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<h1 id="conditionals">Conditionals</h1>
<div class="sourceCode" id="cb1"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb1-1" title="1">(<span class="kw">if</span> (<span class="kw">evenp</span> <span class="dv">7</span>)</a>
<a class="sourceLine" id="cb1-2" title="2"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"7 is even~%"</span>)</a>
<a class="sourceLine" id="cb1-3" title="3"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"7 is odd~%"</span>))</a>
<a class="sourceLine" id="cb1-4" title="4"><span class="co">;; => 7 is odd</span></a>
<a class="sourceLine" id="cb1-5" title="5"><span class="co">;;</span></a>
<a class="sourceLine" id="cb1-6" title="6"><span class="co">;;</span></a>
<a class="sourceLine" id="cb1-7" title="7"><span class="co">;; NIL</span></a></code></pre></div>
<ul>
<li><code>p</code> at the end of <code>evenp</code> function indicates that it is a “predicate” (i.e. returns <code>t</code> or <code>nil</code>).</li>
<li>There are no then or else - the second expression is evaluated if the condition is not <code>nil</code>, the third is evaluated if it is <code>nil</code>.</li>
</ul>
<div class="sourceCode" id="cb2"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb2-1" title="1">(<span class="kw">when</span> (<span class="op">=</span> <span class="dv">0</span> (<span class="kw">mod</span> <span class="dv">8</span> <span class="dv">4</span>))</a>
<a class="sourceLine" id="cb2-2" title="2"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"8 is divisible by 4~%"</span>)</a>
<a class="sourceLine" id="cb2-3" title="3"> (<span class="kw">princ</span> <span class="st">"Not, but really!"</span>))</a>
<a class="sourceLine" id="cb2-4" title="4"><span class="co">;; => 8 is divisible by 4</span></a>
<a class="sourceLine" id="cb2-5" title="5"><span class="co">;; Not, but really!</span></a>
<a class="sourceLine" id="cb2-6" title="6"><span class="co">;;</span></a>
<a class="sourceLine" id="cb2-7" title="7"><span class="co">;; "Not, but really!"</span></a></code></pre></div>
<ul>
<li><code>when</code> lets you run multiple expressions given the condition is not <code>nil</code></li>
<li><code>if</code> would be perfectly valid for a single expression but <code>when</code> is more idiomatic</li>
</ul>
<div class="sourceCode" id="cb3"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb3-1" title="1">(<span class="kw">cond</span> ((<span class="op"><</span> <span class="dv">9</span> <span class="dv">0</span>)</a>
<a class="sourceLine" id="cb3-2" title="2"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"9 is negative"</span>))</a>
<a class="sourceLine" id="cb3-3" title="3"> ((<span class="op"><</span> <span class="dv">9</span> <span class="dv">10</span>)</a>
<a class="sourceLine" id="cb3-4" title="4"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"9 has 1 digit"</span>))</a>
<a class="sourceLine" id="cb3-5" title="5"> (<span class="kw">t</span></a>
<a class="sourceLine" id="cb3-6" title="6"> (<span class="kw">format</span> <span class="kw">t</span> <span class="st">"9 has multiple digits"</span>)))</a>
<a class="sourceLine" id="cb3-7" title="7"><span class="co">;; => 9 has 1 digit</span></a>
<a class="sourceLine" id="cb3-8" title="8"><span class="co">;;</span></a>
<a class="sourceLine" id="cb3-9" title="9"><span class="co">;; NIL</span></a></code></pre></div>
<ul>
<li>Multiple conditions can be written with <code>cond</code>. The first condition that is not <code>nil</code> will be evaluated and returned.</li>
<li>‘Else’, as implemented in other languages can be written as the symbol <code>t</code> which always evaluates to <code>t</code>, <em>which is not <code>nil</code></em>.</li>
</ul>
<div class="sourceCode" id="cb4"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb4-1" title="1">(<span class="kw">defun</span><span class="fu"> what-am-i </span>(i)</a>
<a class="sourceLine" id="cb4-2" title="2"> (<span class="kw">cond</span> ((<span class="kw">stringp</span> i)</a>
<a class="sourceLine" id="cb4-3" title="3"> <span class="st">"I am a string!!"</span>)</a>
<a class="sourceLine" id="cb4-4" title="4"> ((<span class="kw">integerp</span> i)</a>
<a class="sourceLine" id="cb4-5" title="5"> <span class="st">"I am an integer!!"</span>)</a>
<a class="sourceLine" id="cb4-6" title="6"> ((<span class="kw">listp</span> i)</a>
<a class="sourceLine" id="cb4-7" title="7"> <span class="st">"I am a list!!"</span>)))</a>
<a class="sourceLine" id="cb4-8" title="8"><span class="co">;; => WHAT-AM-I</span></a>
<a class="sourceLine" id="cb4-9" title="9"></a>
<a class="sourceLine" id="cb4-10" title="10">(what-am-i <span class="dv">1</span>)</a>
<a class="sourceLine" id="cb4-11" title="11"><span class="co">;; => "I am an integer!!"</span></a>
<a class="sourceLine" id="cb4-12" title="12">(what-am-i <span class="st">"blah-blah"</span>)</a>
<a class="sourceLine" id="cb4-13" title="13"><span class="co">;; => "I am a string!!"</span></a>
<a class="sourceLine" id="cb4-14" title="14">(what-am-i (<span class="kw">list</span> <span class="dv">1</span> <span class="dv">2</span> <span class="dv">3</span>))</a>
<a class="sourceLine" id="cb4-15" title="15"><span class="co">;; => "I am a list!!"</span></a>
<a class="sourceLine" id="cb4-16" title="16">(what-am-i <span class="dv">1</span>/<span class="dv">2</span>)</a>
<a class="sourceLine" id="cb4-17" title="17"><span class="co">;; => NIL</span></a></code></pre></div>
<ul>
<li><code>cond</code> is an expression</li>
<li>There are built-in functions to test the types of variables. As expected (haha) they end in “p”</li>
<li>If none of the conditions match, <code>nil</code> is returned</li>
</ul>
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