rather than returning the result as usual. It is essentially shorthand notation for expressions like <code>foo <- foo %>% bar %>% baz</code>, which boils down to <code>foo %<>% bar %>% baz</code>. Another example is:</p>
|
||
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>iris<span class="sc">$</span>Sepal.Length <span class="sc">%<>%</span> sqrt</span></code></pre></div>
|
||
<p>The <code>%<>%</code> can be used whenever <code>expr <- ...</code> makes sense, e.g. </p>
|
||
<ul>
|
||
<li><code>x %<>% foo %>% bar</code></li>
|
||
<li><code>x[1:10] %<>% foo %>% bar</code></li>
|
||
<li><code>x$baz %<>% foo %>% bar</code></li>
|
||
</ul>
|
||
</div>
|
||
<div id="aliases" class="section level1">
|
||
<h1>Aliases</h1>
|
||
<p>In addition to the <code>%>%</code>-operator, <em>magrittr</em> provides some aliases for other operators which make operations such as addition or multiplication fit well into the <em>magrittr</em>-syntax. As an example, consider:</p>
|
||
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="fu">rnorm</span>(<span class="dv">1000</span>) <span class="sc">%>%</span></span>
|
||
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a><span class="fu">multiply_by</span>(<span class="dv">5</span>) <span class="sc">%>%</span></span>
|
||
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a><span class="fu">add</span>(<span class="dv">5</span>) <span class="sc">%>%</span></span>
|
||
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a>{ </span>
|
||
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"Mean:"</span>, <span class="fu">mean</span>(.), </span>
|
||
<span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a> <span class="st">"Variance:"</span>, <span class="fu">var</span>(.), <span class="st">"</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
||
<span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a> <span class="fu">head</span>(.)</span>
|
||
<span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a>}</span>
|
||
<span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a><span class="co">#> Mean: 5.06493 Variance: 27.05389</span></span>
|
||
<span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a><span class="co">#> [1] 2.477662 -3.999358 8.624820 5.322125 3.014306 10.204276</span></span></code></pre></div>
|
||
<p>which could be written in more compact form as:</p>
|
||
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="fu">rnorm</span>(<span class="dv">100</span>) <span class="sc">%>%</span> <span class="st">`</span><span class="at">*</span><span class="st">`</span>(<span class="dv">5</span>) <span class="sc">%>%</span> <span class="st">`</span><span class="at">+</span><span class="st">`</span>(<span class="dv">5</span>) <span class="sc">%>%</span> </span>
|
||
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a>{</span>
|
||
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">cat</span>(<span class="st">"Mean:"</span>, <span class="fu">mean</span>(.), <span class="st">"Variance:"</span>, <span class="fu">var</span>(.), <span class="st">"</span><span class="sc">\n</span><span class="st">"</span>)</span>
|
||
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">head</span>(.)</span>
|
||
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
|
||
<p>To see a list of the aliases, execute e.g. <code>?multiply_by</code>.</p>
|
||
</div>
|
||
<div id="development" class="section level1">
|
||
<h1>Development</h1>
|
||
<p>The <em>magrittr</em> package is also available in a development version at the GitHub development page: <a href="https://github.com/tidyverse/magrittr">github.com/tidyverse/magrittr</a>.</p>
|
||
</div>
|
||
|
||
|
||
|
||
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|
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