Custom Commands & Environments¶

Topic: LaTeX Lesson: 13 of 16 Prerequisites: Lessons 01-08 (especially mathematical typesetting) Objective: Learn to create custom commands, environments, and personal packages to improve efficiency and maintain consistency

Introduction¶

Custom commands and environments are powerful features that let you:

Avoid repetition: Write \R instead of \mathbb{R} every time
Maintain consistency: Change notation in one place, update everywhere
Improve readability: \norm{\vec{x}} is clearer than \left\|\vec{x}\right\|
Create shortcuts: Define complex formatting as simple commands
Build personal packages: Reuse your definitions across projects

This lesson covers command creation, custom environments, counters, conditional logic, and package creation.

Why Create Custom Commands?¶

Problem: Repetitive Code¶

The vector space $\mathbb{R}^n$ contains vectors in $\mathbb{R}^n$.
For $x, y \in \mathbb{R}^n$, the inner product...

Issues: - Typing \mathbb{R} repeatedly is tedious - If you decide to change to \mathbf{R}, you must edit every occurrence - Risk of inconsistent notation

Solution: Custom Command¶

\newcommand{\R}{\mathbb{R}}

The vector space $\R^n$ contains vectors in $\R^n$.
For $x, y \in \R^n$, the inner product...

Benefits: - Type \R instead of \mathbb{R} - Change definition once to update all occurrences - Consistent notation throughout document

Basic Custom Commands¶

Syntax: `\newcommand`¶

\newcommand{\commandname}{replacement text}

\commandname: Must start with backslash, use only letters (no numbers)
{replacement text}: What LaTeX substitutes when command is used

Examples: Simple Text Replacements¶

\documentclass{article}
\usepackage{amsmath,amssymb}

% Natural numbers, integers, rationals, reals, complex
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\Q}{\mathbb{Q}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\C}{\mathbb{C}}

% Common operators
\newcommand{\dd}{\mathrm{d}}  % differential d
\newcommand{\eps}{\varepsilon}

\begin{document}

For all $x \in \R$, there exists $n \in \N$ such that $n > x$.

The integral $\int_0^1 f(x) \, \dd x$ uses an upright d.

\end{document}

Non-Mathematical Examples¶

\newcommand{\projectname}{Deep Learning Framework}
\newcommand{\version}{v2.1.0}
\newcommand{\email}{contact@example.com}

This is \projectname{} \version.  % {} prevents space issues
Contact: \email

Commands with Arguments¶

Syntax: One Argument¶

\newcommand{\commandname}[1]{replacement with #1}

[1]: Number of arguments (1-9)
#1: First argument placeholder

Examples¶

% Absolute value
\newcommand{\abs}[1]{\left| #1 \right|}

% Norm
\newcommand{\norm}[1]{\left\| #1 \right\|}

% Set notation
\newcommand{\set}[1]{\left\{ #1 \right\}}

% Floor and ceiling
\newcommand{\floor}[1]{\left\lfloor #1 \right\rfloor}
\newcommand{\ceil}[1]{\left\lceil #1 \right\rceil}

% Usage:
$\abs{x} < 1$ and $\norm{\vec{v}} = \sqrt{\abs{x}^2 + \abs{y}^2}$

$S = \set{x \in \R : \abs{x} < 1}$

$\floor{3.7} = 3$ and $\ceil{3.7} = 4$

Multiple Arguments¶

% Inner product
\newcommand{\ip}[2]{\left\langle #1, #2 \right\rangle}

% Derivative
\newcommand{\dv}[2]{\frac{\dd #1}{\dd #2}}

% Partial derivative
\newcommand{\pdv}[2]{\frac{\partial #1}{\partial #2}}

% Binomial coefficient
\newcommand{\binom}[2]{\left(\begin{array}{c} #1 \\ #2 \end{array}\right)}

% Usage:
$\ip{\vec{u}}{\vec{v}} = \sum_{i=1}^n u_i v_i$

$\dv{y}{x} = 2x$ and $\pdv{f}{x} = y^2$

Optional Arguments¶

Syntax¶

\newcommand{\commandname}[total][default for #1]{replacement with #1, #2, ...}

[total]: Total number of arguments
[default for #1]: Default value for first argument (which becomes optional)
Mandatory arguments: #2, #3, etc.

