(\bar is already defined in LaTeX 2ε, therefore below \FOO and \BAR are used instead.)
I can offer a macro
\KeepKthOfLArguments{⟨TeX-⟨number⟩-quantity with integer-value K⟩}%
{⟨TeX-⟨number⟩-quantity with integer-value L⟩}%
⟨list of L undelimited arguments⟩%
which does the following:
In case there is no ⟨K⟩-th argument in the ⟨list of L undelimited arguments⟩, i.e., in case K<1 or K>L:
Does remove the ⟨list of L undelimited arguments⟩ without returning any token.
In case there is a ⟨K⟩-th argument in the ⟨list of undelimited arguments⟩:
Does deliver that ⟨K⟩-th argument with one level of curly braces removed if present.
Examples:
\KeepKthOfLArguments{0}{5}{A}{B}{C}{D}{E} yields no token at all.
\KeepKthOfLArguments{3}{12}{A}{B}{C}{D}{E}{F}{G}{H}{I}{J}{K}{L} yields: C
\KeepKthOfLArguments{3}{4}{A}{B}{CD}{E} yields: CD
\KeepKthOfLArguments{4}{5}{001}{002}{003}{004}{005} yields: 004
\KeepKthOfLArguments{6}{3}{001}{002}{003} yields no token at all.
\KeepKthOfLArguments{4}{5}{001}{002}{003}{{004}}{005} yields: {004}
Due to \romannumeral-expansion the result is delivered after two expansion-steps/by "hitting" \KeepKthOfLArguments with \expandafter twice/by "hitting" the toplevel-expansion of \KeepKthOfLArguments with \expandafter once.
As ⟨TeX-⟨number⟩-quantity with integer-value K⟩ you can use the "variable" \i of a \foreach-loop:
\errorcontextlines=10000
\documentclass[a4paper]{article}
\usepackage{tikz}
\makeatletter
%% Code for \KeepKthOfLArguments:
%%=============================================================================
%% Paraphernalia:
%% \UD@firstoftwo, \UD@secondoftwo, \UD@PassFirstToSecond,
%% \UD@stopromannumeral, \UD@CheckWhetherNull
%%=============================================================================
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@PassFirstToSecond[2]{#2{#1}}%
@ifdefinable\UD@stopromannumeral{\chardef\UD@stopromannumeral=`^^00}%
%%-----------------------------------------------------------------------------
%% 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>
\newcommand\UD@CheckWhetherNull[1]{%
\romannumeral\expandafter\UD@secondoftwo\string{\expandafter
\UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
\UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
\UD@secondoftwo\string}\expandafter\UD@stopromannumeral\UD@secondoftwo}{%
\expandafter\UD@stopromannumeral\UD@firstoftwo}%
}%
%%=============================================================================
%% Keep only the K-th of L consecutive undelimited arguments.
%% ( IF K < 1 OR K > L just remove L consecutive undelimited arguments. )
%%=============================================================================
%% \KeepKthOfLArguments{<integer number K>}%
%% {<integer number L>}%
%% <sequence of L consecutive undelimited arguments>
%%
%% If L < 1 yields nothing.
%% Else:
%% If K >= 1 and K < L yields:
%% <K-th undelimited argument from <sequence of L consecutive undelimited
%% arguments>>
%% If K < 1 or K > L
%% (-> there is no K-th argument in the
%% <sequence of L consecutive undelimited arguments> )
%% yields nothing but removal of <sequence of L consecutive
%% undelimited arguments>
\newcommand\KeepKthOfLArguments[2]{%
\romannumeral
% #1: <integer number K>
% #2: <integer number L>
\expandafter\UD@KeepKthOfLArgumentsKSmallerOneFork
\expandafter{\romannumeral\number\number#1 000\expandafter}%
\expandafter{\romannumeral\number\number#2 000}%
}%
%%-----------------------------------------------------------------------------
\newcommand\UD@KeepKthOfLArgumentsKSmallerOneFork[2]{%
% #1: <K letters m>
% #2: <L letters m >
\UD@CheckWhetherNull{#1}{% K is smaller than one:
\UD@KeepKthOfLArgumentsRemoveNArguments{#2}{\UD@stopromannumeral}{}%
}{% K is not smaller than one:
\expandafter\UD@PassFirstToSecond
\expandafter{%
\UD@firstoftwo{}#1%
}{%
\UD@KeepKthOfLArgumentsEvaluateLMinusKDifferenceLoop{#1}{#2}%
}{#2}%
}%
}%
%%-----------------------------------------------------------------------------
\newcommand\UD@KeepKthOfLArgumentsEvaluateLMinusKDifferenceLoop[4]{%
% #1: <K letters m>
% #2: <L letters m>
% (For detecting whether K>L or K<=L, during the loop letters m will
% be removed both from #1 and #2 until at least one of these arguments
% is empty.
