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|
<html><head>
<title>Lisp Hints</title>
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<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
<hr>
<h1>Lisp Hints</h1>
<hr>
<p>The original document was written by <nobr>Geoffrey J. Gordon</nobr>
[<nobr>ggordon@cs.cmu.edu</nobr>], <nobr>Friday, February 5, 1993</nobr>,
<nobr>for <a href="http://www.cons.org/cmucl/">CMUCL</a></nobr>.</p>
<p>Modified by:</p>
<ul>
<li><nobr>Bruno Haible for <a href="http://www.gnu.org/software/clisp/">CLISP</a>.</nobr></li>
<li><nobr>Tom Almy in 1995 for <a href="http://almy.us/xlisp.html">XLISP Plus</a></nobr>.</li>
<li><nobr>Edgar M. Franke [edgar-rft@web.de] in 2010 for
<a href="http://www.cs.cmu.edu/~music/music.software.html">Nyquist</a></nobr>.</li>
</ul>
<p>All examples tested with <nobr>Nyquist 3.03</nobr> in <nobr>November
2010</nobr>.</p>
<hr>
<h2>Table of Contents</h2>
<hr>
<ol>
<li><nobr><a href="#1">Symbols</a></nobr></li>
<li><nobr><a href="#2">Numbers</a></nobr></li>
<li><nobr><a href="#3">Conses</a></nobr></li>
<li><nobr><a href="#4">Lists</a></nobr></li>
<li><nobr><a href="#5">Functions</a></nobr></li>
<li><nobr><a href="#6">Printing</a></nobr></li>
<li><nobr><a href="#7">Forms and the Top-Level Loop</a></nobr></li>
<li><nobr><a href="#8">Special Forms</a></nobr></li>
<li><nobr><a href="#9">Binding</a></nobr></li>
<li><nobr><a href="#10">Dynamic Scoping</a></nobr></li>
<li><nobr><a href="#11">Arrays</a></nobr></li>
<li><nobr><a href="#12">Strings</a></nobr></li>
<li><nobr><a href="#13">Setf</a></nobr></li>
<li><nobr><a href="#14">Booleans and Conditionals</a></nobr></li>
<li><nobr><a href="#15">Iteration</a></nobr></li>
<li><nobr><a href="#16">Non-local Exits</a></nobr></li>
<li><nobr><a href="#17">Funcall, Apply, and Mapcar</a></nobr></li>
<li><nobr><a href="#18">Lambda</a></nobr></li>
<li><nobr><a href="#19">Sorting</a></nobr></li>
<li><nobr><a href="#20">Equality</a></nobr></li>
<li><nobr><a href="#21">Some Useful List Functions</a></nobr></li>
</ol>
<a name="1"></a>
<hr>
<h2>1 Symbols</h2>
<hr>
<p>A symbol is just a sequence of characters. There are restrictions on what
you can include in a symbol and what the first character can be, but as long
as you stick to letters, digits, and hyphens, you'll be safe. [Except that
if you use only digits and possibly an initial hyphen, Lisp will think you
typed an integer rather than a symbol.] Some examples of symbols:</p>
<pre class="example">
a
b
c1
foo
bar
baaz-quux-garply
</pre>
<p>Some things you can do with symbols follow. Things after a <nobr>'>'
prompt</nobr> are what you type to the Lisp interpreter, while other things
are what the Lisp interpreter prints back to you. The <nobr>semicolon
';'</nobr> is Lisp's comment character. Everything from a ';' to the end of
line is ignored.</p>
<pre class="example">
> (setq a 5) <font color="#008844">; store a number as the value of a symbol</font>
5
> a <font color="#008844">; take the value of a symbol</font>
5
> (let ((a 6)) <font color="#008844">; bind the value of a symbol temporarily to 6</font>
a)
6
> a <font color="#008844">; the value returns to 5 once the let is finished</font>
5
> (+ a 6) <font color="#008844">; use the value of a symbol as an argument to a function</font>
11
> b <font color="#008844">; try to take the value of a symbol which has no value</font>
<font color="#AA0000">error: unbound variable - b</font>
</pre>
<p>There are two special symbols,
<a href="../reference/t.htm"> T </a>
<nobr>and <a href="../reference/nil.htm">NIL</a></nobr>. <nobr>The
value</nobr> of <a href="../reference/t.htm"> T </a> is defined
always to <nobr>be <a href="../reference/t.htm"> T </a></nobr>,
and the value of <a href="../reference/nil.htm">NIL</a></nobr> is defined
always to <nobr>be <a href="../reference/nil.htm">NIL</a></nobr></nobr>.
Lisp uses <a href="../reference/t.htm"> T </a> and
<a href="../reference/nil.htm">NIL</a></nobr> to represent true and false.
<nobr>An example</nobr> of this use is in the if statement, described more
fully later:</p>
<pre class="example">
> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5
</pre>
<p>The last example is odd but correct.
<nobr><a href="../reference/nil.htm">NIL</a> means</nobr> false, and
anything else means true. Unless we have a reason to do otherwise, we use
<a href="../reference/t.htm"> T </a> to mean true, just for the
sake of clarity.</p>
<p>Symbols like <a href="../reference/t.htm"> T </a> and
<a href="../reference/nil.htm">NIL</a> are called
'<nobr>self-evaluating</nobr>' symbols, because they evaluate to themselves.
There is a whole class of <nobr>self-evaluating</nobr> symbols called
'keywords'. Any symbol whose name starts with a colon is a keyword. [See
below for some uses for keywords.] Some examples:</p>
<pre class="example">
> :this-is-a-keyword
:THIS-IS-A-KEYWORD
> :so-is-this
:SO-IS-THIS
> :me-too
:ME-TOO
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="2"></a>
<hr>
<h2>2 Numbers</h2>
<hr>
<p>An integer number [FIXNUM] is a sequence of digits optionally preceded by
a plus <nobr>sign '+'</nobr> or a minus <nobr>sign '-'</nobr>. <nobr>A
floating</nobr> point number [FLONUM] looks like an integer, except that it
has a decimal point and optionally can be written in scientific notation.