Examples¶

% Derivative with optional order
\newcommand{\derivative}[2][1]{\frac{\dd^{#1} #2}{\dd x^{#1}}}

% Usage:
$\derivative{y}$         % first derivative (default)
$\derivative[2]{y}$      % second derivative
$\derivative[n]{f}$      % nth derivative

% Vector with optional dimension
\newcommand{\vect}[2][n]{\mathbf{#2} \in \R^{#1}}

% Usage:
$\vect{x}$        % x ∈ R^n (default)
$\vect[3]{v}$     % v ∈ R^3

`\renewcommand` and `\providecommand`¶

`\renewcommand`: Redefine Existing Commands¶

% Redefine \vec to use bold instead of arrow
\renewcommand{\vec}[1]{\mathbf{#1}}

% Now \vec{x} produces bold x instead of x with arrow

Warning: Only use \renewcommand on commands you understand. Redefining core LaTeX commands can break your document.

`\providecommand`: Define Only If Not Defined¶

\providecommand{\R}{\mathbb{R}}

If \R already exists, nothing happens
If \R doesn't exist, it's defined
Useful in packages to avoid conflicts

Mathematical Command Shortcuts¶

Common Patterns¶

\documentclass{article}
\usepackage{amsmath,amssymb}

% Number sets
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\C}{\mathbb{C}}

% Operators
\newcommand{\dd}{\mathrm{d}}
\newcommand{\Tr}{\operatorname{Tr}}
\newcommand{\rank}{\operatorname{rank}}
\newcommand{\diag}{\operatorname{diag}}

% Delimiters
\newcommand{\abs}[1]{\left| #1 \right|}
\newcommand{\norm}[1]{\left\| #1 \right\|}
\newcommand{\ip}[2]{\left\langle #1, #2 \right\rangle}
\newcommand{\set}[1]{\left\{ #1 \right\}}

% Calculus
\newcommand{\dv}[2]{\frac{\dd #1}{\dd #2}}
\newcommand{\pdv}[2]{\frac{\partial #1}{\partial #2}}
\newcommand{\ppdv}[2]{\frac{\partial^2 #1}{\partial #2^2}}

% Linear algebra
\newcommand{\mat}[1]{\mathbf{#1}}
\newcommand{\transpose}{^{\top}}

\begin{document}

Let $\mat{A} \in \R^{m \times n}$ with $\rank(\mat{A}) = r$.

The trace satisfies $\Tr(\mat{A}\transpose \mat{A}) = \sum_{i,j} a_{ij}^2$.

For $f : \R^n \to \R$, the gradient is $\nabla f = \left( \pdv{f}{x_1}, \ldots, \pdv{f}{x_n} \right)$.

\end{document}

Custom Environments¶

Syntax: `\newenvironment`¶

\newenvironment{envname}[args]
  {begin code}
  {end code}

envname: Environment name (no backslash)
[args]: Optional number of arguments
{begin code}: Executed at \begin{envname}
{end code}: Executed at \end{envname}

Example: Custom Theorem Box¶

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage[most]{tcolorbox}

\newenvironment{mytheorem}[1]
{%
  \begin{tcolorbox}[colback=blue!5,colframe=blue!75!black,title=Theorem: #1]
}{%
  \end{tcolorbox}
}

\begin{document}

\begin{mytheorem}{Pythagorean Theorem}
  For a right triangle with legs $a$, $b$ and hypotenuse $c$:
  \[ a^2 + b^2 = c^2 \]
\end{mytheorem}

\end{document}

Example: Solution Environment¶

\newenvironment{solution}
{%
  \par\medskip\noindent\textbf{Solution:}\par\nopagebreak
  \small
}{%
  \par\medskip
}

% Usage:
\begin{solution}
  Apply the quadratic formula:
  \[ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \]
\end{solution}

Example: Custom Example Box¶

\usepackage{framed}
\usepackage{xcolor}

\definecolor{examplecolor}{rgb}{0.9,0.9,0.95}

\newenvironment{example}[1][]
{%
  \def\exampletitle{#1}%
  \begin{leftbar}
  \noindent\textbf{Example\ifx\exampletitle\empty\else: \exampletitle\fi}\\
}{%
  \end{leftbar}
}

Counters¶

Creating and Using Counters¶

% Define counter
\newcounter{mycounter}

% Set value
\setcounter{mycounter}{0}

% Increment
\stepcounter{mycounter}

% Increment and make referable
\refstepcounter{mycounter}

% Display value
\themycounter
\arabic{mycounter}   % Arabic numerals
\roman{mycounter}    % lowercase roman
\Roman{mycounter}    % uppercase roman
\alph{mycounter}     % lowercase letters
\Alph{mycounter}     % uppercase letters

% Get numeric value
\value{mycounter}

Example: Custom Theorem Counter¶

\documentclass{article}
\usepackage{amsmath}

\newcounter{theorem}
\setcounter{theorem}{0}

\newenvironment{theorem}[1][]
{%
  \refstepcounter{theorem}%
  \par\medskip\noindent\textbf{Theorem \thetheorem\ifx\\#1\\\else\ (#1)\fi.}
  \itshape
}{%
  \par\medskip
}

\begin{document}

\begin{theorem}[Fundamental Theorem of Calculus]
  \label{thm:ftc}
  If $f$ is continuous on $[a,b]$, then...
\end{theorem}

By Theorem~\ref{thm:ftc}, we have...

