Check if a number is in a comma separated list (print or display selected sections from a specific list)

Question

I'm trying to check if a section number belongs to a specified list:

    \ifnum\value{section}=\mylist % check if section number in mylist --- WRONG
        \dosomething
    \fi

The conditional is obviously wrong because it doesn't make sense to compare a number to a list. My sense is that there must be a quick fix (although my TeX knowledge is very limited). Please help!

---

p.s. Here's a more fully-spelled out version of the my question:

I'm trying to create an environment that can print selected sections only. I hope to specify in the beginning of the document a comma separated list of sections to be printed, and then print only selected sections. For example,

    \keepsections{1,5,8}
    \mysection{section 1}{content}
    \mysection{section 2}{content}
    \mysection{section 3}{content}
    \mysection{section 4}{content}
    \mysection{section 5}{content}
    \mysection{section 6}{content}
    \mysection{section 7}{content}
    \mysection{section 8}{content}

Should give us:

This is what I've tried:

    \documentclass{article}

    \newcommand{\keepsections}[1]{ \newcommand{\mylist}{#1} }
    \usepackage{environ}% http://ctan.org/pkg/environ
    \NewEnviron{customsection}{ % new environment
        \stepcounter{section} % environ should use the section counter
        \ifcustomsec % if want to print only select sections from a list
            \ifnum\value{section}=\mylist\relax % check if section in list --- WRONG
                \addtocounter{section}{-1} % use correct counter and print
                \BODY
            \fi
        \else % if want to print all sections,
            \addtocounter{section}{-1} % use correct counter and print
            \BODY
        \fi%
    }
    \newcommand{\mys}[1]{\begin{customsection}#1\end{customsection}}
    \newcommand{\mysection}[2]{\mys{\section{#1}#2}} % to conform to usage above

    \newif\ifcustomsec % Boolean to print only selected sections
    \customsectrue % Bool=True: Tell TeX to print only from the custom list
    \keepsections{1,5,8} % Print only sections 1 and 3

    \begin{document}

    \mysection{section 1}{content}
    \mysection{section 2}{content}
    \mysection{section 3}{content}
    \mysection{section 4}{content}
    \mysection{section 5}{content}
    \mysection{section 6}{content}
    \mysection{section 7}{content}
    \mysection{section 8}{content}

    \end{document}

Although it fails because of the wrong logic pointed out above.

This question -- Create odd-numbered answers, or all answers -- has a very nice solution to printing just even or odd sections that works really well, and that's where I took the idea from.

This question -- Test if a number is in a comma separated range/list of numbers -- discusses defining a command of the sort

    \ifinrange{<num>}{<range>}{<TRUE>}{<FALSE>}% num>=0

It would be nice if I could just replace

    \ifnum\value{section}=\mylist\relax%

portion with

    \ifinrange{\value{section}}{\mylist}{...}{...}

but with my limited knowledge I failed to implement it in this context.

Deeply appreciate any help. Thanks so much.

Answer 1

Using xinttools to loop over the comma separated list of section numbers:

\documentclass{article}
\usepackage{xinttools}

\newcommand{\keepsections}[1]{\newcommand{\mylist}{#1}}

\usepackage{environ}% http://ctan.org/pkg/environ
\NewEnviron{customsection}{% new environment
    \stepcounter{section}% environ should use the section counter
    \ifcustomsec % if want to print only select sections from a list
        \xintFor ##1 in \mylist \do
        {\ifnum\value{section}=##1\relax % check if section in list
            \addtocounter{section}{-1}% use correct counter and print
            \BODY
            \expandafter\xintBreakFor % no need to check farther in the list
        \fi}%
    \else % if want to print all sections,
        \addtocounter{section}{-1}% use correct counter and print
        \BODY
    \fi%
}
\newcommand{\mys}[1]{\begin{customsection}#1\end{customsection}}
\newcommand{\mysection}[2]{\mys{\section{#1}#2}}% to conform to usage above

\newif\ifcustomsec % Boolean to print only selected sections
\customsectrue % Bool=True: Tell TeX to print only from the custom list
\keepsections{1,5,8}% Print only sections 1, 5 and 8

\begin{document}
\mysection{section 1}{content}
\mysection{section 2}{content}
\mysection{section 3}{content}
\mysection{section 4}{content}
\mysection{section 5}{content}
\mysection{section 6}{content}
\mysection{section 7}{content}
\mysection{section 8}{content}
\end{document}