74 lines
2.2 KiB
HTML
74 lines
2.2 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE> More Functions and Predicates</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" More Functions and Predicates">
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<meta name="keywords" value="lp">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<BR> <HR>
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<A HREF="node13.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node2.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node11.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node13.html"> Setf</A>
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<B>Up:</B>
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<A HREF="node2.html"> LISt Processing</A>
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<B> Previous:</B>
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<A HREF="node11.html"> Changing Variable Values</A>
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<BR> <HR> <P>
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<H1> More Functions and Predicates</H1>
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<P>
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To find out the length of a list, there is a function called,
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appropriately enough, length. It takes a single argument which
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should be a list.
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<BLOCKQUOTE>
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<PRE>> (length '(1 2 3))
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3
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> (length a)
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5
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> (length (append a a))
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10
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> (length '(append a a))
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3
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> (length (list a a))
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2
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</PRE>
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</BLOCKQUOTE>
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<P>
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Predicates are functions that always return either t or nil. Atom is a predicate that determines whether its argument is an atom.
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Listp returns t if its argument is a list, and nil otherwise.
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<BLOCKQUOTE>
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<PRE>> (atom 'a)
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T
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> (atom a)
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NIL
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> (listp 'a)
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NIL
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> (listp a)
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T
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</PRE>
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</BLOCKQUOTE>
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Find out for yourself how atom and listp work with the empty list,
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NIL.
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<P>
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Symbolp and numberp are also useful predicates. Experiment with them
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to find out how they work. Constantp is less frequently used, but
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might come in handy some time.
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<P>
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Use the appendix entries, together with the Lisp interpreter, to figure out how these functions and predicates work:
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<P>
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second, third, fourth,..., last, nthcdr, butlast, nbutlast,
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reverse, caar, cddr, cadr, cdar, constantp, integerp.
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<BR> <HR>
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<P>
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<ADDRESS>
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<I>© Colin Allen & Maneesh Dhagat <BR>
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March 2007 </I>
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</ADDRESS>
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</BODY>
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