How to make a key to on, off solutions of exercises in `answers` package?
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Anonymous 1123 
I use `answers` package like this
```
\documentclass[12pt,a4paper]{report}
\usepackage{amsthm,amsfonts,amssymb,amsmath}
\usepackage{answers}
\theoremstyle{definition}
 \Newassociation{sol}{Solution}{ans}
 \newtheorem{ex}{Exercise}[chapter]
\begin{document}
 	\chapter{Try}
 \Opensolutionfile{ans}[ans1]
 \section{Problems}
	 \begin{ex}
		 First exercise
		 \begin{sol}
			 First solution.
			 \end{sol}
	 \end{ex}
\begin{ex}
	 Second exercise
		 \begin{sol}
			 Second solution.
		 \end{sol}
	 \end{ex}
 \Closesolutionfile{ans}
	 \section{Solutions}
	 \input{ans1}
	 \end{document}
```

![ScreenHunter 1075.png](/image?hash=20628d3c9374e5d262bd6ede6c0e0c2fdb7ed21e31b8f33af4b9b81de1eb82d5)

Somtimes, I do not show solutions of exercises. I know, there is a command 
`\usepackage[nosolutionfiles]{answers}` 
In manual write "If the optional argument is given, solutions appear at that point in the text, rather than being written to external files".
Is there a key to on or off  solutions of exercises automatically. That is mean, when I use "off", I do not see solutions at all document, and if I use "on", I see solutions.
Top Answer
samcarter 
You can add your own if statement to switch on/off the solutions. In the following example, they will be off by default. Use `\solutionstrue` to switch them on or `\solutionsfalse` for off again.

```
\documentclass[12pt,a4paper]{report}
\usepackage{amsthm,amsfonts,amssymb,amsmath}
\usepackage{answers}

\newif\ifsolutions
%\solutionstrue % comment this line for no solutions


\theoremstyle{definition}
 \Newassociation{sol}{Solution}{ans}
 \newtheorem{ex}{Exercise}[chapter]
\begin{document}
 	\chapter{Try}
 \Opensolutionfile{ans}[ans1]
 \section{Problems}
	 \begin{ex}
		 First exercise
		 \begin{sol}
			 First solution.
			 \end{sol}
	 \end{ex}
\begin{ex}
	 Second exercise
		 \begin{sol}
			 Second solution.
		 \end{sol}
	 \end{ex}
 \Closesolutionfile{ans}
    \ifsolutions
	 \section{Solutions}
	 \input{ans1}
   \fi
	 \end{document}
```

(you could even use something like https://topanswers.xyz/tex?q=583 to automatically compile both versions)
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