Use of \csname in \input produces mysterious results

Question

I am stuck with the following one where I want to input a file with a name in the line of \jobnameQ.tex.

Here is my MWE excerpted from a bigger picture.

\documentclass{article}

\usepackage{lipsum}

\usepackage{filecontents}

\def\csinput{\expandafter\input{\csname\jobname Q.tex\endcsname}}

\begin{document}

\begin{filecontents}{\jobname Q.tex}
  \lipsum[1]
\end{filecontents}

\csinput

\end{document}

This does not input any file and does not produce any output.

However, I know that if I define \csinput in the manner

\def\csinput#1{\input{\jobname#1.tex}}

and call it like this, \csinput{Q} it will work.

I would like to know why the first methodology does not work. (And how it can be remedied.)

Answer 1

The \csname \jobname Q.tex\endcsname expands to \nameQ.tex, i.e. to the control sequence which is not defined. The undefined control sequences constructed by \csname...\endcsname have \relax meaning. The \expandafter in your code is redundant because { isn't expanable. The \input is a macro in the LaTeX. The \input{something} expands to the primitive \input something. Because the something here has a meaning \relax, the following TeX primitive command is processed:

\input \relax

Because the name of the file in \input primitive command can be delimited by another unexpandable primitive command (for example \relax), we have empty filename here. TeX appends to this empty filename the .tex suffix and performs to read .tex file. This file is present in typical TeX distributions with the contents: \message{File ignored}.

The result: this file is read and the message is printed.

Note I try to explain why the .tex file mentioned above is present in the TeX distributions. TeX stops when it finds an error (in \errorstopmode which is common setting) and waits to the user response. The typical user response is only "press Enter" which continues processing. But there is another type of error: I can't find file .... If user responses by Enter and the .tex doesn't exist then the error is printed again and again because only the real file name must be entered here. This would make a user irritation. The .tex file is read when user hit Enter in this situation and the process continues. This is similar behavior as in "normal" errors.

Answer 2

Suppose \jobname has the value of test. Then \csname\jobname Q.tex\endcsname represents a macro with the name testQ.tex, just as \csname jobname\endcsname represents a macro with the name jobname.

The macro with a name testQ.tex (which would be written \testQ.tex if you could have dots in macro names) doesn't exist, so \input will not get the filename you want to give it.

Basically, you shouldn't use \csname here. You don't need a macro with the same name as the file, you need a macro (well, a macro plus some other stuff) whose value is the filename. Just this:

\def\csinput{\input{\jobname Q.tex}}