487 lines
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487 lines
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</head>
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<body>
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<h1 class="title toc-ignore">Custom knitr language engines</h1>
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<p>Glue provides a few <a href="https://bookdown.org/yihui/rmarkdown-cookbook/custom-engine.html#custom-engine">custom
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language engines</a> for knitr, which allows you to use glue directly in
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knitr chunks.</p>
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<div id="glue-engine" class="section level2">
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<h2><code>glue</code> engine</h2>
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<p>The first engine is the <code>glue</code> engine, which evaluates the
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chunk contents as a glue template.</p>
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<pre class="glue"><code>1 + 1 = {1 + 1}</code></pre>
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<pre><code>## 1 + 1 = 2</code></pre>
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<p>Maybe the most useful use of the <code>glue</code> engine is to set
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the knitr option <code>results = 'asis'</code> and output markdown or
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HTML directly into the document.</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode markdown"><code class="sourceCode markdown"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a><span class="in">```{glue, results = 'asis', echo = FALSE}</span></span>
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<span id="cb3-2"><a href="#cb3-2" tabindex="-1"></a><span class="in">#### mtcars has **{nrow(mtcars)} rows** and _{ncol(mtcars)} columns_.</span></span>
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<span id="cb3-3"><a href="#cb3-3" tabindex="-1"></a><span class="in">```</span></span></code></pre></div>
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<div id="mtcars-has-32-rows-and-11-columns." class="section level4">
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<h4>mtcars has <strong>32 rows</strong> and <em>11 columns</em>.</h4>
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<p>If you want to pass additional arguments into the glue call, simply
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include them as chunk options.</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode markdown"><code class="sourceCode markdown"><span id="cb4-1"><a href="#cb4-1" tabindex="-1"></a><span class="in">```{glue, .open = "<<", .close = ">>", results = 'asis', echo = FALSE}</span></span>
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<span id="cb4-2"><a href="#cb4-2" tabindex="-1"></a><span class="in">The **median waiting time** between eruptions is <<median(faithful$waiting)>>.</span></span>
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<span id="cb4-3"><a href="#cb4-3" tabindex="-1"></a><span class="in">```</span></span></code></pre></div>
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<p>The <strong>median waiting time</strong> between eruptions is 76.</p>
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</div>
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</div>
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<div id="glue_sql-engine" class="section level2">
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<h2><code>glue_sql</code> engine</h2>
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<p>The second engine is <code>glue_sql</code>, which will use
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<code>glue::glue_sql()</code> to generate a SQL query and then run the
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query using the <a href="https://bookdown.org/yihui/rmarkdown/language-engines.html#sql">sql
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engine</a>.</p>
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<p>First we create a new connection to an in-memory SQLite database, and
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write a new table to it.</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a>con <span class="ot"><-</span> DBI<span class="sc">::</span><span class="fu">dbConnect</span>(RSQLite<span class="sc">::</span><span class="fu">SQLite</span>(), <span class="st">":memory:"</span>)</span>
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<span id="cb5-2"><a href="#cb5-2" tabindex="-1"></a>mtcars<span class="sc">$</span>model <span class="ot"><-</span> <span class="fu">rownames</span>(mtcars)</span>
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<span id="cb5-3"><a href="#cb5-3" tabindex="-1"></a>DBI<span class="sc">::</span><span class="fu">dbWriteTable</span>(con, <span class="st">"mtcars"</span>, mtcars)</span></code></pre></div>
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<p>Next define some variables we that we can use with glue to
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interpolate.</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" tabindex="-1"></a>var <span class="ot"><-</span> <span class="st">"mpg"</span></span>
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<span id="cb6-2"><a href="#cb6-2" tabindex="-1"></a>tbl <span class="ot"><-</span> <span class="st">"mtcars"</span></span>
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<span id="cb6-3"><a href="#cb6-3" tabindex="-1"></a>num <span class="ot"><-</span> <span class="dv">150</span></span></code></pre></div>
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<p>Then we can use <code>glue_sql</code> to construct and run a query
