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<html><head><title>Loops</title><meta name="generator" content="DocBook XSL Stylesheets V1.40"/></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="section"><a name="dsssl.expr.loop"/><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="dsssl.expr.loop"/>Loops</h2></div><hr/></div><p>
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DSSSL doesn't have any construct that resembles the
for loop that occurs in most imperative languages like C
and Java. Instead, DSSSL employs a common trick in
functional languages for implementing a loop: tail recursion.
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Loops in DSSSL use a special form of
<tt>let</tt>. This loop counts from 1 to 10:
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(let <a name="dl1"/><img src="../images/callouts/1.png" alt="1" border="0"/>loopvar <a name="dl2"/><img src="../images/callouts/2.png" alt="2" border="0"/>((count 1))
<a name="dl3"/><img src="../images/callouts/3.png" alt="3" border="0"/>(if (> count 10)
<a name="dl4"/><img src="../images/callouts/4.png" alt="4" border="0"/>#t
(<a name="dl5"/><img src="../images/callouts/5.png" alt="5" border="0"/>loopvar <a name="dl6"/><img src="../images/callouts/6.png" alt="6" border="0"/>(+ count 1))))</pre></p><div class="calloutlist"><a name="id2888767"/><table border="0" summary="Callout list"><tr><td width="5%" valign="top" align="left"><a name="id2888774"/><a href="#dl1"><img src="../images/callouts/1.png" alt="1" border="0"/></a> </td><td valign="top" align="left"><p>This variable controls the loop. It is declared without an
initial value, immediately after the <tt>let</tt>
operand.</p></td></tr><tr><td width="5%" valign="top" align="left"><a name="id2767726"/><a href="#dl2"><img src="../images/callouts/2.png" alt="2" border="0"/></a> </td><td valign="top" align="left"><p>
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Any number of additional local variables can be defined after
the loop variable, just as they can in any other
<tt>let</tt> expression.</p></td></tr><tr><td width="5%" valign="top" align="left"><a name="id2862526"/><a href="#dl3"><img src="../images/callouts/3.png" alt="3" border="0"/></a> </td><td valign="top" align="left"><p>If you ever want the loop to end, you have to put some sort of a
test in it.</p></td></tr><tr><td width="5%" valign="top" align="left"><a name="id2862560"/><a href="#dl4"><img src="../images/callouts/4.png" alt="4" border="0"/></a> </td><td valign="top" align="left"><p>This is the value that will be returned.</p></td></tr><tr><td width="5%" valign="top" align="left"><a name="id2770562"/><a href="#dl5"><img src="../images/callouts/5.png" alt="5" border="0"/></a> </td><td valign="top" align="left"><p>Note that you iterate the loop by using the loop variable as if
it was a function name.</p></td></tr><tr><td width="5%" valign="top" align="left"><a name="id2770597"/><a href="#dl6"><img src="../images/callouts/6.png" alt="6" border="0"/></a> </td><td valign="top" align="left"><p>The arguments to this function are the values that
you want the local variables declared in <a href="#dl2" title=""><img src="../images/callouts/2.png" alt="2" border="0"/></a> to have
in the next iteration.</p></td></tr></table></div></div></body></html>