# \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?

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

• 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