66 lines
6.1 KiB
HTML
66 lines
6.1 KiB
HTML
<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Function UNREAD-CHAR</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="f_terpri.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_stream.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_wr_cha.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="unread-char"><I>Function</I> <B>UNREAD-CHAR</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>unread-char</B> <I>character <TT>&optional</TT> input-stream</I> => <I><A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A></I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>character</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A>; must be the last <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> that was read from <I>input-stream</I>. <P>
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<I>input-stream</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#input"><I>input</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#stream_designator"><I>stream designator</I></A>. The default is <A REL=DEFINITION HREF="26_glo_s.htm#standard_input"><I>standard input</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> places <I>character</I> back onto the front of <I>input-stream</I> so that it will again be the next character in <I>input-stream</I>. <P>
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When <I>input-stream</I> is an <A REL=DEFINITION HREF="26_glo_e.htm#echo_stream"><I>echo stream</I></A>, no attempt is made to undo any echoing of the character that might already have been done on <I>input-stream</I>. However, characters placed on <I>input-stream</I> by <A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> are marked in such a way as to inhibit later re-echo by <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A>. <P>
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It is an error to invoke <A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> twice consecutively on the same <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A> without an intervening call to <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A> (or some other input operation which implicitly reads characters) on that <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>. <P>
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Invoking <A REL=DEFINITION HREF="f_peek_c.htm#peek-char"><B>peek-char</B></A> or <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A> commits all previous characters. The consequences of invoking <A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> on any character preceding that which is returned by <A REL=DEFINITION HREF="f_peek_c.htm#peek-char"><B>peek-char</B></A> (including those passed over by <A REL=DEFINITION HREF="f_peek_c.htm#peek-char"><B>peek-char</B></A> that has a <A REL=DEFINITION HREF="26_glo_n.htm#non-nil"><I>non-nil</I></A> <I>peek-type</I>) are unspecified. In particular, the consequences of invoking <A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> after <A REL=DEFINITION HREF="f_peek_c.htm#peek-char"><B>peek-char</B></A> are unspecified. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(with-input-from-string (is "0123")
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(dotimes (i 6)
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(let ((c (read-char is)))
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(if (evenp i) (format t "~&~S ~S~%" i c) (unread-char c is)))))
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>> 0 #\0
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>> 2 #\1
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>> 4 #\2
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=> NIL
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B><P>
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<P>
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<A REL=DEFINITION HREF="v_debug_.htm#STstandard-inputST"><B>*standard-input*</B></A>, <A REL=DEFINITION HREF="v_termin.htm#STterminal-ioST"><B>*terminal-io*</B></A>. <P>
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<P><B>Exceptional Situations:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_peek_c.htm#peek-char"><B>peek-char</B></A>, <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A>, <A REL=CHILD HREF="21_a.htm">Section 21.1 (Stream Concepts)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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<A REL=DEFINITION HREF="#unread-char"><B>unread-char</B></A> is intended to be an efficient mechanism for allowing the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A> and other parsers to perform one-character lookahead in <I>input-stream</I>. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss356.htm">UNREAD-CHAR-AFTER-PEEK-CHAR:DONT-ALLOW</A><LI> <A REL=CHILD HREF="../Issues/iss268.htm">PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR</A><P></UL><HR>
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