Etydhv

I have the following command: newcommandtriangles[2]triangles $triangle #1$ and $triangle #2, such that trianglesABCDEF produces triangles $triangle ABC$ and $triangle DEF$. I would like to expand that command such that it can take any number of arguments and such that trianglesABCDEFGHIJKL produces triangles $triangle ABC$, $triangle DEF$, $triangle GHI$ and $triangle DEF$. I have seen a solution somewhere for a shoppinglist that makes use of makeatletter and some TeX command, but this is more difficult, I think, because before you add the current argument, you need to know whether it is the last argument (if not: put a comma, if yes: put "and").

The shoppinglist you're talking about is at Writing a LaTeX macro that takes a variable number of arguments.

You can adapt it quickly to work with any number of triangles. The case with one triangle is also handled in order to only display "Triangle" instead of "Triangles".

documentclass[11pt]article

makeatletter
 newcommandtriangles[1]%
 checknextarg#1
 newcommandchecknextarg[1]%
 @ifnextcharbgroupTriangles $triangle #1$gobblenextargTriangle $triangle #1$
 newcommandgobblenextarg[1]%
 @ifnextcharbgroup, $triangle #1$gobblenextarg and $triangle #1$
makeatother

begindocument

trianglesABC

trianglesABCDEF

trianglesABCDEFGHI

trianglesABCDEFGHIJKL

enddocument

This is the result obtained:

EDIT

Following the comment by Ulrich Diez, an improved version of this command would be:

documentclass[11pt]article

usepackageltxcmds

makeatletter
 newcommandtriangles[1]%
 checknextarg#1
 newcommandchecknextarg[1]%
 ltx@ifnextchar@nospacebgroupTriangles $triangle #1$gobblenextargTriangle $triangle #1$
 newcommandgobblenextarg[1]%
 ltx@ifnextchar@nospacebgroup, $triangle #1$gobblenextarg and $triangle #1$
makeatother

begindocument

trianglesABC

trianglesABCDEF

trianglesABCDEFGHI

trianglesABCDEFGHIJKL

trianglesABCDEFGHIJKL Large This shall not be part of the command

enddocument

where ltx@ifnextchar@nospace does not skip space and prevent the text after the command to be interpreted as an "argument" of the command triangles.

However, as stated in egreg's answer, this way of doing things should be avoided.

Macros with a variable number of arguments are not the best thing to do with LaTeX; in your case it's simpler to use a comma separated list of triangles.

documentclassarticle
usepackagexparse

ExplSyntaxOn

NewDocumentCommandtrianglesm
 
 bart_triangles:n #1 
 

seq_new:N l__bart_triangles_seq
tl_new:N l__bart_triangles_item_tl

cs_new_protected:Nn bart_triangles:n
 
 seq_clear:N l__bart_triangles_seq
 clist_map_variable:nNn #1 l__bart_triangles_item_tl
 
 __bart_triangles_add:V l__bart_triangles_item_tl
 
 triangle
 int_compare:nF seq_count:N l__bart_triangles_seq < 2 s 
 nobreakspace
 seq_use:Nnnn l__bart_triangles_seq ~and~ ,~ ~and~ 
 

cs_new_protected:Nn __bart_triangles_add:n
 
 seq_put_right:Nn l__bart_triangles_seq $triangle #1$ 
 
cs_generate_variant:Nn __bart_triangles_add:n V 

ExplSyntaxOff

begindocument

trianglesABC

trianglesABC,DEF

trianglesABC,DEF,GHI

enddocument

The comma separated list is mapped and each item is added to a sequence in the form $triangle<vertices>$. Then the sequence is used, with the specified separators (between two, between more than two, between last two).

The “s” is added only if the list contains at least two items.

A listofitems approach. Note: I was taught that for a list of more than 2 items, a comma precedes the "and".

documentclassarticle
usepackagelistofitems
newcommandtriangles[1]%
 setsepchar,%
 readlisttrilist#1%
 foreachitemxintrilist[]%
 $trianglex$%
 ifnumnumexprxcnt+1<listlentrilist[]relax%
 , %
 else%
 ifnumnumexprxcnt+1=listlentrilist[]relax%
 ifnumlistlentrilist[]>2relax,fi
 and %
 fi
 fi%
 %

begindocument
Given trianglesABC.par
Given trianglesABC, DEF.par
Given trianglesABC, DEF,GHI.par
Given trianglesABC, DEF,GHI, JKL.par
enddocument

What about
trianglesABCDEFGHIJKL large This shall not be part of the triangle-command ?