\end{document}

Counter Hierarchy¶

% Create counter that resets with section
\newcounter{example}[section]

% Reset counter
\setcounter{example}{0}

% Automatic reset when section increments

Conditional Commands¶

The `ifthen` Package¶

\usepackage{ifthen}

\ifthenelse{test}{true-code}{false-code}

Tests¶

% Numeric comparison
\ifthenelse{\value{counter} > 5}{Large}{Small}

% String equality
\ifthenelse{\equal{#1}{draft}}{DRAFT MODE}{}

% Boolean
\newboolean{showsolutions}
\setboolean{showsolutions}{true}
\ifthenelse{\boolean{showsolutions}}{Solutions shown}{}

Example: Conditional Solutions¶

\usepackage{ifthen}
\newboolean{solutions}
\setboolean{solutions}{true}  % Change to false to hide

\newcommand{\solution}[1]{%
  \ifthenelse{\boolean{solutions}}{%
    \par\textbf{Solution:} #1\par
  }{}%
}

% Usage:
\textbf{Problem 1:} Solve $x^2 - 5x + 6 = 0$.

\solution{Factor: $(x-2)(x-3)=0$, so $x=2$ or $x=3$.}

Advanced Argument Parsing: `xparse`¶

The xparse package provides more powerful command definitions.

Syntax¶

\usepackage{xparse}

\NewDocumentCommand{\commandname}{argument-spec}{definition}

Argument Specifiers¶

m: Mandatory argument {...}
o: Optional argument [...] (value is \NoValue if not given)
O{default}: Optional with default value
s: Star (produces boolean)
d() or d<>: Delimited argument (...) or <...>

Examples¶

\usepackage{xparse}

% Starred variant: starred uses \|\|, unstarred uses \lVert\rVert
\NewDocumentCommand{\norm}{s m}{%
  \IfBooleanTF{#1}
    {\left\| #2 \right\|}
    {\lVert #2 \rVert}
}

% Usage:
$\norm{x}$     % \lVert x \rVert
$\norm*{x}$    % \| x \|

% Optional subscript
\NewDocumentCommand{\norm}{o m}{%
  \IfNoValueTF{#1}
    {\left\| #2 \right\|}
    {\left\| #2 \right\|_{#1}}
}

% Usage:
$\norm{x}$       % \| x \|
$\norm[2]{x}$    % \| x \|_2
$\norm[\infty]{x}$  % \| x \|_∞

The `etoolbox` Package¶

Provides advanced programming tools for LaTeX.

Conditional Tests¶

\usepackage{etoolbox}

% Check if command is defined
\ifdef{\commandname}{true-code}{false-code}

% Check if command is defined and not empty
\ifdefempty{\commandname}{empty-code}{nonempty-code}

% Check if string is empty
\ifstrempty{#1}{empty-code}{nonempty-code}

Toggle Switches¶

\newtoggle{solutions}
\toggletrue{solutions}   % or \togglefalse{solutions}

\iftoggle{solutions}{Show solutions}{Hide solutions}

Patching Commands¶

% Prepend code to existing command
\pretocmd{\section}{Code before section}{}{}

% Append code
\apptocmd{\section}{Code after section}{}{}

% Replace part of command
\patchcmd{\command}{search}{replace}{success}{failure}

Debugging Custom Commands¶

Show Command Definition¶

\show\mycommand

Prints definition to console/log.

Meaning¶

\meaning\mycommand

Shows full expansion.

