Subfigure: error "Missing number, treated as zero"

Question

\begin{figure} \center
    \begin{subfigure}[b]
        \includegraphics[width=60mm]{a}
        \label{fig:a}
    \end{subfigure} %

    \begin{subfigure}[b]    
        \includegraphics[width=60mm]{b}
        \label{fig:b}    
    \end{subfigure} 
    \caption{my caption}
\end{figure}

I get an error

! Missing number, treated as zero.
<to be read again>
\let
           \includegraphics
                            [width=60mm]{a}

What's wrong?

Answer 1

This is how you should be using it.

\documentclass{article}
\usepackage{graphicx,subfigure}
\begin{document}
\begin{figure}
\centering     %%% not \center
\subfigure[Figure A]{\label{fig:a}\includegraphics[width=60mm]{example-image-a}}
\subfigure[Figure B]{\label{fig:b}\includegraphics[width=60mm]{example-image-b}}
\caption{my caption}
\end{figure}
\end{document}

Note: subfigure is outdated and new one is subfig which introduces subfloat command. You may consider using subfig instead of subfigure.

Answer 2

I believe you're using the subcaption package. The problem you have is that as a second argument to \begin{subfigure} you should tell it how wide you want your figure to be; e.g.

\begin{figure} \centering
    \begin{subfigure}[b]{\linewidth}
        \includegraphics[width=60mm]{a}
        \label{fig:a}
    \end{subfigure} %

    \begin{subfigure}[b]{\linewidth}    
        \includegraphics[width=60mm]{b}
        \label{fig:b}    
    \end{subfigure} 
    \caption{my caption}
\end{figure}

If you have further problems you can refer to the package documentation.

Hope that helps!

Answer 3

This answer is based on this Overleaf article

The subfigure package is long outdated, and will generate a 'Missing number' error when there is no error present. This will generate an error, as the subfigure package does not recognise \textwidth as a number, when it is in fact a number (equivalent to the constant width of the total text block on a page). The way to resolve this is to use the more updated subcaption package, which has replaced subfigure.

The above Overleaf article also provides several other possible causes of the error- "Missing number, treated as zero."

Note: Although this answer addresses the same error, i.e., "Missing number, treated as zero.", but caused by different reasons than the original question. However, I believe the answer will be useful to the community as it addresses the issue in general which is use of sufigure package. Moreover, people usually search the error given by latex not the cause. Had the cause of error known to them, they need not search it on google. LOL

Answer 4

The missing number is the desired width of the subfigure. Change \begin{subfigure}[b] to \begin{subfigure}[b]{60mm} as proposed by @jlfranco.

Answer 5

Maybe you can try to write the label under the \subfigure level, not at the same level.

Right demo work for me:

\begin{figure}[htbp]
\centering
\subfigure[Bearings vibration]{
    \includegraphics[width=5.5cm]{figures}
    }
\quad
\subfigure[Wheel treads vibration]{
    \includegraphics[width=5.5cm]{figures.png}
}
\quad
\subfigure[Rotatory speed]{
    \includegraphics[width=5.5cm]{figures}
\label{fig:label13}} % difference in this line
\caption{examples}\label{fig:label1} 
\end{figure}

Wrong demo not work for me:

\begin{figure}[htbp]
\centering
\subfigure[Bearings vibration]{
    \includegraphics[width=5.5cm]{figures}
    }
\quad
\subfigure[Wheel treads vibration]{
    \includegraphics[width=5.5cm]{figures.png}
}
\quad
\subfigure[Rotatory speed]{
    \includegraphics[width=5.5cm]{figures}
}\label{fig:label13} % difference in this line
\caption{examples}\label{fig:label1} 
\end{figure}