Error: Missing $ inserted in table of latex

Question

The following code:

\documentclass{article}


\begin{document}

\begin{table}[t]
\caption{Practically Used Deep CNN Networks}
\label{table1}
\begin{center}
\begin{tabular}{c|c|c|c|c}
NET_WIT_DATASET  &  CNN_layer  &  in_a_network  &  combining_from_2_networks  &  combining_from_3_networks
\\ \hline \\
LeNet   MNIST            &  conv1       &  20       &   15      &   15 \\
LeNet MNIST            &  conv2     &  50       &  59       &   59 \\
AlexNet CIFAR10 &conv1      &24     &15     &16 \\
AlexNet CIFAR10 &conv2      &96     &107        &107 \\
\end{tabular}
\end{center}
\label{table1}
\end{table}

\end{document}

reported the error:

Missing $ inserted. $ l.204 NET_ WIT_DATASET & CNN_layer &

Answer 1

I suggest you (a) encase the header strings that contain _ (underscore) characters in \url directives to "neuter" the meaning of the special characters, (b) employ a tabularx environment to permit automatic line breaking in the final two columns, and (c) load the xurl package to permit line-breaking in URL strings at arbitrary points.

Incidentally, are these underscore characters really required? Can you maybe make do without them? If you can dispense with them, there would be no need for the \url wrappers, greatly simplifying the typesetting task.

Oh, if your document loads the hyperref package, you should change \url to \nolinkurl (since the strings aren't "real" URL strings...).

\documentclass{article}
\usepackage{xurl,tabularx,ragged2e,booktabs}
\newcolumntype{C}{>{\Centering}X}
\begin{document}

\begin{table}[t]
\urlstyle{same}
\setlength\tabcolsep{4pt}
\caption{Practically Used Deep CNN Networks}
\label{table1}
\begin{tabularx}{\textwidth}{@{} llcCC @{}}
\toprule
\url{NET_WIT_DATASET}  &  
\url{CNN_layer}  &  
\url{in_a_network}  &  
\url{combining_from_2_networks}  &  
\url{combining_from_3_networks} \\ 
\midrule
LeNet MNIST     &  conv1    &  20   &  15   &  15 \\
LeNet MNIST     &  conv2    &  50   &  59   &  59 \\
AlexNet CIFAR10 &  conv1    &  24   &  15   &  16 \\
AlexNet CIFAR10 &  conv2    &  96   & 107   & 107 \\
\bottomrule
\end{tabularx}
\end{table}
\end{document}