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I've seen sometimes that people use a double pound sign (##) when defining/using arguments.
What is the difference between the normal argument, #1, and the double sign one, ##1? Are there any restrictions for its use? Can you list the good practices, if any, for this type of arguments.
It allows using arguments in nested macro definitions.
In
\def\a#1{\def\b#1{...}}
the macro \b would not have an argument, since #1 belongs to \a and would be replaced by its argument.
However,
\def\a#1{\def\b##1{...}}
defines \b with an argument. During expansion
#1 will be replaced by a parameter## becomes #Then \b can use #1 instead of the original ##1.
It follows, that for each level of nesting you need to double the number of # characters:
\def\a#1{\def\b##1{\def\c####1{...}}}
Example:
\documentclass{article}
\def\a#1{\def\b##1{#1 ##1}}
\begin{document}
\a{x} % consequence: \def\b#1{x #1}
\b{y} % prints: x y
\end{document}
In LaTeX syntax this would be:
\newcommand{\a}[1]{%
\newcommand{\b}[1]{#1 ##1}}
or, as \a and \b are already defined, which you would see if you would try it in the small example,
\renewcommand{\a}[1]{%
\renewcommand{\b}[1]{#1 ##1}}
#, ##, #### etc., rather than adding a hash sign for each level, i.e. #, ##, ### etc.?
## is replaced by # just as #1 is replaced by the first argument, and multiple doubling is just a consequence of that.
Aug 11, 2014 at 15:17
\documentclass{article}\def\a#1{\def\b##1{#1 ##1}}\begin{document}\b{y}\end{document} and I got something like and \underbar{y}: any reason why?
\a{…} so the built-in \b was used. Try \documentclass{article}\begin{document}\b{y}\end{document} to see that \b{y} has this \underbar{y} behaviour by default.
Jul 20, 2017 at 8:23