Here are some numbers:</p>
<pre class="example">
5
17
-34
+6
3.1415
1.722e-15
</pre>
<p>The standard arithmetic functions are all available:</p>
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>+</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/addition.htm">addition</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>-</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/subtraction.htm">subtraction</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>*</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/multiplication.htm">multiplication</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>/</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/division.htm">division</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>truncate</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/truncate.htm">truncate</a> a float to an integer</nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>rem</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/rem.htm">remainder</a> of a division</nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>sin</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/sin.htm">sine</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>cos</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/cos.htm">cosine</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>tan</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/tan.htm">tangent</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>sqrt</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr><a href="../reference/sqrt.htm">square root</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>exp</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>natural <a href="../reference/exp.htm">exponent</a></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>expt</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>math <a href="../reference/expt.htm">exponent</a></nobr></td>
</tr>
</tbody></table></p>
<p><nobr><a href="../reference/addition.htm"> + </a> [addition]</nobr>,
<nobr><a href="../reference/subtraction.htm"> - </a> [subtraction]</nobr>,
<nobr><a href="../reference/multiplication.htm"> * </a> [multiplication]</nobr>,
and <nobr><a href="../reference/division.htm"> / </a> [division]</nobr>
accept any number of arguments and return a number according to type
contagion. This means that as long as only integer numbers are given as
arguments the result will always be an integer number, but as soon as at
least one of the arguments is a floating number then the result will be a
floating point number. Here are some examples:</p>
<pre class="example">
> (/ 3 2) <font color="#008844">; integer division causes rounding error</font>
1
> (/ 3 2.0) <font color="#008844">; the 2.0 forces floating point computation</font>
1.5
> (exp 1) <font color="#008844">; e</font>
2.71828
> (exp 3) <font color="#008844">; e * e * e</font>
20.0855
> (expt 3 4.2) <font color="#008844">; exponent with a base other than e</font>
100.904
> (+ 5 6 7 (* 8 9 10)) <font color="#008844">; the functions + - * / accept multiple arguments</font>
738
</pre>
<p>In Nyquist the valid range of integer numbers is limited by the size of a
C 'long' value on the machine on which Nyquist is running.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="3"></a>
<hr>
<h2>3 Conses</h2>
<hr>
<p>A <a href="../reference/cons.htm">cons</a> is just a
<nobr>two-field</nobr> record. <nobr>The fields</nobr> are called
<a href="../reference/car.htm">car</a> and
<a href="../reference/cdr.htm">cdr</a>, for historical reasons.
<nobr>On the</nobr> first machine where Lisp was implemented, there
were two assembly language instructions CAR and CDR which stood for
'contents of address register' and 'contents of decrement register'.
Conses were implemented using these two registers.</p>
<p>Conses are created by the <a href="../reference/cons.htm">cons</a>
function:</p>
<pre class="example">
> (cons 4 5) <font color="#008844">; Allocate a cons. Set the car to 4 and the cdr to 5</font>
(4 . 5)
> (cons (cons 4 5) 6)
((4 . 5) . 6)
> (car (cons 4 5))
4
> (cdr (cons 4 5))
5
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="4"></a>
<hr>
<h2>4 Lists</h2>
<hr>
<p>You can build many structures out of conses. Perhaps the simplest is a
linked list. <nobr>The <a href="../reference/car.htm">car</a></nobr>
<nobr>[the <a href="../reference/first.htm">first</a></nobr> element] of
each cons points to one of the elements of the list, and the
<a href="../reference/cdr.htm">cdr</a> <nobr>[the
<a href="../reference/rest.htm">rest</a></nobr> of the elements] points
either to another cons or to <a href="../reference/nil.htm">NIL</a>.
<nobr>You can</nobr> create such a linked list with the
<a href="../reference/list.htm">list</a> function:</p>
<pre class="example">
> (list 4 5 6)
(4 5 6)
</pre>
<p>Notice that Lisp prints linked lists a special way. <nobr>It omits</nobr>
some of the periods and parentheses. The rule is that if the
<a href="../reference/cdr.htm">cdr</a> of a
<a href="../reference/cons.htm">cons</a> is
<a href="../reference/nil.htm">NIL</a>, Lisp doesn't bother to print the
period or the <a href="../reference/nil.htm">NIL</a>, and if the
<a href="../reference/cdr.htm">cdr</a> of
<nobr><a href="../reference/cons.htm">cons</a> A</nobr> is
<nobr><a href="../reference/cons.htm">cons</a> B</nobr>, then Lisp doesn't
bother to print the period for
<nobr><a href="../reference/cons.htm">cons</a> A</nobr> or the parentheses
for <nobr><a href="../reference/cons.htm">cons</a> B. So:</nobr></p>
<pre class="example">
> (cons 4 nil)
(4) <font color="#008844">; (4 . nil)</font>
> (cons 4 (cons 5 6))
(4 5 . 6) <font color="#008844">; (4 . (5 . 6))</font>
> (cons 4 (cons 5 (cons 6 nil)))
(4 5 6) <font color="#008844">; (4 . (5 . (6 . nil)))</font>
</pre>
<p>The last example is exactly equivalent to the call:</p>
<pre class="example">
(list 4 5 6)
</pre>
<p>Note that <a href="../reference/nil.htm">NIL</a> now means the list with
no elements. <nobr>The <a href="../reference/cdr.htm">cdr</a></nobr> of
<nobr>(a b)</nobr>, a list with <nobr>2 elements</nobr>, <nobr>is
(b)</nobr>, a list with 1 element, and the
<a href="../reference/cdr.htm">cdr</a> of (b), a list with <nobr>1
element</nobr>, <nobr>is <a href="../reference/nil.htm">NIL</a></nobr>,
which therefore must be a list with no elements.</p>
<p><div class="box">
<p>The <a href="../reference/car.htm">car</a> and
<a href="../reference/cdr.htm">cdr</a> of
<a href="../reference/nil.htm">NIL</a> are defined to
<nobr>be <a href="../reference/nil.htm">NIL</a></nobr>.</p>
</div></p>
<p>If you store your list in a variable, you can make it act like a
stack:</p>
<pre class="example">
> (setq a nil)
NIL <font color="#008844">; A = ()</font>
> (push 4 a)
(4) <font color="#008844">; A = (4)</font>
> (push 5 a)
(5 4) <font color="#008844">; A = (5 4)</font>
> (pop a)
5 <font color="#008844">; A = (4)</font>
> (pop a)
4 <font color="#008844">; A = ()</font>
> (pop a)
NIL <font color="#008844">; A = ()</font>
</pre>
<p>See <a href="../reference/pop.htm">pop</a>,
<a href="../reference/push.htm">push</a>,
<a href="../reference/setq.htm">setq</a>.</p>
<a name="list-accessors"></a>
<p><b>List Accessors</b></p>
<p>There are several different approaches to name the accessor
functions for elements of conses and lists. <nobr>The 'traditional'</nobr>
Lisp still uses function names like
<a href="../reference/car.htm">car</a> and
<a href="../reference/cdr.htm">cdr</a> while the 'modern' Lisp uses more
descriptive names like <a href="../reference/first.htm">first</a> and <a
href="../reference/rest.htm">rest</a>. This leads to the situation that in
most Lisps today the list accessor functions are available under different
names for the same functions:</p>
<p><div class="box">
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr>
<td align="right"><nobr>modern</nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr>traditional</nobr></td>
<td></td>
<td>equivalent to</td>
<td><nobr><code> </code></nobr></td>
