71 lines
8.3 KiB
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71 lines
8.3 KiB
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<!-- 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 PEEK-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_wr_by.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_rd_cha.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="peek-char"><I>Function</I> <B>PEEK-CHAR</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>peek-char</B> <I><TT>&optional</TT> peek-type input-stream eof-error-p eof-value recursive-p</I> => <I>char</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>peek-type</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> or <A REL=DEFINITION HREF="a_t.htm#t"><B>t</B></A> or <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>input-stream</I>---<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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<I>eof-error-p</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. The default is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>. <P>
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<I>eof-value</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. The default is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>recursive-p</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. The default is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. <P>
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<I>char</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> or the <I>eof-value</I>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#peek-char"><B>peek-char</B></A> obtains the next character in <I>input-stream</I> without actually reading it, thus leaving the character to be read at a later time. It can also be used to skip over and discard intervening characters in the <I>input-stream</I> until a particular character is found. <P>
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If <I>peek-type</I> is not supplied or <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, <A REL=DEFINITION HREF="#peek-char"><B>peek-char</B></A> returns the next character to be read from <I>input-stream</I>, without actually removing it from <I>input-stream</I>. The next time input is done from <I>input-stream</I>, the character will still be there. If <I>peek-type</I> is <A REL=DEFINITION HREF="a_t.htm#t"><B>t</B></A>, then <A REL=DEFINITION HREF="#peek-char"><B>peek-char</B></A> skips over <A REL=DEFINITION HREF="26_glo_w.htm#whitespace"><I>whitespace</I></A>[2] <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>characters</I></A>, but not comments, and then performs the peeking operation on the next character. The last character examined, the one that starts an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>, is not removed from <I>input-stream</I>. If <I>peek-type</I> is a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A>, then <A REL=DEFINITION HREF="#peek-char"><B>peek-char</B></A> skips over input characters until a character that is <A REL=DEFINITION HREF="f_chareq.htm#charEQ"><B>char=</B></A> to that <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> is found; that character is left in <I>input-stream</I>. <P>
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If an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs and <I>eof-error-p</I> is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, <I>eof-value</I> is returned. <P>
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If <I>recursive-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, this call is expected to be embedded in a higher-level call to <A REL=DEFINITION HREF="f_rd_rd.htm#read"><B>read</B></A> or a similar <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> used by the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A>. <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>, characters that are only peeked at are not echoed. In the case that <I>peek-type</I> is not <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, the characters that are passed by <A REL=DEFINITION HREF="#peek-char"><B>peek-char</B></A> are treated as if by <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A>, and so are echoed unless they have been marked otherwise by <A REL=DEFINITION HREF="f_unrd_c.htm#unread-char"><B>unread-char</B></A>. <P>
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<P><B>Examples:</B><P>
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<PRE>
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(with-input-from-string (input-stream " 1 2 3 4 5")
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(format t "~S ~S ~S"
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(peek-char t input-stream)
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(peek-char #\4 input-stream)
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(peek-char nil input-stream)))
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>> #\1 #\4 #\4
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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_rdtabl.htm#STreadtableST"><B>*readtable*</B></A>, <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><P>
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<P>
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If <I>eof-error-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> and an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_end_of.htm#end-of-file"><B>end-of-file</B></A> is signaled. <P>
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If <I>peek-type</I> is a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A>, an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs, and <I>eof-error-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_end_of.htm#end-of-file"><B>end-of-file</B></A> is signaled. <P>
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If <I>recursive-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> and an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs, an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_end_of.htm#end-of-file"><B>end-of-file</B></A> is signaled. <P>
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<P><B>See Also:</B> None.
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<P>
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<P><B>Notes:</B> None.
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<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/iss268.htm">PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR</A><LI> <A REL=CHILD HREF="../Issues/iss013.htm">ARGUMENTS-UNDERSPECIFIED:SPECIFY</A><P></UL><HR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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