Hide custom environment content based on boolean

Question

I have a custom environment which adds some text around its content, but I also want it to be able to show or hide its contents depending on a boolean.

I have tried doing the following:

\newif\ifshow
\showfalse
\newenvironment{myenv}
{\ifshow\textbf{Content of myenv:}}
{\fi}

But the first \begin{myenv} raises an Incomplete \iffalse; all text was ignored after line X. It would seem that the \iffalse is causing the \end{myenv} to be ignored, so the \fi is never written.

Any idea on how could I accomplish this?

Answer 1

I usually do it the other way around, i.e. define "different environments" based on the if. I use the comment package for excluding the contents of the environment.

\usepackage{comment}
\ifshow
  \newenvironment{myenv}{\textbf{Content of myenv:}}{}
\else
  \excludecomment{myenv}
\fi

Answer 2

Another way to do this is to use the environ package and conditionally keep or toss the \BODY.

\usepackage{environ}
\NewEnviron{myenv}{
  \ifshow
    \textbf{Content of myenv: \BODY}
  \fi}

Answer 3

I use

\newcommand\suppress[1]{} 

in combination with

\newcommand\myOptElse[3]{\ifthenelse{\boolean{#1}}%
{#2\suppress{#3}}%
{\suppress{#2}#3}}% if #1 then #2 else #3

and a boolean variable. This enables me to change between #2 and #3 with an ON/OFF switch #1.

Example:

%Declare the boolean variable    
\newboolean{showmyenv} %Testblöcke anzeigen
%
%Set to OFF
\setboolean{showmyenv}{false}
\myOptElse{showmyenv}{MyEnv content}{Alternative content} 
%the above will show "Alternative content", if you do not want any, leave it empty
%
%Set to ON
\setboolean{showmyenv}{true}
\myOptElse{showmyenv}{MyEnv content}{Alternative content} 
%the above will show "MyEnv content"

Answer 4

---

This is an implementation of the non-catcode-changing method (bullet point 2 in David's answer below).

\documentclass{article}
\usepackage{environ}

\ExplSyntaxOn

\NewEnviron{disposecontentinside}{}

\newif\ifshow

\newenvironment{testenv}{
    \ifshow
        \let\execute_this_when_environment_end\relax  % = do nothing when environment end
        % then do nothing
    \else
        % in this case, we want to execute \disposecontentinside here, and \enddisposecontentinside when the environment ends:
        \let\execute_this_when_environment_end\enddisposecontentinside
        \expandafter\disposecontentinside   % see https://tex.stackexchange.com/questions/21262/what-do-firstoftwo-and-secondoftwo-do/ for explanation for the expandafters
    \fi
}{
    \execute_this_when_environment_end
}

\ExplSyntaxOff

\begin{document}

\showtrue
\begin{testenv}
    show this
\end{testenv}

\showfalse
\begin{testenv}
    hide this
\end{testenv}
\end{document}

Advantages...

• keep synctex (unlike this other answer)
• can use \verb inside (except when it's something like \verb+}+ then you get problems with brace balancing if \showfalse)
• can use inside the argument of another commands etc.
• don't need to copy paste the code of environ package to grab the body
• unlike this other answer, it can be used even when the conditional is only known when the environment begins

---

Other solutions include

• Use a (plain TeX) box. See https://tex.stackexchange.com/a/73632/250119
• Use a lrbox environment which is a (thin) wrapper over the above.

The 2 methods above has the property that the inner content are still executed instead of being "skipped over".

• The solutions mentioned in the bullet points in https://tex.stackexchange.com/a/73634/250119.

Answer 5

When doing that for exercises/exams/etc, I want also to have a separate numbering for equations and floats (figures and tables). So my solution is to have this:

% requires ifthenelse package
\newboolean{Solucion} % True/False to show the solution
\setboolean{Solucion}{true} %

% define the color we use to typeset the solution, requires xcolor
\colorlet{darkgreen}{green!40!black}
\colorlet{darkblue}{blue!40!black}
\colorlet{solcolor}{darkblue}

\usepackage{environ}
\newcounter{soleq}
\makeatletter
\NewEnviron{solution}{\ifSolucion\color{solcolor}%
    \let\c@equation\c@soleq\def\theequation{S\thesoleq}%
\expandafter\BODY\fi}
\makeatother

\usepackage{newfloat}
\DeclareFloatingEnvironment[name={\textcolor{solcolor}{Figura}}]{solfigure}
\renewcommand{\thesolfigure}{\textcolor{solcolor}{S\arabic{solfigure}}}
% This is for use with babel[Spanish,English], adapt to your case
\addto\captionsenglish{\floatname{solcolor}{Figure}}
\addto\captionsspanish{\floatname{solcolor}{Figura}}

This outputs the solution in a different color, and the equations are numbered independently from the main part --- so that the equation and float numbers do not change between the document with and without the solution.

You probably need to tweak that a bit (I use Spanish and English with babel, for example), and it could be made better (notice the color of the figure caption), but I think it's a good starting point.