For the sake of unambiguity regarding the question which brace groups belong to triangles and which do not, I strongly recommend a syntax like trianglesABCDEFGHI.

Or a syntax without brace-groups at all, as suggested by egreg, trianglesABC,DEF,GHI

The following example exhibits a romannumeral-expansion-based way of implementing the trianglesABCDEFGHI-syntax:

documentclassarticle

makeatletter
%%----------------------------------------------------------------------
newcommandUD@gobble[1]%
newcommandUD@firstoftwo[2]#1%
newcommandUD@secondoftwo[2]#2%
newcommandUD@exchange[2]#2#1%
newcommandUD@PassFirstToSecond[2]#2#1%
%%-----------------------------------------------------------------------------
%% Extract first inner undelimited argument:
%%.............................................................................
%% UD@ExtractFirstArgABCDE yields A
%%
%% UD@ExtractFirstArgABCDE yields AB
%%
%% !!! The argument of UD@ExtractFirstArg must not be empty. !!!
%% You can check for emptiness via UD@CheckWhetherNull before applying
%% UD@ExtractFirstArg.
%% The argument of UD@ExtractFirstArg may contain the token UD@SelDOm.
%% 
%%.............................................................................
newcommandUD@RemoveTillUD@SelDOm%
longdefUD@RemoveTillUD@SelDOm#1#2UD@SelDOm#1%
newcommandUD@ExtractFirstArg[1]%
 romannumeral0%
 UD@ExtractFirstArgLoop#1UD@SelDOm%
%
newcommandUD@ExtractFirstArgLoop[1]%
 expandafterUD@CheckWhetherNullexpandafterUD@firstoftwo#1%
 UD@exchange#1 %
 expandafterUD@ExtractFirstArgLoopexpandafterUD@RemoveTillUD@SelDOm#1%
%
%%----------------------------------------------------------------------
%% Check whether argument is empty:
%%......................................................................
%% UD@CheckWhetherNull<Argument which is to be checked>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is empty>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is not empty>%
%% The gist of this macro comes from Robert R. Schneck's ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
newcommandUD@CheckWhetherNull[1]%
 romannumeral0expandafterUD@secondoftwostringexpandafter
 UD@secondoftwoexpandafterexpandafterstring#1expandafter
 UD@secondoftwostringexpandafterUD@firstoftwoexpandafterexpandafter
 UD@secondoftwostringexpandafterexpandafterUD@firstoftwo %
 UD@secondoftwoexpandafterexpandafterUD@firstoftwo UD@firstoftwo%
%
%%------------------------------------------------------------------------------
%% Check whether argument is blank (empty or only spaces):
%%..............................................................................
%% -- Take advantage of the fact that TeX discards space tokens when
%% "fetching" _un_delimited arguments: --
%% UD@CheckWhetherBlank<Argument which is to be checked>%
%% <Tokens to be delivered in case that
%% argument which is to be checked is blank>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is not blank%
newcommandUD@CheckWhetherBlank[1]%
 romannumeralexpandafterexpandafterexpandafterUD@secondoftwo
 expandafterUD@CheckWhetherNullexpandafterUD@firstoftwo#1.%
%
%%------------------------------------------------------------------------------
%% The triangles-list:
%%..............................................................................
newcommandtriangles[1]%
 romannumeral0UD@CheckWhetherBlank#1 no triangletriangleloop#1%
%
newcommandtriangleloop[2]%
 UD@CheckWhetherBlank#1 #2%
 expandafterUD@PassFirstToSecondexpandafter%
 romannumeral0expandafterUD@exchangeexpandafter
 %
 romannumeral0expandafterexpandafterexpandafterUD@exchange
 expandafterexpandafterexpandafterUD@ExtractFirstArg#1$ $triangle%
 %
 expandafterUD@CheckWhetherBlankexpandafterUD@gobble#1%
 UD@CheckWhetherBlank#2 triangle triangles #2 and %
 %
 UD@CheckWhetherBlank#2 #2, %
 %
 %
 %
 expandaftertriangleloopexpandafterUD@gobble#1%
 %
 %
%
makeatother


begindocument

trianglespar
trianglesABCpar
trianglesABC DEF GHIpar
trianglesABC DEFpar
trianglesABC DEF GHI JKLpar

expandafterexpandafterexpandafterdef
expandafterexpandafterexpandafterThisTrianglelist
expandafterexpandafterexpandafter%
 trianglesABC DEF GHI JKL%
%

textttstringThisTrianglelist=meaningThisTrianglelist

enddocument

In case you wish a comma before the "and" when the list has more than two items:

