how to select a character (or word) from a list

Question

Is there a way, with the following restrictions:

• No external pacakges (i.e., no substr)
• no conditionals (\ifnum, \ifdefined, etc)

to select a character from a string? i.e.

\CharAt{2}{ABC}   

should yield B. Or at least a word from a delimited parameter? i.e.

\WordAt{2}{A B C} 

should yield B.

The size of the array is fixed, so something like this would work as well:

\WordAt{2}{A}{B}{C}

The emphasis is on making the LaTeX simple, because I am using plastex.

Answer 1

The following works if the position number is 9 or less:

\documentclass{article}
\def\ninthofmany#1#2#3#4#5#6#7#8#9{#9\gobbletorelax}
\def\gobbletorelax#1\relax{}
\def\CharAt#1#2{%
  \expandafter\ninthofmany\romannumeral\numexpr(9000-\number#1000)#2\relax}
\begin{document}
\CharAt{3}{abcdefg}
\CharAt{2}{abcdefg}
\CharAt{1}{abcdefg}
\end{document}

\CharAt{3}{abcdefg}, for example, prepends 6 "m"s (roman numeral for 6000) in front of the string abcdefg, and then picks off the 9th character of the result. Using iterations of the same trick, one could extend the limit on the first parameter as far as needed, at the expense of longer and longer code.

One could easily solve this puzzle for arbitrarily large numbers with a few lines of code if we weren't required to avoid \ifnum (and presumably other \ifs).

Answer 2

It can only work if you are searching within the first 8 characters of the string, or it can get the 9th if the string is only nine characters long. Spaces are ignored in the string argument. It can also parse word lists, where the first argument is specified as {{word1}{word2}{word3}...}

\documentclass{article}
\def\nextchar#1#2#3#4#5#6#7#8#9\relax{%
  \def\1{#1}%
  \def\2{#2}%
  \def\3{#3}%
  \def\4{#4}%
  \def\5{#5}%
  \def\6{#6}%
  \def\7{#7}%
  \def\8{#8}%
  \def\9{#9}%
}
\newcommand\findchar[2]{%
  \nextchar#1\relax\relax\relax\relax\relax\relax\relax\relax\relax%
  \csname#2\endcsname%
}
\begin{document}
\findchar{abc}{2}
should give a b

\findchar{thisisatest}{7}
gives the same result as
\findchar{this is a test}{7}

\findchar{123456789}{9}
can find the ninth character, since the string is only 9 in length

\findchar{{This}{is}{a}{test}{of}{the}{emergency}{broadcast}{system}}{7}
can be found among word lists too.
\end{document}

Answer 3

I looked a bit at the plastex pdf documentation but could not find any concrete explanation of what is acceptable and what is not.

As a rhetorical exercise, here is a macro \NthElt {N}{list} which returns (expandably) the Nth element of the list (spaces gobbled), the list may be a macro expanding to some string, or some other list of braced things; the N has to be an explicit positive integer.

My constraint was not to use any \if but as it has been used in other answers I allowed myself delimited macros and \romannumeral. Use of \romannumeral could be removed entirely; delimited macros are at the heart of the method.

And this is e-TeX as \numexpr is crucially used.

\documentclass{article}

\makeatletter
\def\gobble@     {}%
\def\gobble@i    #1{}%
\def\gobble@ii   #1#2{}%
\def\gobble@iii  #1#2#3{}%
\def\gobble@iv   #1#2#3#4{}%
\def\gobble@v    #1#2#3#4#5{}%
\def\gobble@vi   #1#2#3#4#5#6{}%
\def\gobble@vii  #1#2#3#4#5#6#7{}%

\def\gobtoundef    #1\undef {}% 
\def\gobtonine     #19{}%
%\def\FirstOfTwo   #1#2{#1}%   here, LaTeX: just use \@firstoftwo
%\def\ThirdOfThree #1#2#3{#3}% here, LaTeX: just use \@thirdofthree

% EXPANDABLY RECOVERS THE #1th ITEM
% #1 MUST BE AN EXPLICIT POSITIVE INTEGER  (not a count or not even a macro)
%     
\def\NthElt #1#2%
{%
    \romannumeral0\expandafter\NthElta\expandafter {\romannumeral-`0#2}{#1}%
}%
\def\NthElta #1#2%
{%
    \NthEltb {#2}#1\undef\undef\undef\undef\undef\undef\undef\undef\UNDEF
}%
\def\NthEltb #1%
{%
    \IfMoreThanEight #1\undef\@firstoftwo {\NthEltc}{\NthEltGetIt}%
    {#1}%
}%
\def\IfMoreThanEight #1#2%
{% 
    \gobtoundef #2\PerhapsAtMostEight #1%
}%
\def\PerhapsAtMostEight #1%
{%
    \gobtonine #1\gobble@ii 9\@thirdofthree
}%
\def\NthEltc #1#2#3#4#5#6#7#8#9%
{%
    \gobtoundef #9\NthEltSilentEnd\undef
    \expandafter\NthEltb\expandafter{\the\numexpr #1-8\relax}%
}%
\def\NthEltSilentEnd #1\UNDEF { }%  space stops the \romannumeral0
\def\NthEltGetIt #1%
{%
    \expandafter\expandafter\expandafter\NthEltFinish
    \csname gobble@\romannumeral\numexpr#1-1\endcsname
}%
\def\NthEltFinish #1#2\UNDEF  {\gobtoundef #1\expandafter\space
                                             \gobble@iii\undef\space #1}% 
% space stops the \romannumeral0
\makeatother

\begin{document}    
\NthElt {1}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

\NthElt {7}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

\NthElt {26}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

+++\NthElt {27}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}+++

% 0 is not legal as index, but acts as if 1.
+++\NthElt {0}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}+++

\newcount\cnta
\loop
   \advance\cnta 1
      \expandafter\NthElt\expandafter{\the\cnta}{ABCDEFGHIJKLMNOPQRSTUVWXYZ}.%
   \ifnum\cnta < 30
\repeat

\edef\z {\NthElt
  {37}{ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJ{THIS IS ME!}LMNOPQRSTUVWXYZ}}
\texttt{\meaning\z}

\end{document}