Input a parameter from another file

Question

Thank to the people asking and answering this question for how to import a part of a file into Latex. I want to extend this question a bit more.

What if you have such a file resulted from a C++ simulation:

parmeters.tex

time=5 sec
max_velocity=3 m/s
tolerance=$10^{-6}$
start_time=349872034 sec
stop_time=349872039 sec
distance=11 m

In my latex, I would like to type

The trajectory travels the distance of \paraminput[distance]{parameters.tex} in 
\paraminput[time]{parameters.tex}. The simulation stops when the relative error is
less than \paraminput[tolerance]{parameters.tex}. The maximum observed velocity 
is \paraminput[max_velocity]{parameters.tex}. ...

And the result

The trajectory travels the distance of 11 m in 
5 sec. The simulation stops when the relative error is
less than $10^{-6}$. The maximum observed velocity 
is 3 m/s. ...

So I do not need to change the report everytime I run the simulation.

How is it possible in Latex?

Answer 1

An expl3 based solution, reading the file, storing the contents and matching the first [#1] parameter.

It would be better to read the file once instead of opening and closing it all the time (I will provide another solution later on)

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\ior_new:N \ar_param_file

\seq_new:N \g_ar_param_seq


\cs_new:Nn \ar_read_by_line:n {%
  \seq_gput_right:Nn \g_ar_param_seq {#1}% Appending the current line to the global sequence buffer
}




\NewDocumentCommand{\paraminput}{O{}m}{%
  \seq_gclear:N \g_ar_param_seq% Clearing the sequence
  \ior_open:Nn \ar_param_file {#2} % Open the input file
  \ior_map_inline:Nn \ar_param_file {\ar_read_by_line:n{##1}}% Reading line by line
  \ior_close:N \ar_param_file% Closing the line
  \seq_map_inline:Nn \g_ar_param_seq {% Traversing through the line
    \seq_set_split:Nnn \l_tmpa_seq {=} {##1} % Splitting each line into 'key=value' pairs
    \seq_if_in:NxT \l_tmpa_seq {#1} {\seq_item:Nn \l_tmpa_seq {2}\seq_map_break:}% Checking if `#1` is in the sequence and display it with \seq_item:Nn..., then break the mapping loop.
  }
}

\ExplSyntaxOff

\begin{document}
The trajectory travels the distance of \paraminput[distance]{parameters.tex} in 
\paraminput[time]{parameters.tex}. The simulation stops when the relative error is
less than \paraminput[tolerance]{parameters.tex}. The maximum observed velocity 
is \paraminput[max_velocity]{parameters.tex}. ...
\end{document}

Update

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\ior_new:N \ar_param_file

\seq_new:N \g_ar_param_seq


\cs_new:Nn \ar_read_by_line:n {%
  \seq_gput_right:Nn \g_ar_param_seq {#1}
}

\tl_new:N \g_paramfile_path_tl

\NewDocumentCommand{\parampath}{m}{%
  \tl_gset:Nn \g_paramfile_path_tl {#1}
}





\NewDocumentCommand{\paraminput}{O{}m}{%
  \seq_gclear:N \g_ar_param_seq
  \ior_open:Nn \ar_param_file {\g_paramfile_path_tl #2}
  \ior_map_inline:Nn \ar_param_file {\ar_read_by_line:n{##1}}
  \ior_close:N \ar_param_file
  \seq_map_inline:Nn \g_ar_param_seq {%
    \seq_set_split:Nnn \l_tmpa_seq {=} {##1}
    \seq_if_in:NxT \l_tmpa_seq {#1} {\seq_item:Nn \l_tmpa_seq {2}\seq_map_break:}
  }
}


\ExplSyntaxOff

\begin{document}

The trajectory travels the distance of \paraminput[distance]{parameters.tex} in 
\paraminput[time]{parameters.tex}. The simulation stops when the relative error is
less than \paraminput[tolerance]{parameters.tex}. The maximum observed velocity 
is \paraminput[max_velocity]{parameters.tex}. ...


\parampath{/tmp/output_from_simulation/}
The trajectory travels the distance of \paraminput[distance]{parameters.tex} in 
\paraminput[time]{parameters.tex}. The simulation stops when the relative error is
less than \paraminput[tolerance]{parameters.tex}. The maximum observed velocity 
is \paraminput[max_velocity]{parameters.tex}. ...
\end{document}

Answer 2

Quite simple with LuaLaTeX:

\documentclass{article}

\usepackage{luacode}

\begin{document}

\begin{luacode*}
function read_param(filename,paramname)
    for line in io.lines(filename) do
        param,value=string.match(line,"(.*)=(.*)")
        if param==paramname then 
            tex.print(value)
        end
    end
end
\end{luacode*}

\newcommand{\paraminput}[2][]{\directlua{read_param("#2","#1")}}

The trajectory travels the distance of \paraminput[distance]{parmeters.tex} in 
\paraminput[time]{parmeters.tex}. The simulation stops when the relative error is
less than \paraminput[tolerance]{parmeters.tex}. The maximum observed velocity 
is \paraminput[max_velocity]{parmeters.tex}.

\end{document}

Answer 3

Provided your parameters.tex looks like this (with "")

time="5 sec"
max_velocity="3 m/s"
tolerance="$10^{-6}$"
start_time="349872034 sec"
stop_time="349872039 sec"
distance="11 m"

you could create a noweb file (so.Rnw) like

\documentclass[a4paper,12pt]{article}
\usepackage{Sweave}

\begin{document}

<<echo=FALSE>>=
source( "parameters.tex" )
@

This is an easy - can be done in \Sexpr{time} or less.\\
There is a tolerance of \Sexpr{tolerance} over a distance of \Sexpr{distance}. 

\end{document}

and compile it with

R CMD Sweave so.Rnw && pdflatex so.tex

which gives you

This may look more complicated but it gives you easy access to real variables and actually is set up rather easily.