I need to go and rework a bunch of notation in a large body of work. But I might change my mind later, so I want to move to defining the notation a bit more semantically. E.g. writing \v x to represent a vector, so I can later decide to format vectors in different ways.

On thing that is currently wrong with it is that I write A_i to represent that $ i^{\text {th}}$ column vector. After looking at other similar works, I have concluded that A_{:,i} would be clearer. So I have defined a macro to smartly determine if a : should be added.

I called it \i to be concise since I will be writing it often.

Code + Example

\documentclass{article}  \usepackage{xparse,xstring,etoolbox}  \renewcommand{\v}[1]{\tilde{#1}} % a vector   \newcommand{\ind}[2]{#1_{#2}} % indexed  %% Smart indexing and naming code  \newcommand{\ifupper}[3]{     \normalexpandarg     \exploregroups     \StrCount{ABCDEFGHIJKLMNOPQRSTUVWXYZ}{#1}[\uppercount]     \ifnumgreater{\uppercount}{0}{#2}{#3} }  %smart index \DeclareDocumentCommand{\i}{u{_} m}{     \ifupper{#1}%     {% just a single uppercase character, i.e. a matrix         %make sure the index is the right length         \StrCount{#2}{,}[\indcount]         \ifnumgreater{\indcount}{0}         { % Got multiple indexes so all good             \ind{#1}{#2}         }         { % Only 1 index so grab the column             \ind{#1}{{:,#2}}         }     }%     {% Not just a single upper case character         \ind{#1}{#2}     } }  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Example document \begin{document}          \begin{tabular}{r l} 		$  \i {\v x}_i$   & the $  i$  th element of the vector $  \v x$  \ 		$  \i{X}_{i,j}$   & the row $  i$   and column $  j$   element of the matrix $  X$  \ 		$  \i{X}_i$   & the $  i$  th column vector of the matrix $  X$  \ 		$  \i{X}_{i,:}$   & the $  i$  th row vector of the matrix $  X$  \ 	\end{tabular}  \end{document} 

Example document output

Crossref: https://tex.stackexchange.com/q/406963/5834