<td>(1 2 3 4 5 6 7 8)</td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/first.htm">first</a></nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr><a href="../reference/car.htm">car</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nth.htm">nth</a> 0 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>1</td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/second.htm">second</a></nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr><a href="../reference/caar.htm">cadr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nth.htm">nth</a> 1 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>2</td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/third.htm">third</a></nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr><a href="../reference/caaar.htm">caddr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nth.htm">nth</a> 2 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>3</td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/fourth.htm">fourth</a></nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr><a href="../reference/caaaar.htm">cadddr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nth.htm">nth</a> 3 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>4</td>
</tr>
<tr>
<td colspan="3"></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nth.htm">nth</a> 4 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>5</td>
</tr>
<tr>
<td colspan="3"></nobr></td>
<td><nobr><code> </code></nobr></td>
<td> ...</td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr>
<td colspan="4"></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 0 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(1 2 3 4 5 6 7 8)</td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/rest.htm">rest</a></nobr></td>
<td><nobr> — </nobr></td>
<td align="left"><nobr><a href="../reference/cdr.htm">cdr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 1 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(2 3 4 5 6 7 8)</td>
</tr>
<tr>
<td colspan="2"></td>
<td align="left"><nobr><a href="../reference/caar.htm">cddr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 2 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(3 4 5 6 7 8)</td>
</tr>
<tr>
<td colspan="2"></td>
<td align="left"><nobr><a href="../reference/cdddr.htm">cdddr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 3 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(4 5 6 7 8)</td>
</tr>
<tr>
<td colspan="2"></td>
<td align="left"><nobr><a href="../reference/cddddr.htm">cddddr</a></nobr></td>
<td><nobr><code> </code></nobr></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 4 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(5 6 7 8)</td>
</tr>
<tr>
<td colspan="4"></td>
<td>(<a href="../reference/nthcdr.htm">nthcdr</a> 5 ... )</td>
<td><nobr><code> </code>→ </nobr></td>
<td>(6 7 8)</td>
</tr>
<tr>
<td colspan="3"></nobr></td>
<td><nobr><code> </code></nobr></td>
<td> ...</td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/last.htm">last</a></nobr></td>
<td colspan="4"></td>
<td><nobr><code> </code>→ </nobr></td>
<td>(8)</td>
</tr>
</tbody></table></p>
</div></p>
<p>The traditional <nobr>c..r-functions</nobr> are available in even more
variations, see <a href="../reference/car.htm">car</a>,
<a href="../reference/cdr.htm">cdr</a>,
<nobr><a href="../reference/cddr.htm">cadr, cddr</a></nobr>,
<nobr><a href="../reference/caaar.htm">caaar...caddr</a></nobr>,
<nobr><a href="../reference/cdddr.htm">cdaar...cdddr</a></nobr>,
<nobr><a href="../reference/caaaar.htm">caaaar...cadddr</a></nobr>,
<nobr>and <a href="../reference/cddddr.htm">cdaaar...cddddr</a></nobr>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="5"></a>
<hr>
<h2>5 Functions</h2>
<hr>
<p>You saw one example of a function above. Here are some more:</p>
<pre class="example">
> (+ 3 4 5 6) <font color="#008844">; this function takes any number of arguments</font>
18
> (+ (+ 3 4) (+ (+ 4 5) 6)) <font color="#008844">; isn't prefix notation fun?</font>
22
> (defun foo (x y) <font color="#008844">; defining a function</font>
(+ x y 5))
FOO
> (foo 5 0) <font color="#008844">; calling a function</font>
10
> (defun fact (x) <font color="#008844">; a recursive function</font>
(if (> x 0)
(* x (fact (- x 1)))
1))
FACT
> (fact 5)
120
> (defun a (x)
(if (= x 0)
t
(b (- x)))) <font color="#008844">; mutually recursive functions</font>
A
> (defun b (x)
(if (> x 0)
(a (- x 1))
(a (+ x 1))))
B
> (a 5)
T
> (defun bar (x) <font color="#008844">; A function with multiple statements</font>
(setq x (* x 3)) <font color="#008844">; in its body. It will return the value</font>
(setq x (/ x 2)) <font color="#008844">; returned by its final statement</font>
(+ x 4))
BAR
> (bar 6)
13
</pre>
<p>See <a href="../reference/addition.htm"> + </a>,
<a href="../reference/subtraction.htm"> − </a>,
<a href="../reference/multiplication.htm"> * </a>,
<a href="../reference/division.htm"> / </a>,
<a href="../reference/number-equal.htm"> = </a>,
<a href="../reference/number-greaterp.htm"> > </a>,
<a href="../reference/defun.htm">defun</a>,
<a href="../reference/if.htm"> if </a>,
<a href="../reference/setq.htm">setq</a>. When we defined 'foo', we
gave it two arguments, 'x' <nobr>and 'y'</nobr>. Now when we call 'foo', we
are required to provide exactly two arguments. The first will become the
value of 'x' for the duration of the call to 'foo', and the second will
become the value of 'y' for the duration of the call. <nobr>In Lisp</nobr>,
most variables are lexically scoped. <nobr>That is</nobr>, if 'foo' calls
'bar' and 'bar' tries to reference 'x', then 'bar' will not get 'foo's value
<nobr>for x</nobr>.</p>
<p><div class="box">
<p>The process of assigning a symbol a value for the duration of some
lexical scope is called 'binding'.</p>
</div></p>
<p>You can specify optional arguments for your functions. Any argument
after the symbol
<a href="../reference/lambda-keyword-optional.htm">&optional</a> is
optional:</p>
<pre class="example">
> (defun bar (x &optional y)
(if y
x
0))
BAR
> (defun baaz (&optional (x 3) (z 10))
(+ x z))
BAAZ
> (bar 5)
0
> (bar 5 t)
5
> (baaz 5)
15
> (baaz 5 6)
11
> (baaz)
13
</pre>
<p>See <a href="../reference/addition.htm"> + </a>,
<a href="../reference/defun.htm">defun</a>,
<a href="../reference/if.htm"> if </a>.
<nobr>It is</nobr> legal to call the function 'bar' with either one or two
arguments. <nobr>If it</nobr> is called with one argument, 'x' will be bound
to the value of that argument and 'y' will be bound <nobr>to
<a href="../reference/nil.htm">NIL</a></nobr>. <nobr>If it</nobr> is called
with two arguments, 'x' and 'y' will be bound to the values of the first and
second argument, respectively.</p>
<p>The function 'baaz' has two optional arguments. <nobr>It specifies</nobr> a
default value for each of them. <nobr>If the</nobr> caller specifies only
one argument, 'z' will be bound <nobr>to 10</nobr> instead of <nobr>to
<a href="../reference/nil.htm">NIL</a></nobr>, and if the caller
specifies no arguments, 'x' will be bound <nobr>to 3</nobr> and <nobr>'z' to
10</nobr>.</p>
<p>You can make your function accept any number of arguments by ending its
argument list with an
<a href="../reference/lambda-keyword-rest.htm">&rest</a> parameter.
Lisp will collect all arguments not otherwise accounted for into a list and
bind the <a href="../reference/lambda-keyword-rest.htm">&rest</a>
parameter to that <nobr>list. So:</nobr></p>
<pre class="example">
> (defun foo (x &rest y)
y)
FOO
> (foo 3)
NIL
> (foo 4 5 6)
(5 6)
</pre>
<p>See <a href="../reference/defun.htm">defun</a>. Finally, you can give
your function another kind of optional argument called a
<a href="../reference/lambda-keyword-key.htm">&key</a> 'keyword'
argument. <nobr>The caller</nobr> can give these arguments in any order,
because they're labelled with keywords:</p>
<pre class="example">
> (defun foo (&key x y)
(cons x y))
FOO
> (foo :x 5 :y 3)
(5 . 3)
> (foo :y 3 :x 5)
(5 . 3)
> (foo :y 3)
(NIL . 3)
> (foo)
(NIL)
</pre>
<p>See <a href="../reference/defun.htm">defun</a>.
