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I tried to use a switch \IfFileExists to define a command differently, but I get the illegal parameter error. I am running LaTeX from TeX Live 2021. A minimal example is:

\documentclass[12pt,fleqn]{article}
\usepackage[dvipsnames]{xcolor}
\IfFileExists{filename}
{\newcommand{\ABC}[1]{{\begin{color}{Red}{#1}\end{color}}}}
{\newcommand{\ABC}[1]{{\begin{color}{Blue}{#1}\end{color}}}}

\begin{document}
\ABC{Why?}
\end{document}
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2 Answers 2

Reset to default
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\IfFileExists doesn't work like “usual” conditionals: it stores the true and false branches in macros, and uses them later. This requires you to double every # in there:

\documentclass[12pt,fleqn]{article}
\usepackage[dvipsnames]{xcolor}
\IfFileExists{filename} %                 VV
{\newcommand{\ABC}[1]{{\begin{color}{Red}{##1}\end{color}}}}
{\newcommand{\ABC}[1]{{\begin{color}{Blue}{##1}\end{color}}}}

\begin{document}
\ABC{Why?}
\end{document}
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  • Thanks, this is great. This was not my real question, unfortunately -- I posted the easier example hoping that it help with the bigger one. Since I can't figure it out, here is the actual true branch I want (which I got online to enable me to input a table as a file and use \midrule etc.): ``` \IfFileExists{filename}{\ExplSyntaxOn \cs_new:Npn \expandableinput #1 { \use:c { @@input } { \file_full_name:n {#1} } } \AddToHook{env/tabular/begin} { \cs_set_eq:NN \input \expandableinput } \AddToHook{env/tabular*/begin} { \cs_set_eq:NN \input \expandableinput } \ExplSyntaxOff}{} ```
    – nicolae
    Apr 7, 2022 at 14:23
  • To be clear, I did try doubling # to ## in the \ExplSyntax construction, it does not work. My goal is to apply this construction when I compile the file on my computer, as opposed to a coauthor running an older latex implementation. (Until he bothers to update..) That's why I used the \IfFileExists.
    – nicolae
    Apr 7, 2022 at 14:29
  • @nicolae That's a very different problem. You are using \ExplSyntaxOn inside the argument of \IfFileExists, but commands like \ExplSyntaxOn don't work (as you'd expect) inside arguments. You were a victim of the XY problem. I would suggest you ask a new question with the actual problem you want to solve
    – Phelype Oleinik
    Apr 7, 2022 at 14:29
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You're best served with a different approach.

\documentclass{article}
\usepackage[dvipsnames]{xcolor}

\newcommand{\ABCdo}[2]{\leavevmode{\color{#1}#2}}

\IfFileExists{filename}
  {\newcommand{\ABC}{\ABCdo{Red}}}
  {\newcommand{\ABC}{\ABCdo{Blue}}}

\begin{document}
\ABC{Why?}
\end{document}

Output with filename (which does not exist):

enter image description here

Output with latex.ltx (which exists):

enter image description here

There is no color environment and, anyway, the text should not be braced.

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