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using those variables into that database. <em>Note</em> you need to
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provide the connection object as a <code>connection</code> chunk
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option.</p>
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<p>In this example there are two type of quotes. The first is a bare
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backtick, these are passed directly to the SQL engine unchanged. The
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second is backticks inside of braces, which are specially interpreted to
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do the proper quoting for the given SQL engine by glue. In this example
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we use the <code>sqlite</code> engine, which uses backticks for quoting,
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but you would use the same backticks inside brace syntax for postgreSQL,
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and <code>glue_sql()</code> would automatically use double quotes for
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quoting instead.</p>
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<div class="sourceCode" id="cb7"><pre class="sourceCode markdown"><code class="sourceCode markdown"><span id="cb7-1"><a href="#cb7-1" tabindex="-1"></a><span class="in">```{glue_sql, connection = con}</span></span>
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<span id="cb7-2"><a href="#cb7-2" tabindex="-1"></a><span class="in">SELECT `model`, `hp`, {`var`}</span></span>
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<span id="cb7-3"><a href="#cb7-3" tabindex="-1"></a><span class="in">FROM {`tbl`}</span></span>
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<span id="cb7-4"><a href="#cb7-4" tabindex="-1"></a><span class="in">WHERE {`tbl`}.`hp` > {num}</span></span>
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<span id="cb7-5"><a href="#cb7-5" tabindex="-1"></a><span class="in">```</span></span></code></pre></div>
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<div class="sourceCode" id="cb8"><pre class="sourceCode sql"><code class="sourceCode sql"><span id="cb8-1"><a href="#cb8-1" tabindex="-1"></a><span class="kw">SELECT</span> `model`, `hp`, `mpg`</span>
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<span id="cb8-2"><a href="#cb8-2" tabindex="-1"></a><span class="kw">FROM</span> `mtcars`</span>
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<span id="cb8-3"><a href="#cb8-3" tabindex="-1"></a><span class="kw">WHERE</span> `mtcars`.`hp` <span class="op">></span> <span class="dv">150</span></span></code></pre></div>
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<div class="knitsql-table">
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<table>
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<caption>Displaying records 1 - 10</caption>
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<thead>
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<tr class="header">
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<th align="left">model</th>
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<th align="right">hp</th>
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<th align="right">mpg</th>
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</tr>
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</thead>
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<tbody>
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<tr class="odd">
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<td align="left">Hornet Sportabout</td>
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<td align="right">175</td>
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<td align="right">18.7</td>
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</tr>
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<tr class="even">
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<td align="left">Duster 360</td>
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<td align="right">245</td>
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<td align="right">14.3</td>
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</tr>
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<tr class="odd">
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<td align="left">Merc 450SE</td>
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<td align="right">180</td>
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<td align="right">16.4</td>
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</tr>
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<tr class="even">
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<td align="left">Merc 450SL</td>
|
|
<td align="right">180</td>
|
|
<td align="right">17.3</td>
|
|
</tr>
|
|
<tr class="odd">
|
|
<td align="left">Merc 450SLC</td>
|
|
<td align="right">180</td>
|
|
<td align="right">15.2</td>
|
|
</tr>
|
|
<tr class="even">
|
|
<td align="left">Cadillac Fleetwood</td>
|
|
<td align="right">205</td>
|
|
<td align="right">10.4</td>
|
|
</tr>
|
|
<tr class="odd">
|
|
<td align="left">Lincoln Continental</td>
|
|
<td align="right">215</td>
|
|
<td align="right">10.4</td>
|
|
</tr>
|
|
<tr class="even">
|
|
<td align="left">Chrysler Imperial</td>
|
|
<td align="right">230</td>
|
|
<td align="right">14.7</td>
|
|
</tr>
|
|
<tr class="odd">
|
|
<td align="left">Camaro Z28</td>
|
|
<td align="right">245</td>
|
|
<td align="right">13.3</td>
|
|
</tr>
|
|
<tr class="even">
|
|
<td align="left">Pontiac Firebird</td>
|
|
<td align="right">175</td>
|
|
<td align="right">19.2</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
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