<nobr>An <a href="../reference/lambda-keyword-key.htm">&key</a></nobr>
parameter can have a default <nobr>value too:</nobr></p>
<pre class="example">
> (defun foo (&key (x 5))
x)
FOO
> (foo :x 7)
7
> (foo)
5
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="6"></a>
<hr>
<h2>6 Printing</h2>
<hr>
<p>Some functions can cause output. The simplest one is
<a href="../reference/print.htm">print</a>, which
prints its argument and then <nobr>returns it:</nobr></p>
<pre class="example">
> (print 3)
3 <font color="#008844">; screen output</font>
3 <font color="#008844">; return value</font>
</pre>
<p>The first 3 above was <a href="../reference/print.htm">print</a>ed, the
second was returned.</p>
<p>If you want more complicated output, you will need to use
<a href="../reference/format.htm">format</a>.
Here's an example:</p>
<pre class="example">
> (format t "An atom: ~S~%and a list: ~S~%and an integer: ~A~%"
nil (list 5) 6)
An atom: NIL <font color="#008844">; screen output</font>
and a list: (5) <font color="#008844">; screen output</font>
and an integer: 6 <font color="#008844">; screen output</font>
NIL <font color="#008844">; return value</font>
</pre>
<p>See <a href="../reference/list.htm">list</a>. <nobr>The first</nobr>
argument to <a href="../reference/format.htm">format</a> is either
<a href="../reference/t.htm"> T </a>,
<a href="../reference/nil.htm">NIL</a>, or a stream.
<nobr><a href="../reference/t.htm"> T </a> specifies</nobr> output
to the terminal. <nobr><a href="../reference/nil.htm">NIL</a> means</nobr>
not to print anything but to return a string containing the output instead.
Streams are general places for output <nobr>to go</nobr>. They can specify a
file, or the terminal, or a printer device. This tutorial will not describe
streams in any further detail.</p>
<p>The second argument is a formatting template, which is a string
optionally containing formatting directives. <nobr>All remaining</nobr>
arguments may be referred to by the formatting directives. Lisp will replace
the directives with some appropriate characters based on the arguments to
which they refer and then print the resulting string.</p>
<p>The format function always returns <a href="../reference/nil.htm">NIL</a>
unless its first argument is <a href="../reference/nil.htm">NIL</a>, in
which case it prints nothing and returns a string.</p>
<p>There are several different directives available:</p>
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr>
<td><nobr><code> </code></nobr></td>
<td valign="top">
<table cellpadding="0" cellspacing="0" width="100%"><tbody>
<tr valign="top">
<td class="button"><nobr><code>~S</code></nobr></td>
</tr>
</tbody></table>
</td>
<td valign="top"><nobr> - </nobr></td>
<td width="100%">[standard] - accepts any Lisp object and replaces it by
the same printed representation which is produced by the
<a href="../reference/print.htm">print</a> function.</td>
</tr>
<tr>
<td><nobr><font size="-2"> </font></nobr></td>
</tr>
<tr>
<td><nobr><code> </code></nobr></td>
<td valign="top">
<table cellpadding="0" cellspacing="0" width="100%"><tbody>
<tr valign="top">
<td class="button"><nobr><code>~A</code></nobr></td>
</tr>
</tbody></table>
</td>
<td valign="top"><nobr> - </nobr></td>
<td width="100%">[aestethic] - tries to '<nobr>pretty-print</nobr>'
its argument.</td>
</tr>
<tr>
<td><nobr><font size="-2"> </font></nobr></td>
</tr>
<tr>
<td><nobr><code> </code></nobr></td>
<td valign="top">
<table cellpadding="0" cellspacing="0" width="100%"><tbody>
<tr valign="top">
<td class="button"><nobr><code>~%</code></nobr></td>
</tr>
</tbody></table>
</td>
<td valign="top"><nobr> - </nobr></td>
<td width="100%">[linebreak] - is always replaced by a linebreak character
or character sequence of the underlying operation system.</td>
</tr>
<tr>
<td><nobr><font size="-2"> </font></nobr></td>
</tr>
<tr>
<td><nobr><code> </code></nobr></td>
<td valign="top">
<table cellpadding="0" cellspacing="0" width="100%"><tbody>
<tr valign="top">
<td class="button"><nobr><code>~~</code></nobr></td>
</tr>
</tbody></table>
</td>
<td valign="top"><nobr> - </nobr></td>
<td width="100%">[tilde] - is replaced by a single '~' character.</td>
</tr>
</tbody></table></p>
<p>If the last character in a line in a
<a href="../reference/format.htm">format</a> template is a tilde, then
the linebreak is ignored and the template continues with the next
<nobr>non-whitespace</nobr> character in the next line.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="7"></a>
<hr>
<h2>7 Forms and the Top-Level Loop</h2>
<hr>
<p>The things which you type to the Lisp interpreter are called 'forms'.
<nobr>The Lisp</nobr> interpreter repeatedly
<a href="../reference/read.htm">read</a>s a form,
<a href="../reference/eval.htm">eval</a>uates it, and
<a href="../reference/print.htm">print</a>s the result. This procedure
is therefore called the '<nobr>read-eval-print' loop</nobr>, or REPL for
short.</p>
<p>Some forms will cause errors. After an error, Lisp will put you into the
debugger so you can try to figure out what caused the error.</p>
<p>In general, a form is either an <a href="../reference/atom.htm">atom</a>,
<nobr>[for example</nobr> a symbol, an integer, or a string] or <nobr>a
<a href="../reference/list.htm">list</a></nobr>. <nobr>If the</nobr> form is
an <a href="../reference/atom.htm">atom</a>, Lisp evaluates it immediately.
Symbols evaluate to their value, integers and strings evaluate to
themselves. <nobr>If the</nobr> form is a
<a href="../reference/list.htm">list</a></nobr>, Lisp treats its first
element as the name of a function. <nobr>It evaluates</nobr> the remaining
elements recursively, and then calls the function with the values of the
remaining elements as arguments. </p>
<p>For example, if Lisp sees the form:</p>
<pre class="example">
(+ 3 4)
</pre>
<p>then it treats <a href="../reference/addition.htm"> + </a> as
the name of a function. <nobr>It then</nobr> evaluates 3 to <nobr>get
3</nobr> and 4 to <nobr>get 4</nobr>, finally it calls <a
href="../reference/addition.htm"> + </a> with 3 and 4 as the
arguments. <nobr>The <a href="../reference/addition.htm"> + </a>
function</nobr> <nobr>returns 7</nobr>, which Lisp prints.</p>
<p><div class="box">
<p><b>Nyquist:</b> <nobr>A description</nobr> of the debugger can be found
in the <nobr><a href="../manual/xlisp.htm#break-loop">Break Command Loop</a></nobr>
section and a detailed description of the evaluation process can be found in
the <a href="../manual/xlisp.htm#the-evaluator">Evaluator</a> section of the XLISP
manual.</p>
</div></p>
<p>The <nobr>top-level</nobr> loop provides some other conveniences.
