TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. It only takes a minute to sign up.
Sign up to join this community
I am looking for ideas how to make TeX understand that when # appears both in the if-else and in the else-fi part of a conditional in an alignment tab, in the end there will never be more than one # in the tab. Here is some sample code:
\def\table{
\halign{
\iftrue\hfil##\else##\hfil\fi\cr
Bla-bla\cr}}
\table
You can pretend to have two # in the template by using a macro:
\newif\ifright
\def\table{%
\halign{%
\def\temp{##}\ifright\hfil\temp\else\temp\hfil\fi\cr
Bla-bla\cr
aaaaaaaaaaaaaaaaaaa\cr
}%
}
% \rightfalse
\table
\righttrue
\table
\bye
Alternatively, use \span
\newif\ifright
\def\table{%
\halign{%
\span\ifright\hfil##\span\else##\hfil\span\fi\cr
Bla-bla\cr
aaaaaaaaaaaaaaaaaaa\cr
}%
}
% \rightfalse
\table
\righttrue
\table
\bye
\newif and \span: \halign{\def\hash{#}\iftrue\hfil\hash\else\hash\hfil\fi\cr Bla-bla\cr}
\halign{\span\iftrue\hfil#\span\else#\hfil\span\fi\cr Bla-bla\cr}
\halign inside a macro.
\halign inside a macro. However, I cannot get the point of \newif\ifright and \def\temp{##} when just \span seems to suffice (both when \halign is in a macro and when it is not).
Are you looking for something like the following
\let\hash\relax
\def\table#1{%
\edef\preamble{\iftrue\hfil\hash\else\hash\fi\cr}%
\let\hash##
\expandafter\halign\expandafter{\preamble #1}%
\let\hash\relax
}
\table{a\cr b\cr}
\bye
The idea here is that during the 'construction' phase we represent # using a non-expandable token; only during the actual table construction is \hash equal to #. In this way there is never a conditional containing multiple # tokens.
tabular environment uses a similar idea, with the name \@sharp for # (see \@mkpream and so on).
\edef will expand everything it can, and if \hash is undefined you'll get an error.
As an alternative to Joseph's you can hide the duplication in a macro (at the expense of passing each cell through a macro expansion)
\def\foo#1{\iftrue\hfil#1\else#1\hfil\fi}
\def\table{%
\halign{\foo{##}\cr
Bla-bla\cr}}
\table
\bye
I'am using toks register \tabdata for such purposes where the \halign preamble is created step by step. The advatage of this is that the macro \table can create arbitrary \halign preamble according to special language offered to user. The algorithm can scan such language and it can create \halign preamble using \addtabdata macro.
\newif\ifright
\newtoks\tabdata
\def\table{\tabdata={}% the \tabdata idea inspired from OPmac
\ifright \addtabdata{\hfil##}\else\addtabdata{##\hfil}\fi
\halign\expandafter{\the\tabdata\cr
ddghsghdfs\cr fghdg\cr}
}
\def\addtabdata#1{\tabdata=\expandafter{\the\tabdata#1}}
\table
\righttrue
\table
\end
