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<html><head><title>XLISP do</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
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<a href="reference-index.htm">Reference</a>
<hr>
<h1>do</h1>
<hr>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>(do ([<i>binding</i> ... ]) (<i>test-expr</i> [<i>result</i>]) [<i>expr</i> ... ])</dt>
<dd><i>binding</i> - a variable binding which is can take one of the following forms:<br>
<dl><dd><i>symbol</i><br>
(<i>symbol init-expr</i> [<i>step-expr</i>])
<dl><dd><i>symbol</i> - a symbol<br>
<i>init-expr</i> - an initialization expression for <i>symbol</i><br>
<i>step-expr</i> - an expression with that <i>symbol</i> is updated
at the end of each loop</dd></dl></dd></dl>
<i>test-expr</i> - iteration ends when this expression returns a
non-<a href="nil.htm">NIL</a> value<br>
<i>result</i> - an optional expression for the returned result<br>
<i>expr</i> - expressions comprising the body of the loop which may
contain <a href="return.htm">return</a>s,
<a href="go.htm">go</a>s or tags for
<a href="go.htm">go</a><br>
returns - the value of the last result expression</dd>
</dl>
<h2>Description</h2>
<p>The 'do' special form is basically a 'while' looping construct that
contains symbols [with optional initializations and updates], a loop test
[with an optional return value] and a block of code [expressions] to
evaluate. The 'do' form evaluates its initializations and updates in no
specified order [as opposed to <a href="do-star.htm">do*</a> which
does it in sequential order]. The sequence of these events is:</p>
<pre>
<i>init-expr</i> execution
while not <i>test-expr</i> do
loop code execution
<i>step-expr</i> execution
end-while
return <i>result</i>
</pre>
<p> The first form after the 'do' is the 'binding' form. It contains a
series of 'symbols' or 'bindings'. The 'binding' is a 'symbol' followed by
an initialization expression 'init-expr' and an optional 'step-expr'. If
there is no 'init-expr', the 'symbol' will be initialized to
<a href="nil.htm">NIL</a>. There is no specification as to the
order of execution of the bindings or the step expressions, except that
they happen all together.</p>
<p>The 'do' form will go through and create and initialize the symbols.
This is followed by evaluating the 'test-expr'. If 'test-expr' returns a
non-<a href="nil.htm">NIL</a> value, the loop will terminate.
If 'test-expr' returns a <a href="nil.htm">NIL</a> value then
the 'do' will sequentially execute the 'exprs'. After execution of the loop
'exprs', the 'symbols' are set to the 'step-exprs' [if the 'step-exprs'
exist]. Then, the 'test-expr' is re-evaluated, and so on. The value of the
'result' expression is evaluated and returned. If no 'result' is specified,
<a href="nil.htm">NIL</a> is returned. When the 'do' is finished
execution, the 'symbol's that were defined will no longer exist or retain
their values.</p>
<h2>Examples</h2>
<pre class="example">
(do (i) <font color="#008844">; DO loop with var I</font>
((eql i 0) "done") <font color="#008844">; test and result</font>
(print i) (setq i 0) (print i)) <font color="#008844">; prints NIL 0</font>
<font color="#008844">; returns "done"</font>
(do (i) <font color="#008844">; DO loop with var I</font>
((eql i 0)) <font color="#008844">; test but no result</font>
(print i) (setq i 0) (print i)) <font color="#008844">; prints NIL 0</font>
<font color="#008844">; returns NIL</font>
(do <font color="#008844">; DO loop</font>
((i 0 (setq i (1+ i))) <font color="#008844">; var I=0 increment by 1</font>
(j 10 (setq j (1- j)))) <font color="#008844">; var J=10 decrement by 1</font>
((eql i j) "met in the middle") <font color="#008844">; test and result</font>
(princ i) (princ " ") <font color="#008844">; prints 0 10</font>
(princ j) (terpri)) <font color="#008844">; 1 9</font>
<font color="#008844">; 2 8</font>
<font color="#008844">; 3 7</font>
<font color="#008844">; 4 6</font>
<font color="#008844">; returns "met in the middle"</font>
</pre>
<p>See the
<a href="../manual/xlisp-man-020.htm#do">do</a>
special form in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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