\forlistloop of etoolbox reference requested

Question

I am having a problem of using the command of \forlistloop of etoolbox

unlike the way of typical C program where

int a[5]
for ( i = 0; i <5; i++)
{
    a[i]
}

one could access i I am not sure how to access the individual elements of the list using the command.

I see that \forlistloop<handler><list> is the basic sequence that is given.

I tried the following and get errors of undefined sequence.

\def\myrowlist{}
\forcsvlist{\listadd\myrowlist}{r,\theta,z}

$   \newcounter{itemcount}
    %\begin{itemize}
        \forlistloop{\stepcounter{itemcount}\itemRaone}{\myrowlist}
        \itemRaone
    %\item Total: \number\value{itemcount} items
    %\end{itemize}
$

My expectation is that I should be able to extract r, \theta and z separately each time.

Answer 1

Try the routines \grabitem{number} to get the item from the list. It's not failsafe, of course.

In general, it's better to use wrapper commands instead of doing stuff like \forcsvlist{\stepcounter...}{..} directly.

\documentclass{article}

\usepackage{etoolbox}


\listadd\myrowlist{}

\newcounter{itemcount}
\newcommand{\additem}[2][\myrowlist]{%
  \listadd{#1}{#2}%
  \stepcounter{itemcount}%
}%

\newcommand{\addtolist}[2][\myrowlist]{%
  \setcounter{itemcount}{0}
  \let #1\relax
  \listgadd #1{}%
  \forcsvlist{\additem[#1]}{#2}%
}

\makeatletter
\newcounter{temp@@count}

\newcommand{\getitem}[2]{%
  \stepcounter{temp@@count}%
    \ifnumequal{\value{temp@@count}}{#1}{%
      \ensuremath{#2}%
      \listbreak
    }{}
}%

\newcommand{\grabitem}[2][\myrowlist]{%
  \setcounter{temp@@count}{0}
  \ifnumgreater{#2}{\value{itemcount}}{%
    \GenericError{Error}{Number #2 for grabbing item too large}{There are only \number\value{itemcount} items}{Check your code!}
  }{% All is ok.
    \forlistloop{\getitem{#2}}{#1}%
  }
}

\makeatother

\newcommand{\showlist}[1]{%
$#1$

}%



\begin{document}

\addtolist{r,\theta,z,\Gamma,\Delta}

\forlistloop{\showlist}{\myrowlist} 

\grabitem{1}

\grabitem{5}


\begin{itemize}
  \forcsvlist{\item \grabitem[\myrowlist]}{1,3,5,4,5,2}
\end{itemize}



\end{document}

Answer 2

If you are willing to switch from etoolbox to the more powerful expl3 programming environment, you could do like this:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\newlist}{m}
 {
  \seq_new:c { g_sumanta_lists_#1_seq }
 }
\newlist{myrow} % default

\NewDocumentCommand{\additems}{O{myrow} m}
 {
  \clist_map_inline:nn { #2 }
   {
    \seq_gput_right:cn { g_sumanta_lists_#1_seq } { ##1 }
   }
 }

\NewDocumentCommand{\getitem}{O{myrow} m O{\use:n}}
 {
  \int_compare:nTF { #2 > \seq_count:c { g_sumanta_lists_#1_seq } }
   {
    NO ITEM
   }
   {
    #3 { \seq_item:cn { g_sumanta_lists_#1_seq } { #2 } }
   }
 }

\NewDocumentCommand{\uselist}{O{\use:n}m}
 {
  \seq_map_inline:cn { g_sumanta_lists_#2_seq } { #1 { ##1 } }
 }
\ExplSyntaxOff

\newcommand{\makeitem}[1]{\item $#1$}

\begin{document}

\additems{r,\theta,z,\Gamma}
\additems{\Delta}

\getitem{1}

\getitem{2}[\ensuremath]

\getitem{99}

\begin{itemize}
\uselist[\makeitem]{myrow}
\end{itemize}

\newlist{foo}

\additems[foo]{a,b,c}

\begin{enumerate}
\uselist[\makeitem]{foo}
\end{enumerate}


\end{document}

The optional argument to \additems is a list name (default myrow), the mandatory one is a comma separated list of items.

With \getitem you obtain the item whose number is specified by the mandatory argument; the first optional argument is a list name, the trailing optional argument is a one parameter macro that wraps the item (default is just producing the item). In case the number exceeds the available items, NOITEM is printed; this could be turned into an error message.

The \uselist command takes as optional argument a one parameter macro that should wrap the item. The default is just producing the item.

Answer 3

For iterating over an elementary list of items, you can use an array-like structure/interface (similar to what one would expect in programming languages like C) provided by arrayjobx:

\documentclass{article}
\usepackage{multido,arrayjobx}

\newarray\myrowlist\readarray{myrowlist}{r & \theta & z}

\begin{document}

\newcommand{\itemdelim}{\renewcommand{\itemdelim}{,\,}}% http://tex.stackexchange.com/a/89187/5764
$\multido{\i=1+1}{3}{\itemdelim\myrowlist(\i)}$

\end{document}

Elements within the array is specified using \readarray{<array>} where <array> is an &-delimited sequence.

multido has an easy enough interface to iterate over some \integer sequence.