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    <title>Booleans</title>
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    <section class="sect1" title="Booleans" epub:type="subchapter" id="Tutorial---Booleans">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both">Booleans</h2>
          </div>
        </div>
      </div>
      <p>Scheme uses the syntax <code class="literal">#t</code> and <code class="literal">#f</code>
for Boolean true and false value, respectively.  For example, the
“less-than” function is named <code class="literal">&lt;</code>.
Its result is true if the first argument is less than the second (or, if
there are more than two arguments, that they are in increasing order):
</p>
      <pre class="screen">(&lt; 3 4) ⇒ #t
(&lt; -3 -4) ⇒ #f
(&lt; 2 3 5 7 11)) ⇒ #t
</pre>
      <p>The <code class="literal">if</code> special form takes two or three sub-expressions:
It evaluates the first expression.
If that is true it evaluates the second expression;
otherwise it evaluates the third expression, if provided:
</p>
      <pre class="screen">(if (&lt; 3 4) (+ 5 5) (+ 5 6)) ⇒ 10
</pre>
      <p>We call <code class="literal">if</code> a special form rather than a function,
because for a function all the arguments are evaluated before the
function is called, but in a special form that is not neceassarily the case.
</p>
      <p>In addition to <code class="literal">#t</code> any value except <code class="literal">#f</code>
counts as “true” when evaluating the first expression of an <code class="literal">if</code>:
</p>
      <pre class="screen">(if 0 (+ 5 5) (+ 5 6)) ⇒ 11
</pre>
      <p>You can use <code class="literal">and</code>, <code class="literal">or</code>,
and <code class="literal">not</code> to create complex boolean expressions.
Of these <code class="literal">and</code> and <code class="literal">or</code>
are special forms that only evaluate as many of the sub-expressions as needed.
</p>
      <pre class="screen">(if (not (and (&gt;= i 0) (&lt;= i 9)))
    (display "error"))
</pre>
      <p>You can use the <code class="literal">cond</code> form as an alternative to
<code class="literal">if</code>:
</p>
      <pre class="screen">(cond ((&lt; 3 3) 'greater)
      ((&gt; 3 3) 'less)
      (else ’equal))       ⇒ equal
</pre>
      <p>The null value (written as <code class="literal">#!null</code> in Kawa or <code class="literal">null</code> in Java)
is also considered as false.
</p>
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