><code class="sourceCode r"><a class="sourceLine" id="cb7-1" data-line-number="1">x1 &lt;-<span class="st"> </span><span class="kw"><a href="https://www.rdocumentation.org/packages/base/topics/data.frame">data.frame</a></span>(<span class="dt">id =</span> <span class="dv">1</span><span class="op">:</span><span class="dv">4</span>, <span class="dt">bla=</span>letters[<span class="dv">1</span><span class="op">:</span><span class="dv">4</span>],</a>
<a class="sourceLine" id="cb7-2" data-line-number="2">       