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133 lines
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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: Issue MAP-INTO Writeup</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="../Issues/iss238_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss239.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss240_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue MAP-INTO Writeup</H2>
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<PRE><B>Status:</B> Passed, Jun 89 X3J13<P>
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<B>Issue:</B> <A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A><P>
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<B>References:</B> none<P>
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Related issues: BIT-ARRAY-FUNCTIONS<P>
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<B>Category:</B> ADDITION<P>
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<B>Edit history:</B> 23-May-89, version 1 by Moon<P>
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19-Jun-89, version 2 by Moon (fix arglist)<P>
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23-Jun-89, version 3 by Moon (include BarMar's suggestions)<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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The function <A REL=DEFINITION HREF="../Body/f_map.htm#map"><B>MAP</B></A> is very useful but can be a source of inefficiency<P>
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because it conses the result. Sometimes the user has storage<P>
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already allocated in which the result could be stored.<P>
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<P>
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<B>Proposal (MAP-INTO:ADD-FUNCTION):<P>
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</B><P>
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Add the following function:<P>
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<P>
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<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> result-sequence function &rest sequences [Function]<P>
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<P>
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Destructively modifies the result-sequence to contain the results of<P>
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applying function to each element in the argument sequences in turn.<P>
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Returns result-sequence.<P>
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<P>
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The arguments result-sequence and each element of sequences can each be<P>
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either a list or a vector (one-dimensional array). Note that <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is<P>
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considered to be a sequence, of length zero. If result-sequence and<P>
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each element of sequences are not all the same length, the iteration<P>
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terminates when the shortest sequence is exhausted. If result-sequence<P>
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is a vector with a <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A>, the <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> is ignored when<P>
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deciding how many iterations to perform, and afterwards the<P>
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<A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> is set to the number of times function was applied.<P>
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<P>
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If result-sequence is longer than the shortest element of sequences,<P>
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extra elements at the end of result-sequence are left unchanged.<P>
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<P>
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<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> differs from <A REL=DEFINITION HREF="../Body/f_map.htm#map"><B>MAP</B></A> in that it modifies an existing sequence<P>
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rather than creating a new one. In addition, <A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> can be called<P>
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with only two arguments, while <A REL=DEFINITION HREF="../Body/f_map.htm#map"><B>MAP</B></A> requires at least three arguments.<P>
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If result-sequence is <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, <A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> immediately returns <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, since<P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is a sequence of length zero.<P>
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<P>
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If BIT-ARRAY-FUNCTIONS:NO-NEW-FUNCTIONS passes, then <A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> will<P>
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allow result-sequence and each element of sequences to be mappables<P>
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all of the same rank.<P>
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<P>
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The function must take at least as many arguments as there are<P>
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sequences provided.<P>
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<P>
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If function has side effects, it can count on being called first on all<P>
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of the elements with index 0, then on all of those numbered 1, and so<P>
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on.<P>
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<P>
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<B>Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>map-into</B></A> x #'+ x y)<P>
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(<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>map-into</B></A> q #'<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>cons</B></A> keys vals)<P>
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(<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>map-into</B></A> syms #'<A REL=DEFINITION HREF="../Body/f_gensym.htm#gensym"><B>gensym</B></A>)<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_map_in.htm#map-into"><B>MAP-INTO</B></A> is a simple way to express reuse of storage that is<P>
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stylistically consistent with the rest of Common Lisp.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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Symbolics Genera 7.2 implements the proposal.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Small.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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Small.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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More expressive language.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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User programs won't have to write the above examples as<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_loop.htm#loop"><B>loop</B></A> for xx on x <A REL=DEFINITION HREF="../Body/a_and.htm#and"><B>and</B></A> yy in y <A REL=DEFINITION HREF="../Body/m_do_do.htm#do"><B>do</B></A><P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>car</B></A> xx) (+ (<A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>car</B></A> xx) yy)))<P>
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<P>
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or something else about equally horrible.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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None.<P>
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</PRE>
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<HR>
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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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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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