<nobr>One particularly</nobr> convenient convenience is the ability to talk
about the results of previously typed forms. Lisp always saves its most
recent three results, it stores them as the values of the symbols
<a href="../manual/xlisp.htm#command-loop"> * </a>,
<a href="../manual/xlisp.htm#command-loop"> ** </a>,
<nobr>and <a href="../manual/xlisp.htm#command-loop"> *** </a></nobr>.
<nobr>For example:</nobr></p>
<pre class="example">
> 3
3
> 4
4
> 5
5
> ***
3
> ***
4
> ***
5
> **
4
> *
4
</pre>
<p>See <a href="../manual/xlisp.htm#command-loop"> * </a>,
<a href="../manual/xlisp.htm#command-loop"> ** </a>,
<a href="../manual/xlisp.htm#command-loop"> *** </a>,
<a href="../manual/xlisp.htm#command-loop"> + </a>,
<a href="../manual/xlisp.htm#command-loop"> ++ </a>,
<a href="../manual/xlisp.htm#command-loop"> +++ </a>,
<a href="../manual/xlisp.htm#command-loop"> − </a>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="8"></a>
<hr>
<h2>8 Special Forms</h2>
<hr>
<p>There are a number of special forms which look like function calls but
aren't. These include control constructs such as
<a href="../reference/if.htm"> if </a> statements and
<a href="../reference/do.htm">do</a> loops, assignments like
<a href="../reference/setq.htm">setq</a>,
<a href="../reference/setf.htm">setf</a>,
<a href="../reference/push.htm">push</a>, and
<a href="../reference/pop.htm">pop</a>, definitions such as
<a href="../reference/defun.htm">defun</a>, and binding constructs such
<nobr>as <a href="../reference/let.htm">let</a></nobr>. <nobr>Not all</nobr>
of these special forms have been mentioned yet, see below for examples.</p>
<p>One useful special form is the <a href="../reference/quote.htm">quote</a>
form. <nobr>The <a href="../reference/quote.htm">quote</a></nobr> function
prevents its argument from being evaluated. <nobr>For example:</nobr></p>
<pre class="example">
> (setq a 3)
3
> a
3
> (quote a)
A
> 'a <font color="#008844">; 'a is an abbreviation for (quote a)</font>
A
</pre>
<p>Another similar special form is the
<a href="../reference/function.htm">function</a> form, it causes its
argument to be interpreted as a function rather than being evaluated.
<nobr>For example:</nobr></p>
<pre class="example">
> (setq + 3)
3
> +
3
> '+
+
> (function +)
#<Subr-+: #88b44d5e>
> #'+ <font color="#008844">; #'+ is an abbreviation for (function +)</font>
#<Subr-+: #88b44d5e>
</pre>
<p>The <a href="../reference/function.htm">function</a> special form is
useful when you want to pass a function as an argument to another function.
<nobr>See below</nobr> for some examples of functions which take functions
as arguments.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="9"></a>
<hr>
<h2>9 Binding</h2>
<hr>
<p>Binding is lexically scoped assignment. <nobr>It happens</nobr> to the
variables in a function's parameter list whenever the function is called.
<nobr>The formal</nobr> parameters are bound to the actual parameters for
the duration of the function call. <nobr>You can</nobr> bind variables
anywhere in a program with the <a href="../reference/let.htm">let</a>
special form, which looks <nobr>like this:</nobr></p>
<pre class="example">
(let ((<font color="#0000CC">variable-1 value-1</font>)
(<font color="#0000CC">variable-2 value-2</font>)
<font color="#008844">...</font> )
<font color="#0000CC">body</font>)
</pre>
<p>The <a href="../reference/let.htm">let</a> function binds
'<nobr>variable-1</nobr>' to '<nobr>value-1</nobr>',
'<nobr>variable-2</nobr>' to '<nobr>value-2</nobr>', and so forth.
<nobr>Then it</nobr> executes the statements in its body. The body of a
<a href="../reference/let.htm">let</a> follows exactly the same rules that
a function body does. Some examples:</p>
<pre class="example">
> (let ((a 3)) (+ a 1))
4
> (let ((a 2)
(b 3)
(c 0))
(setq c (+ a b))
c)
5
> (setq c 4)
4
> (let ((c 5))
c)
5
> c
4
</pre>
<p>See <a href="../reference/addition.htm"> + </a>,
<a href="../reference/let.htm">let</a>,
<a href="../reference/setq.htm">setq</a>. <nobr>Instead of:</nobr></p>
<pre class="example">
(let ((a nil)
(b nil))
<font color="#008844">...</font> )
</pre>
<p>you can write:</p>
<pre class="example">
(let (a b)
<font color="#008844">...</font> )
</pre>
<p>The '<nobr>value-1</nobr>', '<nobr>value-2</nobr>', etc. inside a
<a href="../reference/let.htm">let</a> form cannot reference the variables
'<nobr>variable-1</nobr>', '<nobr>variable-2</nobr>', etc. that the
<a href="../reference/let.htm">let</a> form is binding. <nobr>For
example:</nobr></p>
<pre class="example">
> (let ((x 1)
(y (+ <font color="#AA0000">x</font> 1))) <font color="#008844">; x is still unbound here</font>
y)
<font color="#AA0000">error: unbound variable - x</font>
</pre>
<p>If the symbol 'x' already has a global value, stranger happenings will
result:</p>
<pre class="example">
> (setq x 7)
7
> (let ((x 1)
(y (+ <font color="#AA0000">x</font> 1))) <font color="#008844">; references to the global x</font>
y)
8
</pre>
<p>The <a href="../reference/let-star.htm">let*</a> special form is just
like <a href="../reference/let.htm">let</a> except that it allows values to
reference variables defined earlier in the
<a href="../reference/let-star.htm">let*</a> form.
<nobr>For example:</nobr></p>
<pre class="example">
> (setq x 7)
7
> (let* ((x 1)
(y (+ x 1))) <font color="#008844">; references to x in the line before</font>
y)
2
</pre>
<p>The <a href="../reference/let-star.htm">let*</a> form:</p>
<pre class="example">
(let* ((x a)
(y b))
<font color="#008844">...</font> )
</pre>
<p>is equivalent to the following <a href="../reference/let.htm">let</a>
construct:</p>
<pre class="example">
(let ((x a))
(let ((y b))
<font color="#008844">...</font> ))
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="10"></a>
<hr>
<h2>10 Dynamic Scoping</h2>
<hr>
<p>The <a href="../reference/let.htm">let</a> and
<a href="../reference/let-star.htm">let*</a> forms provide lexical scoping,
which is what you expect if you're used to programming in C or Pascal.
Dynamic scoping is what you get in BASIC. <nobr>If you</nobr> assign a value
to a dynamically scoped variable, every mention of that variable returns
that value until you assign another value to the same variable.</p>
<p>In Lisp, dynamically scoped variables are called 'special' variables.
