Embedding if statements in environments

Question

I am trying to write an environment so that I can toggle on and off the answers to questions on a hand out. I also want the answers to appear red and then the main text go back to black after the question.

My code is as follows,

\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{geometry}
\usepackage{siunitx}
\usepackage[dvipsnames]{xcolor}
\usepackage{gensymb}

\newif\ifstudent  % This controls if it is a student version or not 

\studenttrue 

%\studentfalse

\newenvironment{answer}[1]
    {\ifstudent
    \\
    \vspace{#1}
    \\
    \else\\
    \color{red}
    }
    { 
    \color{black}\\
    \fi
    }

\begin{document}

At 18\degree C, gold has a resistance of 180 $\si{\Omega}$. When the temperature is increased, the resistance increases to 200 $\si{\Omega}$. Calculate the new temperature if the temperature coefficient of resistivity for gold is 0.0034.

\begin{answer}{40mm}
$$R_T=R_0\left[1+\alpha(T-T_0)\right]$$
\\
$$T=\frac{\frac{R_T}{R_0}-1}{\alpha}+T_0=\frac{\frac{200\;\si{\Omega}}{250\;\si{\Omega}}-1}{0.0034}+18\degree\si{C}=116\degree\si{C}$$
\end{answer}
\end{document}

Unfortunately I get an error of <inserted text> \fi <*> main.tex if i run it like it is. It runs fine if i switch it to true.

Thank you in advance.

Answer 1

May I suggest the following.

Notes: It is always good to start and end with a \par. The best way to hide something is to put it into a savebox (which is probably what \phantom does). For an environment, the logical choice is lrbox. Inside an environment, group, or savebox, you don't need to restore the color.

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage[utf8]{inputenc}

\usepackage{geometry}
\usepackage{siunitx}

\usepackage{gensymb}

\newif\ifstudent  % This controls if it is a student version or not 
\studenttrue 
%\studentfalse

\newenvironment{answer}[1]{\par
    \ifstudent\vspace{#1}\fi% #1 can only be used in first half of environments
    \begin{lrbox}{0}\minipage{\linewidth}\color{red}}%
{\endminipage\end{lrbox}%
    \ifstudent\else\usebox0\par\fi
}

\begin{document}

At 18\degree C, gold has a resistance of 180 $\si{\Omega}$. When the temperature is increased, the resistance increases to 200 $\si{\Omega}$. Calculate the new temperature if the temperature coefficient of resistivity for gold is 0.0034.

\begin{answer}{40mm}
$$R_T=R_0\left[1+\alpha(T-T_0)\right]$$
\\
$$T=\frac{\frac{R_T}{R_0}-1}{\alpha}+T_0=\frac{\frac{200\;\si{\Omega}}{250\;\si{\Omega}}-1}{0.0034}+18\degree\si{C}=116\degree\si{C}$$
\end{answer}
\hrule% show bottom ov space
\end{document}