Nehtdx

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").

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.

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.