Debug Messages¶

% Print to console and log
\typeout{Debug: value is \themycounter}

% Print only to log
\message{Internal state: ...}

Tracing¶

\tracingmacros=1  % Show macro expansions in log
\tracingmacros=0  % Turn off

Creating a Personal `.sty` Package¶

Why Create a Package?¶

Reusability: Use same commands across multiple documents
Organization: Keep preamble clean
Sharing: Give colleagues your notation conventions
Maintenance: Update commands in one place

Steps¶

Create file: mystyle.sty
Add package header:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mystyle}[2026/02/15 My Custom Commands]

% Load required packages
\RequirePackage{amsmath}
\RequirePackage{amssymb}
\RequirePackage{xparse}

% Define commands
\newcommand{\R}{\mathbb{R}}
\newcommand{\C}{\mathbb{C}}
\newcommand{\norm}[1]{\left\| #1 \right\|}
\NewDocumentCommand{\ip}{m m}{\left\langle #1, #2 \right\rangle}

% Define environments
\newenvironment{theorem}[1][]
  {\par\medskip\noindent\textbf{Theorem\ifx\\#1\\\else\ (#1)\fi.}\itshape}
  {\par\medskip}

\endinput

Use package: Place mystyle.sty in same directory as .tex file (or in your local texmf tree)

\documentclass{article}
\usepackage{mystyle}

\begin{document}
  $\norm{x} < 1$ for all $x \in \R$.
\end{document}

Local `texmf` Tree¶

To make package available globally:

# Find your home texmf directory
kpsewhich -var-value TEXMFHOME

# Example: ~/texmf
# Place mystyle.sty in:
~/texmf/tex/latex/mystyle/mystyle.sty

# Update filename database
texhash ~/texmf

Now \usepackage{mystyle} works in any document.

Complete Example: Math Paper Package¶

File: mathpaper.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mathpaper}[2026/02/15 Math Paper Macros]

% Required packages
\RequirePackage{amsmath,amssymb,amsthm}
\RequirePackage{xparse}

% Number sets
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\Q}{\mathbb{Q}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\C}{\mathbb{C}}

% Operators
\DeclareMathOperator{\Tr}{Tr}
\DeclareMathOperator{\rank}{rank}
\DeclareMathOperator{\diag}{diag}
\DeclareMathOperator{\span}{span}

% Delimiters with optional size
\NewDocumentCommand{\abs}{s m}{%
  \IfBooleanTF{#1}{\left| #2 \right|}{\lvert #2 \rvert}%
}
\NewDocumentCommand{\norm}{s m}{%
  \IfBooleanTF{#1}{\left\| #2 \right\|}{\lVert #2 \rVert}%
}
\NewDocumentCommand{\ip}{s m m}{%
  \IfBooleanTF{#1}{\left\langle #2, #3 \right\rangle}{\langle #2, #3 \rangle}%
}

% Calculus
\newcommand{\dd}{\mathrm{d}}
\NewDocumentCommand{\dv}{m m}{\frac{\dd #1}{\dd #2}}
\NewDocumentCommand{\pdv}{m m}{\frac{\partial #1}{\partial #2}}

% Theorem environments
\theoremstyle{plain}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{proposition}[theorem]{Proposition}
\newtheorem{corollary}[theorem]{Corollary}

\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}
\newtheorem{example}[theorem]{Example}

\theoremstyle{remark}
\newtheorem{remark}[theorem]{Remark}

\endinput

Usage:

\documentclass{article}
\usepackage{mathpaper}

\begin{document}

\section{Vector Spaces}

\begin{definition}
  A \emph{normed vector space} is a pair $(V, \norm{\cdot})$ where $V$ is a vector space over $\R$ and $\norm{\cdot} : V \to [0,\infty)$ satisfies...
\end{definition}

\begin{theorem}[Cauchy-Schwarz]
  For all $x, y \in V$, we have $\abs*{\ip{x}{y}} \leq \norm{x} \norm{y}$.
\end{theorem}

\end{document}

Best Practices¶

Do's¶

Use descriptive names: \norm{x} is better than \n{x}
Consistent conventions: Stick to one notation style
Comment definitions: Especially in .sty files
Group related commands: Keep number sets together, operators together
Use \ensuremath for commands that work in both text and math:

\newcommand{\R}{\ensuremath{\mathbb{R}}}
Now \R works in text and $\R$ in math.

Don'ts¶

Don't redefine core commands unless you know what you're doing
Don't use too many arguments: More than 3-4 becomes hard to read
Don't make commands too complex: If definition is >5 lines, reconsider
Don't use single-letter names for non-mathematical commands
Don't hardcode formatting: Use semantic commands

% Bad: hardcoded color
\newcommand{\important}[1]{\textcolor{red}{#1}}

% Good: semantic command
\newcommand{\important}[1]{\emph{#1}}  % Can change later

Exercises¶

Exercise 1: Basic Math Shortcuts¶

Create custom commands for: - Set builder notation: \setbuilder{x \in \R}{x > 0} - Probability: \Prob{X = 1}, \Expect{X}, \Var{X} - Big-O notation: \bigO{n^2}, \bigOmega{n}, \bigTheta{n \log n}

