Iterate over part of a list

Question

I know that the package etoolbox has a \foreach command.

I want to set a list once an use it multiple times. In the example, I would set \mylist equal to the comma separated value list 10,20,30,40,50.

I want to have a command like

\forparteach 3 \var \in {10,20,30, 40,50} {
  \var
}

is the same as

\foreach \var \in {10,20,30} {
   \var
}

Or even

\forparteach {1,3,4,2,5,5} \var \in {10,20,30,40,50} {
   \var
}

is the same as

\foreach \var \in {10,30,40,20,50,50} {
   \var
}

Is there any package out there that does this or an easy solution?

Thanks!

Answer 1

You can implement your own foreach cycle.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\xforeach}{sO{}mm}
 {
  \IfBooleanTF { #1 }
   {
    \marks_xforeach:Vnn #3 { #2 } { #4 }
   }
   {
    \marks_xforeach:nnn { #3 } { #2 } { #4 }
   }
 }

\keys_define:nn { marks/xforeach }
 {
  upto  .int_set:N   = \l_marks_xforeach_upto_int,
  items .clist_set:N = \l_marks_xforeach_items_clist,
 }

\clist_new:N \l__marks_xforeach_main_clist
\seq_new:N \l__marks_xforeach_items_seq
\seq_new:N \l__marks_xforeach_main_seq

\cs_new_protected:Nn \marks_xforeach:nnn
 {
  \clist_set:Nn \l__marks_xforeach_main_clist { #1 }
  \keys_set:nn { marks/xforeach } { upto=-1, items=, #2 }
  \cs_set_protected:Nn \__marks_xforeach_cycle:n { #3 }

  \seq_clear:N \l__marks_xforeach_items_seq
  \clist_if_empty:NTF \l_marks_xforeach_items_clist
   {% items not specified
    \int_compare:nT { \l_marks_xforeach_upto_int = -1 }
     {
      \int_set:Nn \l_marks_xforeach_upto_int
       { \clist_count:N \l__marks_xforeach_main_clist }
     }
    \int_step_inline:nn { \l_marks_xforeach_upto_int }
     {
      \seq_put_right:Nn \l__marks_xforeach_items_seq { ##1 }
     }
   }
   {% items
    \seq_set_from_clist:NN \l__marks_xforeach_items_seq \l_marks_xforeach_items_clist
   }
  \seq_clear:N \l__marks_xforeach_main_seq
  \seq_map_inline:Nn \l__marks_xforeach_items_seq
   {
    \seq_put_right:Nx \l__marks_xforeach_main_seq
     {
      \clist_item:Nn \l__marks_xforeach_main_clist { ##1 }
     }
   }
  \seq_map_function:NN \l__marks_xforeach_main_seq \__marks_xforeach_cycle:n
 }
\cs_generate_variant:Nn \marks_xforeach:nnn { V }

\ExplSyntaxOff

\begin{document}

\newcommand{\myarray}{10,20,30,40,50}

\xforeach{10,20,30,40,50}{(Item: #1) }

\xforeach*{\myarray}{(Item: #1) }

\xforeach[upto=3]{10,20,30,40,50}{(Item: #1) }

\xforeach*[upto=3]{\myarray}{(Item: #1) }

\xforeach[items={1,3,4,2,5,5}]{10,20,30,40,50}{(Item: #1) }

\xforeach*[items={1,3,4,2,5,5}]{\myarray}{(Item: #1) }

\end{document}

Some points to note: differently from \foreach, the cycles are not performed in groups; the current item is denoted by #1 rather than by designating a macro holding it. The *-variant accepts as second argument a macro instead of an explicit list.

One could rely on the presence of braces like \foreach, but in my opinion using options and specific commands is cleaner.

Answer 2

Both of the things can be achieved with the current \foreach in a straightforward way. More specifically, \breakforeach allows you to break a loop, which allows you to restrict the loop to the first n entries. And you can store the list in an array and access its elements. Notice that I did not even try to write a new macro \forparteach because \foreach is very delicate and I do not dare to mess around with it.

\documentclass{article}
\usepackage{pgffor}
\begin{document}
\section*{Cutting a loop off at some number}
\foreach \var [count=\n] in {10,20,30,40,50} 
{ \var 
\ifnum\n>2 
\breakforeach 
\fi
}
\section*{Selecting a subset of items from a list/an array}
\def\myarray{{10,20,30,40,50}}
\foreach \var [evaluate=\var as \myindex using {int(\var-1)}] in {1,3,4,2,5,5} {
\pgfmathparse{\myarray[\myindex]}\pgfmathresult
}
\end{document}

Answer 3

You can do these things with xinttools. The \xintFor/\xintFor* loops do not create groups. On the other hand, they do not currently implement some nice \foreach syntax like 1, 2, ..., 5. And there is no easy way to create key=value selection as xinttools is not a full-fledged programming interface like provided by LaTeX3, but a small collection of useful macros (usable under Plain TeX, too).

\documentclass{article}

\usepackage{xinttools}
\usepackage{parskip}
\begin{document}\pagestyle{empty}\thispagestyle{empty}

\section*{Cutting a loop off at some number}

First three:

\begin{itemize}
% this uses \xintFor* as argument will be {item1}{item2}....
% i.e. a "list" in the sense of xinttools doc, not a
% "comma separated list"
 \xintFor* #1 in {\xintKeep{3}{\xintCSVtoList{10,20,30,40,50}}}
 \do 
  {\item (Item: #1)}
\end{itemize}

Last two:

\begin{itemize}
 \xintFor* #1 in {\xintKeep{-2}{\xintCSVtoList{10,20,30,40,50}}}
 \do 
  {\item (Item: #1)}
\end{itemize}

Trim first two:

\begin{itemize}
 \xintFor* #1 in {\xintTrim{2}{\xintCSVtoList{10,20,30,40,50}}}
 \do 
  {\item (Item: #1)}
\end{itemize}

Trim last two:

\begin{itemize}
 \xintFor* #1 in {\xintTrim{-2}{\xintCSVtoList{10,20,30,40,50}}}
 \do 
  {\item (Item: #1)}
\end{itemize}

Nested keep/trim (keep 2 after having trimmed 2):

\begin{itemize}
 \xintFor* #1 in {\xintKeep{2}{\xintTrim{2}{\xintCSVtoList{10,20,30,40,50}}}}
 \do 
  {\item (Item: #1)}
\end{itemize}


\clearpage
\section*{Selecting a subset of items from a list}

Selecting items 1, 3, 4, 2, 5, 5:

% for better efficiency, let's convert comma separated values
% in a "list" (in xinttools terminology) first, once and for all
% because \xintNthElt macro expects such as list

\oodef\MyListOfBracedItems{\xintCSVtoList{10,20,30,40,50}}

\begin{itemize}
% this uses the non-starred \xintFor, which accepts comma separated values
 \xintFor #1 in {1, 3, 4, 2, 5, 5}
 \do 
  {\item (Item: \xintNthElt{#1}{\MyListOfBracedItems})}
\end{itemize}

It is also possible to create an ``array'', for better efficiency afterwards:

\xintAssignArray\xintCSVtoList{10, 20, 30, 40, 50}\to\MyArray

\begin{itemize}
% this uses the starred \xintFor, which accepts braced items (or
% single tokens; spaces skipped)
 \xintFor* #1 in {1 3 4 2 5 5}
 \do 
  {\item (Item: \MyArray{#1})}
\end{itemize}


\end{document}

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