Add text to document without changing formatting or spacing

Question

I am a teacher. When I write an exam, I would like to be able to print the student version of the exam and then print the exam answer key. My students write their answers directly on the exam, so the space I've left them to write their answer will typically be large enough to contain anything I want to have in the answer key. The problem is how to print the two versions of the exam.

Note that I am familiar with the EXAM package (https://ctan.org/pkg/exam?lang=en), and am not currently interested in using it. Looking through other answers, I think what I want is something based on smash (What does \smash do, and where is it documented?).

Shown below is an MWE that has two implementations of \answerkey. Version A is my attempt to do what I want -- optionally include the answer, do it in red, and smash it so that it doesn't take up any space. Unfortunately, for the first use below, the vertical spacing is still changing with and without the answers. For the second use below, it breaks totally because the answer has multiple paragraphs.

The optimal answer is a single \answerkey command that can handle both of the uses below. The goal is that the spacing of the document not change when \answerstrue or \answersfalse is used.

\documentclass[10pt]{article}
\usepackage{xcolor}
\usepackage{tikz}
\newif\ifanswers
\answerstrue
%\answersfalse
\newcommand{\answerkeyA}[1]{%
  \ifanswers%
   \textcolor{red}{\smash{\textbf{\parbox[t]{\linewidth}{#1}}}}%
  \fi%
}
\newcommand{\answerkeyB}[1]{%
  \ifanswers%
    #1%
  \fi%
}
\begin{document}
\begin{enumerate}
\item Answer the following question with words.
\answerkeyA{ I do not like them in a house.  I do not like them with
    a mouse.  I do not like them here or there.  I do not like them
    anywhere.  I do not like green eggs and ham. }
\vspace*{1in}
\item Draw a picture to illustrate the following problem.
\answerkeyB{
  Answer:
\begin{tikzpicture}[scale=0.3,font=\sffamily]
    \draw [step=1.0, thin, gray!50] (-3, -3) grid (3, 3);
    \draw [arrows={latex-latex}] (-3,0) -- coordinate (x axis mid) (3,0);
    \draw [arrows={latex-latex}] (0,-3) -- coordinate (y axis mid) (0,3);
  \end{tikzpicture}
}
\vspace*{2in}
\item The next problem should be here.
\end{enumerate}
\end{document}

Answer 1

You could do \smash followed by \vspace, but \parbox has parameters for controlling its height as well as width.

Here is what I would do, given the logical switch \ifanswers and specified space for the students to write answers.

\newcommand\answer[2]{% #1 = size  #2 = answer
  \par \noindent
  \parbox[t][#1][s]{\linewidth}{%
     \ifanswers \color{red}#2\fi
     \par\vfill
  }\par
  \setlength\prevdepth{0pt}\medskip
}

The [s] argument for "stretch" and the \vfill at the end of the box combine to be like a [t] argument, except with [t] there is no warning when the text can't fit in the allotted space. This version gives an overfull box warning when the answer text is too large, and the answer can print over the next question.

There is a little bit of extra space above the answer, which may be just what is needed. (It is caused by having the top edge of the answer be at the "baseline".) Also, there is optional \medskip extra space after the answer to balance.

With a little more, the \answer macro can be tightened to give regular baseline spacing between the question and the answer.

\newcommand\answer[2]{% #1 = size  #2 = answer
  \par \noindent
  \parbox[t][#1][s]{\linewidth}{%
     % the next line preserves the top baseline alignment
     \vskip-\ht\strutbox \noindent\strut
     \ifanswers \color{red}\ignorespaces#2\fi
     \par\vfill
  }\par
  \setlength\prevdepth{0pt}%
}

In that version the paragraph is started before the answer text, and a "strut" plus a negation is used to bring baseline alignment to where it belongs. The \ignorespaces allows an answer to be written like

\answer{3cm}{
   this is the answer...
}

without introducing spurious spaces.

Answer 2

If you want the spacing to not change, then you need to still do something with the text even in the student version, so that TeX knows what the spacing should be. \vphantom will leave the appropriate vertical spacing, but it also breaks with paragraphs. This solution is to write the answers anyway, but in invisible ink (aka, white). The only catch is that the tikzpicture also sets colors, so I need a new color command in there as well: \answercolor, which is either the color you give it, or white. (Also, \textcolor{white}{#1} doesn't like paragraph breaks either, so I switched to {\color{white}#1}.) (Note that the text is still present, so that this would not be suitable for giving as a pdf - you can copy and paste the invisible text. But once printed, the text is no longer there.

\documentclass[10pt]{article}
\usepackage{xcolor}
\usepackage{tikz}
\newif\ifanswers
%\answerstrue
\answersfalse
\newcommand{\answerkeyA}[1]{%
  \ifanswers%
   \textcolor{red}{\smash{\textbf{\parbox[t]{\linewidth}{#1}}}}%
  \else%
   \textcolor{white}{\smash{\textbf{\parbox[t]{\linewidth}{#1}}}}%
  \fi%
}
\newcommand{\answerkeyB}[1]{%
  \ifanswers%
    #1%
  \else%
   {\color{white}#1}%
  \fi%
}
\newcommand{\answercolor}[1]{%
 \ifanswers%
  #1%
 \else%
  white%
 \fi%
}
\begin{document}
\begin{enumerate}
\item Answer the following question with words.
\answerkeyA{ I do not like them in a house.  I do not like them with
    a mouse.  I do not like them here or there.  I do not like them
    anywhere.  I do not like green eggs and ham. }
\vspace*{1in}
\item Draw a picture to illustrate the following problem.
\answerkeyB{
  Answer:
\begin{tikzpicture}[scale=0.3,font=\sffamily]
    \draw [step=1.0, thin, \answercolor{gray!50}] (-3, -3) grid (3, 3);
    \draw [arrows={latex-latex}] (-3,0) -- coordinate (x axis mid) (3,0);
    \draw [arrows={latex-latex}] (0,-3) -- coordinate (y axis mid) (0,3);
  \end{tikzpicture}
}
\vspace*{2in}
\item The next problem should be here.
\end{enumerate}
\end{document}

Answer 3

I think this is probably what you want. This solution provides answer environment, and you only need to put your answers in there. You can switch between teacher/student version by activating \answerfalse.

