Command already defined, but is unrecognised

Question

During a project involving plenty angle brackets, I tried to redefine \rangle as \> using \newcommand to make things a little easier.

However, this called the error "command \> already defined". When removing the definition, it does run, although the symbol \> appears pink and is listed an unrecognised symbol.

I turned off all packages and there doesn't seem to be any clashing with \>. I also defined \< as \langle in exactly the same way with no such issue occurring.

I'm unsure if this is a bug or if I am missing something subtle. Using TexStudio.

Answer 1

TeXStudio is just an editor; the fact that it doesn't highlight a macro doesn't necessarily mean it isn't defined, especially if it's an internal macro (i.e. not intended for direct use in a document).

Now \< is undefined as standard, but \> is defined as \mskip \medmuskip. It's hard to know where this might be used by the kernel or by a package, so changing it would be very unwise. It would be better to define a command such as

\newcommand\ang[1]{\langle #1 \rangle}

Then you can place an object in angle brackets using \ang{whatever}. Depending on your usage case, you might want to use more arguments, for example an inner product might be defined as

\newcommand\ip[2]{\langle #1 , #2 \rangle}

Finally, you might want to use \left and \right in your definitions, to allow the bracket sizes to change.

Answer 2

It's impossible for editors to keep track of every command that packages may define (there are thousands of them).

Actually it's a bit surprising that \> is ignored, because it is a command defined in the LaTeX kernel with a dual function:

1. an alias to \: for back compatibility with plain TeX (and very old LaTeX) documents;

2. a main command inside the tabbing environment, which also (re)defines \<.

If you don't plan to use tabbing, then redefining \> might be safe. However, it pops out in some packages:

skak xskak blog qsymbols ednotes clrscode3e clrscode zxjatype knitting pax
qqru-shipunov gn-logic14 polytable catoptions bxcjkjatype AlProTex program
sgamevar alphabeta textalpha 

where it is redefined. I guess you're unlikely to use most of them, but the last two might bite you.

You can do differently:

\usepackage{mathtools}

\DeclarePairedDelimiter{\foo}{\langle}{\rangle}

and then use

\foo{x} \foo[\big]{x} ... \foo[\Bigg]{x}

where the optional argument sets the size, or \foo*{x} for \left and \right automatic sizing (use sparingly).

Find a better name than \foo, of course.

Answer 3

For inner products, you can load mathtools and xparseand define an \innerp{a,b} command this way:

\DeclarePairedDelimiterX{\innerp}[1]{\langle}{\rangle}{\innpargs{#1}}
\NewDocumentCommand{\innpargs}{ >{\SplitArgument{1}{,}}m }
 {\innpargsaux#1}
\NewDocumentCommand{\innpargsaux}{ m m }
 {#1\,,#2}%

Using \innerp*{a,b} will behave as though there was a pair of \left\langle \right\rangle. For fine-tuning the size of the delimiters, you can use an optional argument instead: \innerp[\big]{a,b}, \innerp[\Big]{a,b}, &c., which behaves as though there was a pair of \bigl\langle \bigr\rangle or \Bigl\langle Bigr\rangle, and so on.