Specifying coordinates in tikzcd arrows with a foreach loop

Question

I'm trying to use a foreach loop to draw a whole bunch of arrows in a tikzcd environment by using the foreach loop to specify coordinates but I can't get it to compile. Here's a minimal example of what I'm trying:

\documentclass{article}

\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}
  A & B & C \\
  \foreach \x in {2,3}{
    \arrow[from=1-1, to=1-\x]
  }
\end{tikzcd}

\end{document}

Which should appear as an arrow from A to B and another from A to C. Instead I'm recieving this error:

ERROR: Undefined control sequence.

--- TeX said ---

pgf@sh@ns@\tikzcdmatrixname -1-\x

l.12 \end{tikzcd}

I think I could get it to work in a plain tikz environment but I'd like to use tikzcd if possible. Thanks for any help.

Answer 1

Try with a \draw in execute at end picture:

\documentclass{article}

\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}[
    every matrix/.append style={name=mymatrix},
    execute at end picture={
        \foreach \x in {2,3}{
            \draw[->] (mymatrix-1-1) -- (mymatrix-1-\x);
        }
    }
]
  A & B & C \\
\end{tikzcd}

And Zarko's example:

\begin{tikzcd}[
    every matrix/.append style={name=mymatrix},
    execute at end picture={
         \foreach \x [count=\xx] in {2,3}{ \draw[->] (mymatrix-1-\xx) -- (mymatrix-1-\x); 
         }
    }
]
  A & B & C \\
\end{tikzcd}
\end{document}

Answer 2

This is a classical expansion problem.

The \arrow macro just takes everything between [ and ] and stores it as-is in a long list which gets processed much later. At that point \x isn't 2 or 3 anymore.

I provide three solutions:

1. An \arrowx[<opt1>][<opt2>] macro that expands <opt1> before it processes it. The <opt2> will get forwarded as-is and not expanded. That will only be really necessary when you have to use fragile node content.

2. The \arloop{<opt>}{<list>} macro uses a \foreach loop (i.e. the ... syntax is allowed) over <list> to construct \arrow[<opt>] statements where #1 in <opt> is replaced by the elements of your list.

3. The end arrows key that executes its argument directly (after the nodes have been constructed, of course) when there's no expansion issue anymore.

(The PGFPlots package also provides various ungrouped and/or invoking loop constructs but these seem easy enough to implement just for TikZ-CD.)

Code

\documentclass{standalone}
\usepackage{tikz-cd}
\makeatletter
\tikzcdset{every diagram/.append code=\let\arrowx\tikzcd@arrowx\let\arloop\tikzcd@arloop}
\NewDocumentCommand{\tikzcd@arrowx}{O{}O{}}{%
  \edef\tikzcd@temp{#1}%
  \expandafter\arrow\expandafter[\expandafter{\tikzcd@temp,#2}]}
\NewDocumentCommand{\tikzcd@arloop}{mm}{%
  \pgfkeys{/utils/temp/.code=\arrow[{#1}],/utils/temp/.list={#2}}}
\NewDocumentCommand{\tikzcd@arrowlate}{O{}}{%
  \path[{/tikz/commutative diagrams/.cd,every arrow,#1}]
    (\tikzcd@ar@start\tikzcd@startanchor)to(\tikzcd@ar@target\tikzcd@endanchor);}
\tikzcdset{
  end arrows/.style={
    /tikz/commutative diagrams/every matrix/.append style={
      append after command=
        \pgfextra{\pgfutil@g@addto@macro\tikzcd@savedpaths{\let\arrow\tikzcd@arrowlate#1}}}}}
\makeatother
\begin{document}
\begin{tikzcd}
  A & B & C
  \foreach \x in {2, 3}{
    \arrowx[from=1-1, to=1-\x, bend right]
  }
\end{tikzcd}
\begin{tikzcd}
  A & B & C
  \arloop{from=1-1, to=1-#1, bend left}{2,3}
\end{tikzcd}
\begin{tikzcd}[
  end arrows={
    \foreach \x in {2, 3}{
      \arrow[from=1-1, to=1-\x, bend right]
    }
  }
]
  A & B & C
\end{tikzcd}
\end{document}

Output