Teacher version:

Student version:

More details about this solution:

1. I am using xparse to capture the body of answer environment. Note that this requires newer version of xparse. If you do not have access to it, please consider using environ package instead.
2. The content is put into a breakable tcolorbox to measure height. After putting the content into input stream, tcolorbox will take care of breaking the content across several pages.
3. It is assumed that you will not change \baselinestretch - you may need to modify the calculations if line spacing is changed.
4. When \answerfalse is activated, I am filling the tcolorbox with new lines whose height is equal to original content.
5. Because the body of the environment is captured as argument, you will not be able to use verbatim environments inside answer (e.g. align,align*,listing,etc.)

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath, amssymb}
\usepackage{expl3}
\usepackage{xparse}
\usepackage{tikz}
\usepackage{blindtext}
\usepackage[breakable, skins]{tcolorbox}
\ExplSyntaxOn
\bool_new:N \g_show_answer_bool
\coffin_new:N \g_answer_coffin
\dim_new:N \g_answer_height_dim
\dim_new:N \g_answer_parbox_height_dim
% unit height of each \parbox for tcolorbox's
% page breaking algorithm to work with
\dim_gset:Nn \g_answer_parbox_height_dim {\baselineskip}
\newcommand{\answertrue}{\bool_gset_true:N \g_show_answer_bool}
\newcommand{\answerfalse}{\bool_gset_false:N \g_show_answer_bool}
% shows answer by default
\answertrue
\cs_set:Npn \my_tcbox:nn #1#2 {
    \begin{tcolorbox}[breakable, enhanced, left=0pt, right=0pt, top=0pt, bottom=0pt, boxsep=0pt, boxrule=0pt, colback=white, colframe=white, width=\linewidth, #2]
        #1
    \end{tcolorbox}
}
\cs_generate_variant:Nn \my_tcbox:nn {Vn}
\cs_set:Npn \my_height_box:n #1 {
    \exp_not:n{\parbox{\linewidth}{\rule{0pt}{#1}}}
}
% +b (body capture) requires xparse package to be no older than 2019-03-05 release
% or one can use environ package to achieve similar effects
\DeclareDocumentEnvironment{answer}{+b}{
    \bool_if:NTF \g_show_answer_bool {
        % shows answer
        \my_tcbox:nn {#1} {}
    } {
        % otherwise, use content that has identical height instead
        % put content in a minipage to measure height
        \hcoffin_gset:Nn \g_answer_coffin {
            \my_tcbox:nn {#1} {}
        }
    % get height of content
    \dim_gset:Nn \g_answer_height_dim {
        \coffin_dp:N \g_answer_coffin + 
        \coffin_ht:N \g_answer_coffin +
        \baselineskip
    }

    % check if height of content is longer than current page
    \fp_compare:nNnT {\g_answer_height_dim} > {\pagegoal - \pagetotal} {
        % if so, increase \g_answer_height_dim
        \dim_gadd:Nn \g_answer_height_dim {\baselineskip}
    }

    % compute how many units we need
    \fp_set:Nn \l_tmpa_fp {floor(\g_answer_height_dim / \g_answer_parbox_height_dim)}
    % create token list to fill vspace
    \tl_set:Nn \l_tmpa_tl {}
    \fp_step_inline:nnnn {1} {1} {\l_tmpa_fp} {
        \tl_put_right:Nn \l_tmpa_tl {\par\phantom{}}
    }
    % compute space less than \g_answer_parbox_height_dim
    \fp_set:Nn \l_tmpb_fp {\g_answer_height_dim - \l_tmpa_fp * \g_answer_parbox_height_dim}
    % fill remaining space
    \tl_put_right:Nx \l_tmpa_tl {\exp_not:N\vspace*{\fp_eval:n {\l_tmpb_fp} pt}}
    %\tl_show:N \l_tmpa_tl
    \my_tcbox:Vn \l_tmpa_tl {}
}

}{}
\ExplSyntaxOff
% toggle version w/ or w/out answer
%\answerfalse
\begin{document}
\begin{enumerate}
\item Answer the following question with words.
\begin{answer}
I do not like them in a house.  I do not like them with a mouse.

I do not like them here or there.  I do not like them anywhere.

I do not like green eggs and ham.
\end{answer}
\item Draw a picture to illustrate the following problem.
\begin{answer}
\begin{tikzpicture}[scale=0.3,font=\sffamily]
    \draw [step=1.0, thin, gray!50] (-3, -3) grid (3, 3);
    \draw [arrows={latex-latex}] (-3,0) -- coordinate (x axis mid) (3,0);
    \draw [arrows={latex-latex}] (0,-3) -- coordinate (y axis mid) (0,3);
\end{tikzpicture}
\end{answer}
\item Write a long and useless article.
\begin{answer}
\Blindtext[4]
\end{answer}
\end{enumerate}
Space to top page top: \the\pagetotal
\end{document}