# \newcommand and comma separated parameters

I would like to learn to manipulate comma separated lists in Latex. To give a concreet but artificial example, I would like to have this command:

\swap{1,10}


expand to this:

{10,1}


How can I achieve it?

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I think you should ask what you want in full generality. For your concrete example,

\newcommand*\swap[1]{\doswap#1\relax}
\def\doswap#1,#2\relax{{#2,#1}}


should work, although I didn't test it.

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I didn't know how to ask before seeing the answer. Can you elaborate on what \relax does and why do I need separate \def ? –  Łukasz Lew Sep 8 '10 at 13:39
Normally, \relax tells TeX to do nothing. Here, I was just using it as a delimiter for the second argument to \doswap. As for why you need the separate \def, I cannot think of a way to do this without defining a second control sequence. \swap takes a single parameter and so I defined it so that \swap{1,10} expands to \doswap 1,10\relax. Then \doswap reads 1 as its first argument and 10 as its second argument and so it expands to {10,1}, exactly as you wanted. –  TH. Sep 8 '10 at 13:50
This is really useful! How would one go about writing a \newcommand variant that does all this behind the scenes? –  Neil Olver Oct 27 '10 at 12:29
\newcommand doesn't allow you to define a macro with delimited arguments, so you cannot. –  TH. Oct 27 '10 at 14:05