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<TITLE>Common LISP Hints: Setf</TITLE>
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<H2><A NAME="s16">16. Setf</A></H2>
<P>Certain forms in LISP naturally define a memory location. For example,
if the value of <CODE>x</CODE> is a structure of type <CODE>foo</CODE>, then
<CODE>(foo-bar x)</CODE> defines
the <CODE>bar</CODE> field of the value of <CODE>x</CODE>. Or, if the value of <CODE>y</CODE>
is a one-dimensional array, <CODE>(aref y 2)</CODE> defines the third element
of <CODE>y</CODE>. </P>
<P>The <CODE>setf</CODE> 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. For example,</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
> (setq a (make-array 3))
#(NIL NIL NIL)
> (aref a 1)
NIL
> (setf (aref a 1) 3)
3
> a
#(NIL 3 NIL)
> (aref a 1)
3
> (defstruct foo bar)
FOO
> (setq a (make-foo))
#s(FOO :BAR NIL)
> (foo-bar a)
NIL
> (setf (foo-bar a) 3)
3
> a
#s(FOO :BAR 3)
> (foo-bar a)
3
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P><CODE>Setf</CODE> is the only way to set the fields of a structure or the elements
of an array.</P>
<P>Here are some more examples of <CODE>setf</CODE> and related functions.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
> (setf a (make-array 1)) ;setf on a variable is equivalent to setq
#(NIL)
> (push 5 (aref a 1)) ;push can act like setf
(5)
> (pop (aref a 1)) ;so can pop
5
> (setf (aref a 1) 5)
5
> (incf (aref a 1)) ;incf reads from a place, increments,
6 ;and writes back
> (aref a 1)
6
</PRE>
</CODE></BLOCKQUOTE>
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