compose an array of strings as a macro

Question

I can use the \foreach loop to iterate over an array of strings:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\foreach \v in {A, B}{
  \input{the\v file.txt}
}
\end{document}

Now, I would like to compose a string as a macro as in:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\newcommand\var{A, B} % <<< macro
\foreach \v in \var{
  \input{the\v file.txt}
}
\end{document}

Then I get a problem:

! LaTeX Error: File `theA, Bfile.txt' not found.

Using \def instead of \newcommand creates the same problem using Latex version pdfTeXk, Version 3.141592-1.40.3 (Web2C 7.5.6).

How to solve?

Answer 1

I can reproduce the problem with TikZ/PGF version 1.7 (2006/10/11, twelve years ago), but not with later versions.

I guess that, starting with version 2.0, the syntax of \foreach has been extended to cope with the cases

\foreach \v in {<list>}{...}
\foreach \v in \cs{...}

but the latter case was not possible in previous versions.

Solution: update your TeX system.

For the old system you can do, which will also work in newer releases, as follows.

\documentclass{article}
\usepackage[pdftex]{graphicx}
\usepackage{tikz}
\begin{document}
\newcommand\var{A, B} % <<< macro
\expandafter\foreach\expandafter\v\expandafter i\expandafter n\expandafter{\var}{
  \input{the\v file.txt}
}
\end{document}

^{I had to go back to TeX Live 2007, the earliest release I have on one of my machines, to reproduce the issue: This is pdfTeXk, Version 3.1415926-1.40.9 (Web2C 7.5.7)}

Answer 2

Your \newcommand defines one string, so when called it produces that string. For more about the differences, see https://tex.stackexchange.com/a/658/114249. What you should use is the \def command:

\documentclass{article}
\usepackage{tikz}
\begin{document}
    \def\var{A, B} % <<< macro
    \foreach \v in \var{
        \input{the\v file.txt}
    }
\end{document}

This tries to open theAfile.txt and theBfile.txt as requested.

Answer 3

Maybe you use some package releases where \foreach's list-argument must not come from a macro but must be delivered fully expanded.

If so, then something like the following might help:

\documentclass{article}
\usepackage{tikz}
\newcommand\PassFirstToSecond[2]{#2{#1}}%
\newcommand\OneLevelExpandedSecond[2]{%
  \expandafter\PassFirstToSecond\expandafter{#2}{#1}%
}%
\begin{document}
\newcommand\var{A, B} % <<< macro
\OneLevelExpandedSecond{\foreach \v in }{\var}{% <- We don't want unneeded spaces due to \endlinechar!
  \input{the\v file.txt}%  <- We don't want unneeded spaces due to \endlinechar!
}% <- We don't want unneeded spaces due to \endlinechar!
\end{document}