Expected output format for set builder: { x ∈ ℝ : x > 0 }

Exercise 2: Custom Environment¶

Create a warning environment that: - Has an optional title argument - Displays a warning symbol (use \textbf{⚠} or \textbullet) - Uses a colored background (load xcolor, use \colorbox) - Has slightly smaller text

Exercise 3: Conditional Compilation¶

Create a command \version{student}{instructor} that: - Uses a boolean toggle studentversion - Displays first argument if toggle is true - Displays second argument if toggle is false - Use it to show/hide solutions in a problem set

Exercise 4: Derivative Command¶

Create a sophisticated \deriv command using xparse that: - \deriv{f}{x} → df/dx - \deriv[2]{f}{x} → d²f/dx² - \deriv*{f}{x} → uses \left\lfloor instead of regular fraction - Has a starred variant for display style

Exercise 5: Personal Package¶

Create mymath.sty containing: - Your favorite number set commands - At least 5 mathematical operators - At least 3 delimiter commands (abs, norm, etc.) - A custom theorem environment - Proper package documentation comments

Use it in a sample document with a theorem and proof.

Exercise 6: Counter-Based Numbering¶

Create an exercise environment that: - Auto-numbers exercises - Has an optional difficulty parameter: [easy], [medium], [hard] - Displays the difficulty in parentheses after number - Resets counter each section

Exercise 7: Dynamic Content¶

Create a command \matlab that: - Checks if matlab is defined as a toggle - If true: typesets code in MATLAB style - If false: typesets code in generic style - Allows easy switching between conference submission (no colors) and final version (colorful)

Summary¶

Custom commands and environments are essential for:

Efficiency: Type less, accomplish more
Consistency: Uniform notation throughout document
Maintainability: Change definitions in one place
Reusability: Create personal packages for multiple projects
Readability: Semantic commands make LaTeX source clearer

Key takeaways: - \newcommand{\name}[args]{def} for simple commands - \newenvironment{name}{begin}{end} for custom environments - \newcounter, \stepcounter, \refstepcounter for numbering - xparse for advanced argument parsing - Create .sty files for reusable command collections

Mastering custom commands transforms LaTeX from a typesetting system into a personalized authoring environment tailored to your needs.

Navigation

Previous: 12_Graphics_with_TikZ.md
Next: 14_Document_Classes.md

Custom Commands & Environments¶

Introduction¶

Why Create Custom Commands?¶

Problem: Repetitive Code¶

Solution: Custom Command¶

Basic Custom Commands¶

Syntax: \newcommand¶

Examples: Simple Text Replacements¶

Non-Mathematical Examples¶

Commands with Arguments¶

Syntax: One Argument¶

Examples¶

Multiple Arguments¶

Optional Arguments¶

Syntax¶

Examples¶

\renewcommand and \providecommand¶

\renewcommand: Redefine Existing Commands¶

\providecommand: Define Only If Not Defined¶

Mathematical Command Shortcuts¶

Common Patterns¶

Custom Environments¶

Syntax: \newenvironment¶

Example: Custom Theorem Box¶

Example: Solution Environment¶

Example: Custom Example Box¶

Counters¶

Creating and Using Counters¶

Example: Custom Theorem Counter¶

Counter Hierarchy¶

Conditional Commands¶

The ifthen Package¶

Tests¶

Example: Conditional Solutions¶

Advanced Argument Parsing: xparse¶

Syntax¶

Argument Specifiers¶

Examples¶

The etoolbox Package¶

Conditional Tests¶

Toggle Switches¶

Patching Commands¶

Debugging Custom Commands¶

Show Command Definition¶

Meaning¶

Debug Messages¶

Tracing¶

Creating a Personal .sty Package¶

Why Create a Package?¶

Steps¶

Local texmf Tree¶

Complete Example: Math Paper Package¶

Best Practices¶

Do's¶

Don'ts¶

Exercises¶

Exercise 1: Basic Math Shortcuts¶

Exercise 2: Custom Environment¶

Exercise 3: Conditional Compilation¶

Exercise 4: Derivative Command¶

Exercise 5: Personal Package¶

Exercise 6: Counter-Based Numbering¶

Exercise 7: Dynamic Content¶

Summary¶

Syntax: `\newcommand`¶

`\renewcommand` and `\providecommand`¶

`\renewcommand`: Redefine Existing Commands¶

`\providecommand`: Define Only If Not Defined¶

Syntax: `\newenvironment`¶

The `ifthen` Package¶

Advanced Argument Parsing: `xparse`¶

The `etoolbox` Package¶

Creating a Personal `.sty` Package¶

Local `texmf` Tree¶