% When the loop terminates with 0<K<=L, #1 will be empty and #2
% will hold an amount of letters m corresponding to the the
% difference L-K.
% When the loop terminates with K>L, #1 will not be empty and #2
% will be empty.
% )
% #3: <K-1 letters m>
% #4: <L letters m>
% (#3 and #4 will be left untouched during the loop so they can be
% used for performing appropriate action when loop terminates as
% it is known whether K>L.)
\UD@CheckWhetherNull{#1}{% We have K<=L:
\UD@KeepKthOfLArgumentsRemoveNArguments{%
#3%
}{%
\UD@KeepKthOfLArgumentsRemoveNArguments{#2}{\UD@stopromannumeral}%
}{}%
}{%
\UD@CheckWhetherNull{#2}{% We have K>L:
\UD@KeepKthOfLArgumentsRemoveNArguments{#4}{\UD@stopromannumeral}{}%
}{% We don't know yet whether K<=L or K>L, thus remove letters m and
% do another iteration:
\expandafter\UD@PassFirstToSecond
\expandafter{%
\UD@firstoftwo{}#2%
}{%
\expandafter\UD@KeepKthOfLArgumentsEvaluateLMinusKDifferenceLoop
\expandafter{%
\UD@firstoftwo{}#1%
}%
}{#3}{#4}%
}%
}%
}%
%%-----------------------------------------------------------------------------
%% \UD@KeepKthOfLArgumentsRemoveNArguments{<N letters m>}%
%% {<argument 1>}%
%% {<argument 2>}%
%% <sequence of consecutive
%% undelimited arguments>
%%.............................................................................
%% Removes the first N undelimited arguments from the <sequence of
%% consecutive undelimited arguments>, then inserts
%% <argument 1><argument 2>
%%
%% On the one hand when providing <argument 2> empty, you can use
%% <argument 1> for nesting calls to \UD@KeepKthOfLArgumentsRemoveNArguments.
%% On the other hand you can provide a <space token> for stopping
%% \romannumeral-expansion as <argument 1> and have the
%% macro grab the <K-th undelimited argument> from the <sequence of L
%% consecutive undelimited arguments> as <argument 2>.
%%
\newcommand\UD@KeepKthOfLArgumentsRemoveNArguments[3]{%
%% #1: <N letters m>
%% #2: <Argument 1>
%% #3: <Argument 2>
\UD@CheckWhetherNull{#1}{#2#3}{%
\UD@firstoftwo{%
\expandafter\UD@KeepKthOfLArgumentsRemoveNArguments
\expandafter{%
\UD@firstoftwo{}#1%
}{#2}{#3}%
}%
}%
}%
%%-----------------------------------------------------------------------------
%% End of code for \KeepKthOfLArguments.
\makeatother
\newcommand\FOO[1]{%
%if #1 has some property
\par \noindent Argument is: #1%
%\fi
}
\newcommand\BAR[9]{%
\foreach \i in {1,...,9}{%
%if (expansion of) \i has some property
\expandafter\expandafter\expandafter\FOO
\expandafter\expandafter\expandafter{%
\KeepKthOfLArguments{\i}{9}{#1}{#2}{#3}{#4}{#5}{#6}{#7}{#8}{#9}%
}%
%\fi
}%
}%
\begin{document}
\BAR{A}{B}{C}{D}{E}{F}{G}{H}{I}%
\end{document}
\tl_map_inline:nnsyntax. Is it part of the (La)TeX standard? – user242525 May 19 '21 at 20:15\tl_map_inline:nnis part ofexpl3(you can find its description ininterface3). Since last year,expl3is part of LaTeX, so no packages are needed. Before that you just have to load\usepackage{expl3}. – Phelype Oleinik May 19 '21 at 20:21