Can I make a LaTeX macro 'return' a filename?

I'm writing my thesis/dissertation and since its an on-going work I don't always have the actual images ready for the figures I put into my document, but for various reasons want to automatically have it substitute a dummy figure in place when the included graphics file doesn't exist. E.g. I can do something like \includegraphics[width=8cm]{\chapdir/figures/fluxcapacitor} (where \chapdir is a macro for my 'current' chapter directory, e.g. \def\chapdir{./ch_timetravel} and if there's no ./ch_timetravel/figures/fluxcapacitor.jpg it'll insert ./commands/dummy.jpg instead.

I've structured my macros (perhaps naïvely?) so that I have a macro (\figFileOrDummy) that determines the appropriate file to include by checking if the argument provided to it exists, so that I can call \includegraphics[properties]{\figFileOrDummy{\chapdir/figures/fluxcapacitor}}. Except I'm getting various errors depending on how I try to call this, which seem to suggest that I'm approaching the problem in a fundamentally flawed way as far as 'good LaTeX programming' goes.

Here's the macro to check if the file exists (and 'return' either filename or the dummy filename):

\newcommand{\figFileOrDummy}[1]{%
    % Figure base name (no extension) to be used if the file exists
    \def\fodname{#1}%
    \def\dummyfig{commands/dummy}%
    % Check if output is PS (.EPS) or PDF (.JPG/.PDF/.PNG/...) figures
    \ifx\pdfoutput\undefined%
        % EPS figures only
        \IfFileExists{\fodname.eps}{}{\def\fodname{\dummyfig}}%
    \else%
        % Check existence of various extensions: PDF, TIF, TIFF, JPG, JPEG, PNG, MPS
        \def\figtest{0}% flag below compared to this value
        \IfFileExists{\fodname.pdf}{\def\figfilenamefound{1}}{\def\figfilenamefound{0}}%
        \IfFileExists{\fodname.jpg}{\def\figfilenamefound{1}}{}%
        \IfFileExists{\fodname.png}{\def\figfilenamefound{1}}{}%
        % and so on...
        % If no files found matching the filename (flag is 0) then use the dummy figure
        \ifx\figfilenamefound\figtest%
            \def\fodname{\dummyfig}%
        \fi%
    \fi%
    % 'return' the filename
    \fodname%
}%

Alternatively, here's a much simpler version which seems to have similar problems:

\newcommand{\figFileOrDummy}[1]{%
    \def\dummyfig{commands/dummy}%
    \dummyfig%
}

The \def commands seems to be processed after the expansion of the macro they're trying to define, so it ends up being \def {commands/dummy}... (note the space after \def) and obviou开发者_如何学Pythonsly complains.

Also it seems to treat the literal contents of the macro as the filename for \includegraphics, rather than resolving/expanding it first, so complains that the file '\def {commands/dummy}... .png' doesn't exist..

I've tried also doing something like \edef\figfilename{\figFileOrDummy{\chapdir/figures/fluxcapacitor}} to try to force it to make \figfilename hold just the value rather than the full macro, but I get an Undefined control sequence error complaining the variables I'm trying to \def in the \figFileOrDummy macro are undefined.

So my question is either

1. How do I make this macro expand properly?; or
2. If this is the wrong way of structuring my macros, how should I actually structure such a macro, in order to be able to insert dummy/real figures automatically?; or
3. Is there a package that already handles this type of thing nicely that I've overlooked?

I feel like I'm missing something pretty fundamental here...

I think the point is that \expandafter is only interested in its arguments as a string representing a filename, so doesn't evaluate it — macro languages are lazy! Try \expandafter {\includegraphics[width=8cm]}{\chapdir/figures/fluxcapacitor}.

Two points of style:

1. You don't need to put % at the end of a line to stop spurious whitespace if the line ends with a control sequence: the control sequence gobbles up all following whitespace, including the end of line. This makes the code much more readable, to my taste. Note that, in particular, to Tex's "mouth" both \def\newcs{abc} and \def \newcs {abc} are identical: they are exactly the same sequence of tokens.
2. I dropped the code around \figtest: you get better error reporting -always at a premium with Tex- if you use either \newif primitive (create new test with \newif\figexists, set/reset with \figexiststrue, \figexistsfalse, and test with \iffigexists...) or the Latex ifthenelse package (to keep with orthodoxy).

Cleaned-up code

I first thought the problem lay elsewhere, so wrote something prettier:

\def\dummypath{commands/dummy}%
\ifx\pdfoutput\undefined
\def\figFileOrDummy#1{\IfFileExists
    {#1.eps}{#1}\dummypath}
\else
\def\figFileOrDummy#1{\IfFileExists
    {#1.pdf}{#1}{\IfFileExists
      {#1.jpg}{#1}{\IfFileExists
        {#1.png}{#1}\dummypath}}} %or have more graphics types, if you like.
\fi

Alright, so I've found a possible answer to #2, by restructuring the way the macros work (and sort of using some suggestions from Charles Stewart's answer — I'll admit I don't like the 'look' of what seems to be widely considered good LaTeX code, I'm perhaps too ingrained in my C/C++ ways to be a real LaTeX programmer).

Anyway, my answer...

Instead of trying to produce the file name in a macro to pass to the \includegraphics macro, make a macro that wraps \includegraphics and passes it the real or dummy file name. This seems to avoid passing (as an argument) a long script/macro, though I don't see any good reason why it should have to be written this way. But it does work...

% Dummy figure file
\def\dummyfigure{commands/dummy}%

% Includegraphics wrapper macro to include either dummy or real figure
\ifx\pdfoutput\undefined
\newcommand{\incgfx}[2]{%
    \def\testfile{\chapdir/fig/#2}%
    \IfFileExists{\testfile.eps}%
        {\includegraphics[#1]{\testfile}}% test file found
        {\includegraphics[#1]{\dummyfigure}}% test file not found
}
\else
\newcommand{\incgfx}[2]{%
    \def\figfilename{\dummyfigure}
    \def\testfile{\chapdir/fig/#2}
    \IfFileExists{\testfile.jpg}{\def\figfilename{\testfile}}{}
    \IfFileExists{\testfile.png}{\def\figfilename{\testfile}}{}
    \IfFileExists{\testfile.pdf}{\def\figfilename{\testfile}}{}
    \IfFileExists{\testfile.jpeg}{\def\figfilename{\testfile}}{}
    \IfFileExists{\testfile.tif}{\def\figfilename{\testfile}}{}
    \IfFileExists{\testfile.tiff}{\def\figfilename{\testfile}}{}
    \includegraphics[#1]{\figfilename}
}
\fi

This allows one to use it as intended:

\begin{figure}
    \begin{center}
        \incgfx{height=3cm}{\chapdir/fig/fluxcapacitor}
        \caption{...}\label{fig:...}
    \end{center}
\end{figure}

Again, I'd like to think there's a way to make the original idea work rather than having to make a wrapper for existing functions, but this will do for now...

Answer to #3: For this purpose, I find very useful the todonotes package. It does not provide the level of automation that your code is aiming to offer, but it has a very nice \missingfigure command that lets you put a dummy box for, you guess it, a missing figure.