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My document has an empty citation \cite{}. I'd expected to get some warning about it.

Is it a bug? (if not - why?) Is it possible to enable checking for such cases?

MWE:

\documentclass{article} \begin{document} \cite{Goodfellow2014a} \cite{} \bibliography{main} \bibliographystyle{plain} \end{document}

and main.bib:

@misc{Goodfellow2014a, author = {Goodfellow, I J and Shlens, J and Szegedy, C}, title = {{Explaining and harnessing adversarial examples}}, year = {2014} }

I'm using pdflatex and bibtex.

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    I'm almost tempted to say it is a bug: if you go \cite{Goodfellow2014a,somethingelse,} then you do get a warning about the empty entry from the trailing , but a completely empty list is silently accepted Commented Mar 8, 2022 at 11:44
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    I think it classifies as a bug and should be fixed because people may rely on fixing cites later and being warned that something is not resolved. Can you please open an issue at github.com/latex3/latex2e/issues ? Commented Mar 9, 2022 at 12:43
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    @FrankMittelbach I opened an issue - github.com/latex3/latex2e/issues/790 Commented Mar 10, 2022 at 9:36

2 Answers 2

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We can't change \@for but given that it looks like wrong behavior perhaps we can fix it by checking for an empty argument before calling \@citex (we can't do in that macro because too many styles redefine that).

E.g., something like

\documentclass{article} \makeatletter \DeclareRobustCommand\cite{% \@ifnextchar [{\@tempswatrue\@citexfix}{\@tempswafalse\@citexfix[]}} \def\@citexfix[#1]#2{% \IfBlankTF {#2}% {\@citex[#1]{\space}}% {\@citex[#1]{#2}}% } \ExplSyntaxOn \cs_new_eq:NN \IfBlankTF \tl_if_blank:nTF \ExplSyntaxOff \makeatother \begin{document} \cite{Goodfellow2014a} \cite{} \cite{ } \cite{ } %tab \bibliography{main} \bibliographystyle{plain} \end{document}
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Hmmm \cite takes a comma separated list of cite keys, if you use \cite{aaa,bbb,} then you get a warning about an empty entry from the trailing comma, but if the list is completely empty then the whole thing is skipped.

This is actually a feature of LaTeX's \@for loop macro:

\documentclass{article} \makeatletter \begin{document} \@for \tmp:=aa,bb,\do{\typeout{1 [\tmp]}} \@for \tmp:=\do{\typeout{2 [\tmp]}} \end{document}

produces terminal and log output

1 [aa] 1 [bb] 1 []

where the first loop includes an iteration with an empty value but the second loop has no iterations at all.

This might be a bit unexpected but \@for has been that way since the earliest latex releases, so almost 40 years now and it is used in multiple places in the latex format and in packages.

\@for is documented as accepting (and by implication skipping) lists of length 0

% \@for NAME := LIST \do {BODY} : Assumes that LIST expands to A1,A2, % ... ,An . % Executes BODY n times, with NAME = Ai on the i-th iteration. % Optimized for the normal case of n = 1. Works for n=0.

so arguably the unexpected behaviour is that the trailing comma isn't ignored.

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