documentclassarticle

makeatletter
%%----------------------------------------------------------------------
newcommandUD@gobble[1]%
newcommandUD@firstoftwo[2]#1%
newcommandUD@secondoftwo[2]#2%
newcommandUD@exchange[2]#2#1%
newcommandUD@PassFirstToSecond[2]#2#1%
%%-----------------------------------------------------------------------------
%% Extract first inner undelimited argument:
%%.............................................................................
%% UD@ExtractFirstArgABCDE yields A
%%
%% UD@ExtractFirstArgABCDE yields AB
%%
%% !!! The argument of UD@ExtractFirstArg must not be empty. !!!
%% You can check for emptiness via UD@CheckWhetherNull before applying
%% UD@ExtractFirstArg.
%% The argument of UD@ExtractFirstArg may contain the token UD@SelDOm.
%% 
%%.............................................................................
newcommandUD@RemoveTillUD@SelDOm%
longdefUD@RemoveTillUD@SelDOm#1#2UD@SelDOm#1%
newcommandUD@ExtractFirstArg[1]%
 romannumeral0%
 UD@ExtractFirstArgLoop#1UD@SelDOm%
%
newcommandUD@ExtractFirstArgLoop[1]%
 expandafterUD@CheckWhetherNullexpandafterUD@firstoftwo#1%
 UD@exchange#1 %
 expandafterUD@ExtractFirstArgLoopexpandafterUD@RemoveTillUD@SelDOm#1%
%
%%----------------------------------------------------------------------
%% Check whether argument is empty:
%%......................................................................
%% UD@CheckWhetherNull<Argument which is to be checked>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is empty>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is not empty>%
%% The gist of this macro comes from Robert R. Schneck's ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
newcommandUD@CheckWhetherNull[1]%
 romannumeral0expandafterUD@secondoftwostringexpandafter
 UD@secondoftwoexpandafterexpandafterstring#1expandafter
 UD@secondoftwostringexpandafterUD@firstoftwoexpandafterexpandafter
 UD@secondoftwostringexpandafterexpandafterUD@firstoftwo %
 UD@secondoftwoexpandafterexpandafterUD@firstoftwo UD@firstoftwo%
%
%%------------------------------------------------------------------------------
%% Check whether argument is blank (empty or only spaces):
%%..............................................................................
%% -- Take advantage of the fact that TeX discards space tokens when
%% "fetching" _un_delimited arguments: --
%% UD@CheckWhetherBlank<Argument which is to be checked>%
%% <Tokens to be delivered in case that
%% argument which is to be checked is blank>%
%% <Tokens to be delivered in case that argument
%% which is to be checked is not blank%
newcommandUD@CheckWhetherBlank[1]%
 romannumeralexpandafterexpandafterexpandafterUD@secondoftwo
 expandafterUD@CheckWhetherNullexpandafterUD@firstoftwo#1.%
%
%%------------------------------------------------------------------------------
%% The triangles-list:
%%..............................................................................
newcommandtriangles[1]%
 romannumeral0UD@CheckWhetherBlank#1 no triangletriangleloop#1%
%
newcommandtriangleloop[3]%
 UD@CheckWhetherBlank#1 #2%
 expandafterUD@exchangeexpandafter%
 romannumeral0UD@exchange expandafterexpandafterexpandafter
 expandafterexpandafterexpandafterexpandafter%
 expandafterUD@exchangeexpandafter%
 romannumeral0%
 expandafterUD@CheckWhetherBlankexpandafterUD@gobble#1 %
 UD@CheckWhetherBlank#2 ,%
 %
 expandafter%
 romannumeral0expandafterUD@exchangeexpandafter
 %
 romannumeral0expandafterexpandafterexpandafterUD@exchange
 expandafterexpandafterexpandafterUD@ExtractFirstArg#1$ $triangle%
 %
 expandafterUD@CheckWhetherBlankexpandafterUD@gobble#1%
 UD@CheckWhetherBlank#2 triangle triangles #2#3 and %
 %
 UD@CheckWhetherBlank#2 #2, %
 %
 %
 %
 %
 expandaftertriangleloopexpandafterUD@gobble#1%
 %
 %
%
makeatother


begindocument

trianglespar
trianglesABCpar
trianglesABC DEFpar
trianglesABC DEF GHIpar
trianglesABC DEF GHI JKLpar

expandafterexpandafterexpandafterdef
expandafterexpandafterexpandafterThisTrianglelist
expandafterexpandafterexpandafter%
 trianglesABC DEF GHI JKL%
%

textttstringThisTrianglelist=meaningThisTrianglelist

enddocument