Understanding a \@for loop

Question

I'm having trouble with a \@for loop. I've got a macro that takes a comma delimited list and puts each element into a row of an array. When I do this using a \@for loop, I get an extra row that I don't understand. I can manually get rid of it with a negative space, (as in the commented line below) but that can't be the right way to do it, so I must be doing something wrong. Here's a minimal example. For comparison, if I construct what I expect the result of the loop to be manually, the extra row doesn't appear.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\fbun}[1]
   {\ensuremath{\left[\begin{array}{c}
      \@for\xx:=#1\do {\text{\xx}\\}
%     \\ [-2.75ex] % why is this required?
      \end{array}\right]}}
\makeatother
\begin{document}
$\left[\begin{array}{c}
      A\\
      B\\
      C\\
      D\\
      \end{array}\right]$
% same array, but generated by a macro
\fbun{A,B,C,D}
\end{document}

Answer 1

As Altermundus said, you cannot add extra {} or \relax or other invisible material after the last row of the tabular. However, there is some extra material added after \@for. The implementation of \@for is not very good for this.

Since the \@for in LaTeX2e kernel has restrictions, I suggest \forcsvlist from etoolbox:

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}
\def\addrow#1{\text{#1}\\}
\newcommand{\fbun}[1]
   {\ensuremath{\left[\begin{array}{c}
      \forcsvlist\addrow{#1}%
      \end{array}\right]}}

\begin{document}
$\left[\begin{array}{c}
      A\\
      B\\
      C\\
      D\\
      \end{array}\right]$

\fbun{A,B,C,D}
\end{document}

Answer 2

In "User's Guide and Reference Manual" Leslie Lamport writes page 45:

There must be no &after the last item in a row and no \\ after the last row.

$\left[\begin{array}{c}
      A\\
      B\\
      C\\
      D\\{}%
\end{array}\right]$  

This code gives a new line with an empty group. Each cell of an array is in a group.

\@foris a no developable loopso you need to write something like

\fbun{A,B,C} 
D

or you can create a developable loop but it's more complicated :

\makeatletter
\newcommand*{\For}[1]{\noalign{\gdef\@Do##1{#1}}\@For}
\newcommand*{\@For}[3]{%
  \unless\ifnum#1 \ifnum#3>0 >\else <\fi #2
    \@Do{#1}%
    \expandafter\@For\expandafter{\number\numexpr#1+#3}{#2}{#3}%
  \fi
} 
\makeatother

$\left[\begin{array}{c}
  \For{\@Alph{#1}\\}{1}{4}{1}    
\end{array}\right]$ 

\For is a developable loop for example, you can try :

\begin{tabular}{ll}
  \For{A number & #1\\}{1}{20}{1}
\end{tabular}

Answer 3

The problem is that TeX finds something following the last \\ which starts a new row (it's neither \noalign nor \crcr that wouldn't).

A solution might be to use a token register: we develop the loop while in the first cell, so there's no problem of \xx not being defined any more after TeX has seen \\ (or &).

\usepackage{amsmath}
\makeatletter
\newcommand{\fbun}[1]{%
  \begin{bmatrix}
  \toks@={\@gobble}%
  \@for\next:=#1\do
    {\toks@=\@xp{\the\@xp\toks@\@xp\\\@xp\text\@xp{\next}}}%
  \the\toks@
  \end{bmatrix}}
\makeatother

I've used \@xp (from amsmath) that's a shorthand for \expandafter; if the \text around the entry wasn't required, then

\toks@=\expandafter{\the\expandafter\toks@\next}

would have sufficed. Initializing \toks@ to \@gobble has the effect that the first \\ is swallowed.

Instead of array I've used bmatrix (change it if you like) and, of course, I deleted \ensuremath as the first step. :)

Update 2023

We can make the solution much shorter and understandable with expl3 tools.

\documentclass{article}
\usepackage{amsmath}
\ExplSyntaxOn
\NewDocumentCommand{\fbun}{O{c}m}
 {
  \ensuremath
   {
    \left[
    \begin{tabular}{#1}
    \clist_use:nn { #2 } { \ }
    \end{tabular}
    \right]
   }
 }
\ExplSyntaxOff
\begin{document}
$\left[\begin{array}{c}
      \text{A}\
      \text{B}\
      \text{C}\
      \text{D}\
      \end{array}\right]$
% same array, but generated by a macro
\fbun{A,B,C,D}
\fbun[l]{Aaa,Bbbbbb,Cc,Dddd}
\end{document}

Answer 4

I believe the reason you're getting a fifth, empty row when generating the column vector via the for-loop is that the fourth element (like the three before it) gets a \\ affixed to it; this instruction creates a new row which ends up being blank since there's no further content to create.

A (not very elegant) solution would be to redefine \fbun to take two arguments, as follows:

\newcommand{\fbun}[2]
   {\ensuremath{\left[\begin{array}{c}
      \@for\xx:=#1\do {\text{\xx}\\} #2
      \end{array}\right]}}

and to invoke it as

\fbun{A,B,C}{D}

Answer 5

A solution using the \ekvcsvloop provided by expkv. I also added a small key=value interface with expkv-cs, the supported keys are align and formatter (the latter will get the text of each row as argument). There are also a few shortcuts in the key=value interface, c is align=c, l is align=l, r is align=r, and every unknown key without a =value is considered a formatter.

\documentclass{article}
\usepackage{amsmath}
\usepackage{expkv-cs}
\makeatletter
\newcommand\fbun[1][]{\fbun@kv{#1}}
% this is the output, if you don't need the bling bling of a key=value interface
% you can reduce the \fbun definition to this.
% With the current definition it'll get two arguments from \fbun@kv and as an
% additional argument reads the comma separated list.
\newcommand\fbun@out[3]
  {%
    \left[\begin{array}{#1}
      % ekvcsvloop will result in \\#2{<#3-element>} for each element in the
      % csv-list #3 (thanks to \expanded in a single step), \@gobble will
      % remove the first \\.
      \expandafter@gobble\expanded{\ekvcsvloop{\#2}{#3}}%
    \end{array}\right]%
  }
\ekvcSplitAndForward\fbun@kv\fbun@out
  {
    % the two keys that will correspond to an argument for \fbun@out
     align     = c
    ,formatter = @firstofone
  }
\ekvcSecondaryKeys\fbun@kv
  {
    % in expkv it is documented behaviour that multiple equals signs are
    % allowed, the first is the one separating key from value
    % nmeta defines the following name to be a shortcut to set whatever is
    % specified in the value.
     nmeta c = align=c
    ,nmeta l = align=l
    ,nmeta r = align=r
  }
\ekvdefunknownNoVal{\string\fbun@kv}{\ekvcPass\fbun@kv{formatter}{#2}}
\makeatother
\begin{document}
$\fbun[\text]{Abc,B,C,D}$
$\fbun[c,\text]{Abc,B,C,D}$
$\fbun[l,\text]{Abc,B,C,D}$
$\fbun[r,\text]{Abc,B,C,D}$
$\fbun{ab,c,d}$
$\fbun[align=@{}c@{}]{ab,c,d}$
\end{document}