Table lines starting with square bracket

this one does not compile:

\documentclass{article}
\begin{document}
\begin{tabular}{ll}
a & b \\
[c] & d \\
\end{tabular}
\end{document}

if required, one can specify the vertical gap between lines in a table using
\\[len]

LaTeX removed the newline and whitespace to interpret the table as having a \\[c] entry, and there exists no length "c".

So the solution is to write the initial square bracket into curly braces. This way it is not interpreted as the beginning of the optional argument of the newline command.

\documentclass{article}
\begin{document}
\begin{tabular}{ll}
a & b \\
{[}c] & d \\
\end{tabular}
\end{document}

Thanks to Werner Grundlingh for showing me the problem source.

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Line spacing in tables

If you need more space between the lines in a table use the array package and put the following line somewhere between \begin{table} and \begin{tabular}

\setlength{\extrarowheight}{length}

Instead of “length” you write the desired space, e.g. “3pt”