<nobr>In Common Lisp</nobr> special variables are declared with the 'defvar'
special form. <nobr>In Nyquist</nobr> there is no 'defvar' form.</p>
<p><div class="box">
<p>Nyquist has no 'dynamic' scoping in the <nobr>Common Lisp</nobr> sense.</p>
</div></p>
<p>In Nyquist every variable assigned with
<a href="../reference/setq.htm">setq</a> or
<a href="../reference/setf.htm">setf</a> at the <nobr>top-level</nobr>,
outside of a function or a <a href="../reference/let.htm">let</a>
binding, is a lexical scoped variable. Here is an example what this means:</p>
<pre class="example">
> (setq *variable* 5) <font color="#008844">; define a global variable</font>
5
> (defun check-variable () <font color="#008844">; define a function in global scope,</font>
*variable*) <font color="#008844">; returning the value of the variable</font>
CHECK-VARIABLE
> (check-variable) <font color="#008844">; the CHECK-VARIABLE function returns</font>
5 <font color="#008844">; the global value of the variable</font>
> (let ((*variable* 10)) <font color="#008844">; create a local binding for the variable</font>
(print (check-variable)) <font color="#008844">; call CHECK-VARIABLE and print the return value</font>
(print *variable*)) <font color="#008844">; print the local value of the variable</font>
5 <font color="#008844">; return value of CHECK-VARIABLE</font>
10 <font color="#008844">; variable value inside of LET</font>
10
</pre>
<p>See <a href="../reference/defun.htm">defun</a>,
<a href="../reference/let.htm">let</a>,
<a href="../reference/print.htm">print</a>,
<a href="../reference/setq.htm">setq</a>. Because the
'<nobr>check-variable</nobr>' function was defined in global
scope and therefore is lexically outside of the
<a href="../reference/let.htm">let</a> form, the
'<nobr>check-variable</nobr>' function returns the variable's global value
<nobr>of 5</nobr>, even if called from inside the
<a href="../reference/let.htm">let</a> form, where the variable has a
<nobr>value of 10</nobr>.</p>
<p><div class="box">
<p><b>Important:</b> In Nyquist there is no way to change the scoping
behaviour of variables, so you must be careful where you define your
variables. With Nyquist it's generally a good idea to prefer local
<a href="../reference/let.htm">let</a> bindings over global variables.</p>
</div></p>
<p>By convention, the name of a global Nyquist variable begins and ends with
a <nobr>star *</nobr> to signal that the variable might behave differently
than the programmer expects.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="11"></a>
<hr>
<h2>11 Arrays</h2>
<hr>
<p>The function
<nobr><a href="../reference/make-array.htm">make-array</a></nobr> makes a
<nobr>1-dimensional</nobr> array.
<nobr>The <a href="../reference/aref.htm">aref</a></nobr> function accesses
its elements. <nobr>All elements</nobr> of an array are initially set
<nobr>to <a href="../reference/nil.htm">NIL</a></nobr>.
<nobr>For example:</nobr></p>
<pre class="example">
> (make-array 4) <font color="#008844">; 1-D array with 4 elements</font>
#(NIL NIL NIL NIL)
</pre>
<p>Array indices always <nobr>start at 0</nobr>. <nobr>See
<a href="#13">below</a></nobr> for how to set the elements of an array.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="12"></a>
<hr>
<h2>12 Strings</h2>
<hr>
<p>A string is a sequence of characters between double quotes. Nyquist
represents a string internally as a <nobr>variable-length</nobr> array of
characters. <nobr>You can</nobr> write a string containing a double quote by
preceding the quote with a backslash. <nobr>A double</nobr> backslash stands
for a single backslash. <nobr>For example:</nobr></p>
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>"abcd"</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>has 4 characters</nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>"\""</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>has 1 character, a quote</nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr valign="top">
<td><nobr><code> </code></nobr></td>
<td class="button"><nobr><code>"\\"</code></nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>has 1 character, a backslash</nobr></td>
</tr>
</tbody></table></p>
<p>Here are some functions for dealing with strings:</p>
<pre class="example">
> (strcat "abcd" "efg")
"abcdefg" <font color="#008844">; STRCAT concatenates strings</font>
> (char "abc" 1)
#\b <font color="#008844">; Lisp writes characters preceded by #\</font>
> (subseq "abc" 0 2)
"ab" <font color="#008844">; SUBSEQ extracts substrings</font>
</pre>
<p>See <a href="../reference/char.htm">char</a>,
<a href="../reference/strcat.htm">strcat</a>,
<a href="../reference/subseq.htm">subseq</a>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="13"></a>
<hr>
<h2>13 Setf</h2>
<hr>
<p>Certain forms in Lisp naturally define a memory location. For example, if
the value of 'x' is a <a href="../reference/list.htm">list</a>, then
<nobr>(<a href="../reference/nth.htm">nth</a> 4 x)</nobr> defines the fifth
element of the <a href="../reference/list.htm">list</a>. <nobr>Or, if</nobr>
the value of 'y' is a <nobr>one-dimensional</nobr>
<a href="../reference/make-array.htm">array</a>,
<nobr>(<a href="../reference/aref.htm">aref</a> y 2)</nobr> defines the
third element of the <a href="../reference/make-array.htm">array</a>.</p>
<p>The <a href="../reference/setf.htm">setf</a> special form uses its first
argument to define a place in memory, evaluates its second argument, and
stores the resulting value in the resulting memory location. <nobr>For
example:</nobr></p>
<pre class="example">
> (setq a (make-array 3))
#(NIL NIL NIL)
> (aref a 1)
NIL
> (setf (aref a 1) 3) <font color="#008844">; store 3 in the second element of a</font>
3
> a
#(NIL 3 NIL)
> (aref a 1) <font color="#008844">; read the second element of a</font>
3
> (setq b (list 1 2 3 4 5))
(1 2 3 4 5)
> (nth 4 b)
5
> (setf (nth 4 b) "five") <font color="#008844">; store "five" in the fifth element of b</font>
"five"
> b
(1 2 3 4 "five")
> (nth 4 b)
"five"
</pre>
<p>The <a href="../reference/setf.htm">setf</a> function is the only way to
set the elements of a list or an array.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="14"></a>
<hr>
<h2>14 Booleans and Conditionals</h2>
<hr>
<p>Lisp uses the <nobr>self-evaluating</nobr> symbol
<a href="../reference/nil.htm">NIL</a> to mean false. Anything other than
<nobr>self-evaluating</nobr> means true. Unless we have a reason not to,
we usually use the <nobr>self-evaluating</nobr> symbol
<a href="../reference/t.htm"> T </a> to stand
<nobr>for true</nobr>.</p>
<p>Lisp provides a standard set of logical functions, for example
<a href="../reference/and.htm">and</a>,
<a href="../reference/or.htm">or</a>, and
<a href="../reference/not.htm">not</a>.
<nobr>The <a href="../reference/and.htm">and</a></nobr> and
<a href="../reference/or.htm">or</a> connectives are
<nobr>short-circuiting</nobr>, <a href="../reference/and.htm">and</a>
will not evaluate any arguments to the right of the first one which
evaluates <nobr>to <a href="../reference/nil.htm">NIL</a></nobr>, while
<a href="../reference/or.htm">or</a> will not evaluate any arguments to the
right of the first one which evaluates
<nobr>to <a href="../reference/t.htm"> T </a></nobr>.</p>
<p>Lisp also provides several special forms for conditional execution. The
simplest of these <nobr>is
<a href="../reference/if.htm"> if </a></nobr>. The first argument
of <a href="../reference/if.htm"> if </a> determines whether the
second or third argument will be executed:</p>
<pre class="example">
> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5
</pre>
<p>If you need to put more than one statement in the 'then' or 'else' clause
of an <a href="../reference/if.htm"> if </a></nobr> statement,
you can use the <a href="../reference/progn.htm">progn</a> special form.
