Get value from \foreach

Question

I build a macro named \fromlist to get value from a list. The value can be typeset, but can not be invoked directly, why and how to deal with it?

Code：

\documentclass[a4paper]{article}
\usepackage{pgffor}
\usepackage{geometry}
\geometry{showframe}
\geometry{left=1cm,right=1cm,top=1cm,bottom=1cm}
\parindent0pt


\begin{document}

\newcommand\fromlist[2]{%
  \foreach \x [count=\i] in #1{%
    \ifnum\i=#2 \x\breakforeach\fi%
  }%
}

% Test:
\def\bbb{2in}\rule{\bbb}{5pt} % This ok.
\def\bbb{\fromlist{{1in,2in,3in}}{2}}\bbb %This is ok.
\rule{\fromlist{{1in,2in,3in}}{2}}{5pt} % This causes error, why?

Answer 1

You need to extract the item by expansion, you can not make assignments within the argument to \rule. Fortunately LaTeX provides such a function out of the box in the clist (comma list) module. More exactlythe nn variant is available, which would work for the explicit list, if you need to expand commands holding a list, you can declare an ee variant as here for the second example.

\documentclass[a4paper]{article}
\usepackage{geometry}
\geometry{showframe}
\geometry{left=1cm,right=1cm,top=1cm,bottom=1cm}
\parindent0pt

\ExplSyntaxOn
\cs_generate_variant:Nn\clist_item:nn{ee}
\newcommand\fromlist{\clist_item:ee}
\ExplSyntaxOff

\newcommand\mylist{1in,2in,3in}
\begin{document}


\rule{\fromlist{1in,2in,3in}{2}}{5pt} 

\rule{\fromlist{\mylist}{3}}{5pt} 


\end{document}

Answer 2

When we say that “a command is (fully) expandable” we mean that it delivers the intended result by performing no assignment or typesetting instruction (among which there is \relax).

So your \bbb works, because its expansion is a length. TeX knows that the first argument to \rule should contain a (rigid) length, and it performs macro expansion until finding something that fits the requirement.

When TeX sees your \fromlist, it realizes it's not a length, so it does an expansion and it sees \foreach. If you do

latexdef -p pgffor -s foreach

from a terminal, you'll get

% pgffor.code.tex, line 60:
\let\foreach=\pgffor@foreach

% pgffor.code.tex, line 45:
\def\pgffor@foreach{%
    \pgffor@atbeginforeach%
    \let\pgffor@assign@before@code=\pgfutil@empty%
    \let\pgffor@assign@after@code=\pgfutil@empty%
    \let\pgffor@assign@once@code=\pgfutil@empty%
    \let\pgffor@remember@code=\pgfutil@empty%
    \let\pgffor@remember@once@code=\pgfutil@empty%
    \pgffor@alphabeticsequencefalse%
    \pgffor@contextfalse%
    %
    \let\pgffor@var=\pgfutil@empty
    %
    \pgffor@vars%
}

and you know that your plan is doomed, because even if \pgffor@atbeginforeach is expandable (it isn't actually), a bunch of \let instructions follows and one is already too many.

Is it necessary to run latexdef to see whether a command is expandable or not? Not really. In general you should not expect a command is expandable, unless somebody tells you it is.

And among the expl3 functions, you can spot those that are (fully) expandable because their name is followed by a star (filled or hollow). The descriptions are found in interface3.pdf.

Your input is a comma separated list of items, so it fits the syntax of a clist. And yes, the function \clist_item:nn has the star after it! So you might do

\ExplSyntaxOn
\cs_set_eq:NN \fromlist \clist_item:nn
\ExplSyntaxOff

but then you'll ask a new question because something like

\def\ccc{1pt,2pt,3pt}
\rule{\fromlist\ccc{2}}{3cm}

doesn't work. How can you accommodate both implicit and explicit comma lists?

My suggestion is to define \fromlist with a *-variant that accepts an implicit list:

\ExplSyntaxOn
\NewExpandableDocumentCommand{\fromlist}{smm}
 {
  \IfBooleanTF{#1}
   {% there is *
    \clist_item:Nn #2 { #3 }
   }
   {% no *
    \clist_item:nn { #2 } { #3 }
   }
 }
\ExplSyntaxOff

A full example.

\documentclass{article}

\ExplSyntaxOn

\NewExpandableDocumentCommand{\fromlist}{smm}
 {
  \IfBooleanTF{#1}
   {% there is *
    \clist_item:Nn #2 { #3 }
   }
   {% no *
    \clist_item:nn { #2 } { #3 }
   }
 }

\ExplSyntaxOff

\newcommand{\mylist}{1in,2in,3in}

\begin{document}

\rule{\fromlist{1in,2in,3in}{2}}{5pt}

\rule{\fromlist*{\mylist}{3}}{5pt}

\end{document}

Answer 3

As I commented, \fromlist is not expandable (for example, try \edef\tmp{\fromlist{{1in,2in,3in}}{2}}). Thus, \rule doesn't see the final result of \fromlist, for use as its input. It therefore throws an error.

A different approach is proposed below, using listofitems, to read and parse the list. Then, particular entries from the list can be expandably recalled. The key is to read the list before passing a list element to \rule.

I first show it as a two step process with \readlist followed by \rule. Alternatively, one can place those two steps in a macro \Zrule, as I do in the second approach.

\documentclass[a4paper]{article}
\usepackage{listofitems}
\usepackage{geometry}
\geometry{showframe}
\geometry{left=1cm,right=1cm,top=1cm,bottom=1cm}
\parindent0pt
\newcommand\Zrule[3]{%
  \readlist\rulearg{#1}%
  \rule{\rulearg[#2]}{#3}
}
\begin{document}

\readlist\rulelen{1cm,2.4cm,3cm}
\rule{\rulelen[2]}{5pt}

\Zrule{1in,2in,3in}{2}{5pt}
\end{document}

Answer 4

This s a variant

\fromlist{⟨tokens to prepend to i-th comma-list-item⟩}%
         {⟨tokens to append to i-th comma-list-item⟩}%
         {⟨TeX-⟨number⟩-quantity of value i⟩}%
         {⟨comma-list⟩}

which—within the local scope of the i-th \foreach-iteration—does

⟨tokens to prepend to i-th comma-list-item⟩{⟨i-th comma-list-item⟩}⟨tokens to append to i-th comma-list-item⟩:

\documentclass[a4paper]{article}
\usepackage{pgffor}
\usepackage{geometry}
\geometry{showframe}
\geometry{left=1cm,right=1cm,top=1cm,bottom=1cm}
\parindent0pt

\makeatletter
\newcommand\PassFirstToSecond[2]{#2{#1}}
\newcommand\fromlist[4]{%
  \foreach \x [count=\i] in #4{%
    \ifnum\i=\expandafter\@firstofone\expandafter{\number#3} %
    \expandafter\@firstofone\else\expandafter\@gobble\fi
    {%
      \breakforeach
      \expandafter\PassFirstToSecond\expandafter{\x}{#1}#2%
    }%
  }%
}%
\makeatother

\begin{document}

List provided explicitly:

\fromlist{\rule}{{5pt}}{1}{{1in,2in,3in}}

\fromlist{\rule}{{5pt}}{2}{{1in,2in,3in}}

\fromlist{\rule}{{5pt}}{3}{{1in,2in,3in}}

\newcommand\commalist{1in,2in,3in}

List provided via macro:

\fromlist{\rule}{{5pt}}{1}{\commalist}

\fromlist{\rule}{{5pt}}{2}{\commalist}

\fromlist{\rule}{{5pt}}{3}{\commalist}

\end{document}