How to Define and Use the TeX Macros

[Eli]

We can use the \def, \newcommand, \renewcommand, \newenvironment, \renewenvironment, and \let commands to create our own macros and environments with \(\LaTeX\)/MathJax.

Let's make \R produce a "bold-faced real number notation", and \bold{...} to put its argument into bold face:


\\( \def\R{{\bf I\!R}}
\def\bold#1{{\bf #1}}/)


Another example is \\(\newcommand{\water}{{\rm H_{2}O}}\\) or \\(\renewcommand{\purewater}{{\rm H_{2}O}}\\), which will output the chemical formula for water when we use the \water or \purewater command. Note that all definitions will be available throughout the rest of the posts.

Howevere, for the TeX macros to work, they must be delimited to evoke the LaTeX environment. The standard delimiters for in-line equations in TeX notation are \\(...\\), or \$...\$, while those for display equations are \$$...\$$ or \\[...\\], but both types of delimiters can be customized as approppriate.

After defining the macros, \( \def\R{{\bf I\!R}}\), \( \def\bold#1{{\bf #1}}\), \(\newcommand{\water}{{\rm H_{2}O}} \), these is what we produce with \R, \water, and \bf{Bold This Text} within \\[ \\] TeX delimiters:

• \[\Huge \R \]
• \[ \Huge \water \]
• \[ \Huge \bf{Bold \ This \ Text} \]

The \newcommand can be used in a more advanced way, for example, as used to draw a commutative diagram here:

Code: [Select all] [Expand/Collapse]

1. % Example one
2. \[ \newcommand{\ra}[1]{\kern-1.5ex\xrightarrow{\ \ #1\ \ }\phantom{}\kern-1.5ex}
3. \newcommand{\ras}[1]{\kern-1.5ex\xrightarrow{\ \ \smash{#1}\ \ }\phantom{}\kern-1.5ex}
4. \newcommand{\da}[1]{\bigg\downarrow\raise.5ex\rlap{\scriptstyle#1}}
5. \begin{array}{c}
6. 0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\
7. \da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\
8. 0 & \ras{h_1} & 0 & \ras{h_2} & E & \ras{h_3} & F & \ras{h_4} & 0 & \ras{h_5} & 0 \\
9. \end{array}
10. \]
11.  
12. %Example Two
13. \[
14. \begin{array}{ccccccccc}  
15. 0 & \xrightarrow{i} & A & \xrightarrow{f} & B & \xrightarrow{q} & C & \xrightarrow{d} & 0\\\
17. \downarrow & \searrow & \downarrow & \nearrow & \downarrow & \searrow & \downarrow & \nearrow & \downarrow\\\  
19. 0 & \xrightarrow{j} & D & \xrightarrow{g} & E & \xrightarrow{r} & F & \xrightarrow{e} & 0  
20. \end{array}  
21. \]
22.  
23. %Example Three
24. $$
25. \newcommand{\ra}[1]{\!\!\!\!\!\!\!\!\!\!\!\!\xrightarrow{\quad#1\quad}\!\!\!\!\!\!\!\!}
26. \newcommand{\da}[1]{\left\downarrow{\scriptstyle#1}\vphantom{\displaystyle\int_0^1}\right.}
27. %
28. \begin{array}{llllllllllll}
29. 0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\
30. \da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\
31. 0 & \ra{h_1} & 0 & \ra{h_2} & E & \ra{h_3} & F & \ra{h_4} & 0 & \ra{h_5} & 0 \\
32. \end{array}
33. $$

This produces


\[ \newcommand{\ra}[1]{\kern-1.5ex\xrightarrow{\ \ #1\ \ }\phantom{}\kern-1.5ex}
\newcommand{\ras}[1]{\kern-1.5ex\xrightarrow{\ \ \smash{#1}\ \ }\phantom{}\kern-1.5ex}
\newcommand{\da}[1]{\bigg\downarrow\raise.5ex\rlap{\scriptstyle#1}}
\begin{array}{c}
0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\
\da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\
0 & \ras{h_1} & 0 & \ras{h_2} & E & \ras{h_3} & F & \ras{h_4} & 0 & \ras{h_5} & 0 \\
\end{array}
\]

\[
\begin{array}{ccccccccc}
0 & \xrightarrow{i} & A & \xrightarrow{f} & B & \xrightarrow{q} & C & \xrightarrow{d} & 0\\\

\downarrow & \searrow & \downarrow & \nearrow & \downarrow & \searrow & \downarrow & \nearrow & \downarrow\\\

0 & \xrightarrow{j} & D & \xrightarrow{g} & E & \xrightarrow{r} & F & \xrightarrow{e} & 0
\end{array}
\]


$$
\newcommand{\ra}[1]{\!\!\!\!\!\!\!\!\!\!\!\!\xrightarrow{\quad#1\quad}\!\!\!\!\!\!\!\!}
\newcommand{\da}[1]{\left\downarrow{\scriptstyle#1}\vphantom{\displaystyle\int_0^1}\right.}
%
\begin{array}{llllllllllll}
0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\
\da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\
0 & \ra{h_1} & 0 & \ra{h_2} & E & \ra{h_3} & F & \ra{h_4} & 0 & \ra{h_5} & 0 \\
\end{array}
$$