<a href="../reference/progn.htm">progn</a> executes each statement in its
body, then returns the value of the <nobr>final one</nobr>:</p>
<pre class="example">
> (setq a 7)
7
> (setq b 0)
0
> (setq c 5)
5
> (if (> a 5)
(progn
(setq a (+ b 7))
(setq b (+ c 8)))
(setq b 4))
13
</pre>
<p>An <a href="../reference/if.htm"> if </a> statement which lacks
either a 'then' or an 'else' clause can be written using the
<a href="../reference/when.htm">when</a> or
<a href="../reference/unless.htm">unless</a> special form:</p>
<pre class="example">
> (when t 3)
3
> (when nil 3)
NIL
> (unless t 3)
NIL
> (unless nil 3)
3
</pre>
<p><a href="../reference/when.htm">when</a> and
<a href="../reference/unless.htm">unless</a>, unlike
<a href="../reference/if.htm"> if </a>, allow any number of
statements in their bodies:</p>
<pre class="example">
(when x
a
b
c)
</pre>
<p>is equivalent to:</p>
<pre class="example">
(if x
(progn
a
b
c))
</pre>
<p>For example:</p>
<pre class="example">
> (when t
(setq a 5)
(+ a 6))
11
</pre>
<p>More complicated conditionals can be defined using the
<a href="../reference/cond.htm">cond</a> special form.
<nobr>A <a href="../reference/cond.htm">cond</a></nobr> form consists of
the symbol cond followed by a number of
<a href="../reference/cond.htm">cond</a> clauses, each of which is a list.
<nobr>The first</nobr> element of a <a href="../reference/cond.htm">cond</a>
clause is the condition, the remaining elements <nobr>[if any]</nobr> are
the actions:</p>
<pre class="example">
(cond (<font color="#0000CC">condition-1 action-1</font>)
(<font color="#0000CC">condition-2 action-2</font>)
...
(t <font color="#0000CC">default-action</font>))
</pre>
<p>The <a href="../reference/cond.htm">cond</a> form finds the first clause
whose condition evaluates to true [does not evaluate <nobr>to
<a href="../reference/nil.htm">NIL</a>]</nobr>. <nobr>It then</nobr>
executes the corresponding action and returns the resulting value. None of
the remaining conditions are evaluated, nor are any actions except the one
corresponding to the selected condition. <nobr>For example</nobr>:</p>
<pre class="example">
> (setq a 3)
3
> (cond
((evenp a) a) <font color="#008844">; if a is even return a</font>
((> a 7) (/ a 2)) <font color="#008844">; else if a is bigger than 7 return a/2</font>
((< a 5) (- a 1)) <font color="#008844">; else if a is smaller than 5 return a-1</font>
(t 17)) <font color="#008844">; else return 17</font>
2
</pre>
<p>If the action in the selected <a href="../reference/cond.htm">cond</a>
clause is missing, then <a href="../reference/cond.htm">cond</a> returns
what the condition <nobr>evaluated to:</nobr></p>
<pre class="example">
> (cond ((+ 3 4)))
7
</pre>
<p>The Lisp <a href="../reference/case.htm">case</a> form is like a C
'switch' statement:</p>
<pre class="example">
> (setq x 'b)
B
> (case x
(a 5)
((d e) 7)
((b f) 3)
(t 9))
3
</pre>
<p>The <a href="../reference/t.htm"> T </a> clause at the end
means that if 'x' is not 'a', '<nobr>d or e</nobr>', or
'<nobr>b or f</nobr>', then the <a href="../reference/case.htm">case</a>
form will <nobr>return 9</nobr>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="15"></a>
<hr>
<h2>15 Iteration</h2>
<hr>
<p>The simplest iteration construct in Lisp is
<a href="../reference/loop.htm">loop</a>.
<nobr>A <a href="../reference/loop.htm">loop</a></nobr> construct repeatedly
executes its body until it hits a
<a href="../reference/return.htm">return</a> special form.
<nobr>For example</nobr>:</p>
<pre class="example">
> (setq a 4)
4
> (loop
(setq a (+ a 1))
(when (> a 7) (return a)))
8
> (loop
(setq a (- a 1))
(when (< a 3) (return)))
NIL
</pre>
<p>The next simplest is <a href="../reference/dolist.htm">dolist</a>.
<nobr>It binds</nobr> a variable to the elements of a list in order and
stops when it hits the end of <nobr>the list</nobr>:</p>
<pre class="example">
> (dolist (x '(a b c))
(print x))
A
B
C
NIL
</pre>
<p><a href="../reference/dolist.htm">dolist</a> always
<nobr>returns <a href="../reference/nil.htm">NIL</a></nobr>. Note that the
value of 'x' in the above example was
<nobr>never <a href="../reference/nil.htm">NIL</a></nobr>.
<nobr>The <a href="../reference/nil.htm">NIL</a></nobr> below the C was the
value that <a href="../reference/dolist.htm">dolist</a> returned, printed
by the <nobr>read-eval-print</nobr> loop.</p>
<p>The most flexible, but also most complicated iteration form is
<nobr>called <a href="../reference/do.htm">do</a></nobr>.
<nobr>A <a href="../reference/do.htm">do</a></nobr> form looks
<nobr>like this</nobr>:</p>
<pre class="example">
> (do ((x 1 (+ x 1)) <font color="#008844">; variable x, initial value 1, update with (+ x 1)</font>
(y 1 (* y 2))) <font color="#008844">; variable y, initial value 1, update with (* y 2)</font>
((> x 5) y) <font color="#008844">; terminate if (> x 5), return the value of y</font>
(print y)
(print 'working))
1
WORKING
2
WORKING
4
WORKING
8
WORKING
16
WORKING
32
</pre>
<p>The first part of a <a href="../reference/do.htm">do</a> form specifies
what variables to bind, what their initial values are, and how to update
them. <nobr>The second</nobr> part specifies a termination condition and a
return value. The last part is the body. <nobr>A
<a href="../reference/do.htm">do</a></nobr> form binds its variables to
their initial values like
<nobr>a <a href="../reference/let.htm">let</a></nobr>, then checks the
termination condition. <nobr>As long</nobr> as the condition is false, it
executes the body repeatedly. When the condition becomes true, it
returns the value of the <nobr>return-value</nobr> form.</p>
<p>The <a href="../reference/do-star.htm">do*</a> form is to
<a href="../reference/do.htm">do</a> as
<a href="../reference/let-star.htm">let*</a> is
<nobr>to <a href="../reference/let.htm">let</a></nobr>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="16"></a>
<hr>
<h2>16 Non-local Exits</h2>
<hr>
<p>The <a href="../reference/return.htm">return</a> special form is an
example of a nonlocal return. Another example is
<nobr><a href="../reference/return-from.htm">return-from</a></nobr>, which
returns a value from the surrounding function:</p>
<pre class="example">
> (defun foo (x)
(return-from foo 3)
x)
FOO
> (foo 17)
3
</pre>
<p>Actually, the <nobr><a
href="../reference/return-from.htm">return-from</a></nobr> form can return
from any named block, it's just that functions are the only blocks which are
named by default. <nobr>You can</nobr> create a named block with the
<a href="../reference/block.htm">block</a> special form:</p>
<pre class="example">
> (block foo
(return-from foo 7)
3)
7
</pre>
<p>The <a href="../reference/return.htm">return</a> special form can return
from any block <nobr>named <a href="../reference/nil.htm">NIL</a></nobr>.
Loops are by default
<nobr>named <a href="../reference/nil.htm">NIL</a></nobr>, but you can
make your own
<nobr><a href="../reference/nil.htm">NIL</a>-named blocks</nobr>:</p>
<pre class="example">
> (block nil
(return 7)
3)
7
</pre>
<p>Another form which causes a nonlocal exit is the
<nobr><a href="../reference/error.htm">error</a> form</nobr>:</p>
<pre class="example">
> (error "This is an error")
<font color="#AA0000">error: This is an error</font>
</pre>
<p>The <a href="../reference/error.htm">error</a> form applies format to its
arguments, then places you in the debugger.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="17"></a>
<hr>
<h2>17 Funcall, Apply, and Mapcar</h2>
<hr>
<p>Earlier I promised to give some functions which take functions as
arguments. Here they are:</p>
<pre class="example">
> (funcall #'+ 3 4)
7
> (apply #'+ 3 4 '(3 4))
14
> (mapcar #'not '(t nil t nil t nil))
(NIL T NIL T NIL T)
</pre>
<p><a href="../reference/funcall.htm">funcall</a> calls its first argument
on its remaining arguments.</p>
<p><a href="../reference/apply.htm">apply</a> is just like
<a href="../reference/funcall.htm">funcall</a>, except that its final
argument should be a list. <nobr>The elements</nobr> of that list are
treated as if they were additional arguments to
<nobr>a <a href="../reference/funcall.htm">funcall</a></nobr>.</p>
<p>The first argument to <a href="../reference/mapcar.htm">mapcar</a> must
be a function of one argument, <a href="../reference/mapcar.htm">mapcar</a>
applies this function to each element of a list and collects the results in
another list.</p>
<p><a href="../reference/funcall.htm">funcall</a> and
<a href="../reference/apply.htm">apply</a> are chiefly useful when their
first argument is a variable. <nobr>For instance</nobr>, a search engine
could take a heuristic function as a parameter and use
<a href="../reference/funcall.htm">funcall</a> or
<a href="../reference/apply.htm">apply</a> to call that function on a
state description. <nobr>The sorting</nobr> functions described later use
<a href="../reference/funcall.htm">funcall</a> to call their comparison
functions.</p>
<p><a href="../reference/mapcar.htm">mapcar</a>, along with nameless
<a href="#18">lambda</a> functions, can replace many loops.</p>
<p><div class="box">
<p><b>Nyquist/XLISP:</b> <nobr>In XLISP</nobr>, a '<nobr>special
form</nobr>' of type FSUBR is not a function. This means that
<a href="../reference/apply.htm">apply</a>,
<a href="../reference/funcall.htm">funcall</a> and
<a href="../reference/mapcar.htm">mapcar</a> only work with functions
of type SUBR [built-in function] or CLOSURE [function defined by
<a href="../reference/defun.htm">defun</a>,
<a href="../reference/flet.htm">flet</a>,
<a href="../reference/labels.htm">labels</a>, or
<a href="../reference/lambda.htm">lambda</a>], but with special forms
of <nobr>type FSUBR</nobr> a 'bad argument type' error is signalled.</p>
</div></p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="18"></a>
<hr>
<h2>18 Lambda</h2>
<hr>
<p>If you just want to create a temporary function and don't want to
bother giving it a name, <a href="../reference/lambda.htm">lambda</a> is
what you need.</p>
<pre class="example">
> #'(lambda (x)
(+ x 3))
#<Closure: #88b71ece>
> (funcall * 5)
8
</pre>
<p>The combination of <a href="../reference/lambda.htm">lambda</a> and
<a href="../reference/mapcar.htm">mapcar</a> can replace many loops.
<nobr>For example</nobr>, the following two forms are equivalent:</p>
<pre class="example">
> (do ((x '(1 2 3 4 5) (cdr x))
(y nil))
((null x) (reverse y))
(push (+ (car x) 2) y))
(3 4 5 6 7)
> (mapcar #'(lambda (x) (+ x 2)) '(1 2 3 4 5))
(3 4 5 6 7)
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="19"></a>
<hr>
<h2>19 Sorting</h2>
<hr>
<p>Lisp provides a primitive for
<nobr>sorting, <a href="../reference/sort.htm">sort</a></nobr>:</p>
<pre class="example">
> (sort '(2 1 5 4 6) #'<)
(1 2 4 5 6)
> (sort '(2 1 5 4 6) #'>)
(6 5 4 2 1)
</pre>
<p>The first argument to <a href="../reference/sort.htm">sort</a> is a list,
the second is a comparison function. <nobr>Be careful</nobr>, because
<a href="../reference/sort.htm">sort</a> is allowed to destroy its argument,
so if the original sequence is important to you, make a copy before sorting
<nobr>the list</nobr>.</p>
<p><div class="box">
<p><b>Bug:</b> In Nyquist 3.03 [November 2010] the XLISP
<a href="../reference/sort.htm">sort</a> function has a bug, so it's better
to store the return value of sort in the original variable <nobr>like
this</nobr>:</p>
<pre class="example">
(setq a '(3 1 4 1 5 9 6 7)) => (3 1 4 1 5 9 6 7)
(setq a (sort a '<)) => (1 1 3 4 5 6 7 9)
a => (1 1 3 4 5 6 7 9)
</pre>
</div></p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="20"></a>
<hr>
<h2>20 Equality</h2>
<hr>
<p>Lisp has many different ideas of equality. Numerical equality is
denoted by =. Two symbols are eq if and only if they are identical. Two
copies of the same list are not eq, but they are equal.</p>
<pre class="example">
> (eq 'a 'a)
T
> (eq 'a 'b)
NIL
> (= 3 4)
T
> (eq '(a b c) '(a b c))
NIL
> (equal '(a b c) '(a b c))
T
> (eql 'a 'a)
T
> (eql 3 3)
T
</pre>
<p>The eql predicate is equivalent to eq for symbols and to = for numbers.</p>
<p>The equal predicate is equivalent to eql for symbols and numbers. It is
true for two conses if and only if their cars are equal and their cdrs are
equal.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="21"></a>
<hr>
<h2>21 Some Useful List Functions</h2>
<hr>
<p>These functions all manipulate lists:</p>
<pre class="example">
> (append '(1 2 3) '(4 5 6)) ;concatenate lists
(1 2 3 4 5 6)
> (reverse '(1 2 3)) ;reverse the elements of a list
(3 2 1)
> (member 'a '(b d a c)) ;set membership -- returns the first tail
(A C) ;whose car is the desired element
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<hr>
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