naming paths inside a TikZ foreach loop

Question

Is it possible to create named paths inside a foreach loop with the name depending on the counter?

I want to write something like

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}
    \foreach \i in {1,2,...,6}
        \draw[name path=line\i] (-3,-\i) -- (3,\i);
    \draw[name intersections={of=line1 and line2}] (intersection-1) circle (5pt);
\end{tikzpicture}

\end{document}

but this throws the following error:

! Missing \endcsname inserted.
<to be read again> 
                   \OT1\i 
l.11    \draw (0,3) circle (3);

Is this a bug or am I doing something wrong? What should I do instead?

Answer 1

It also possible a bit less hacky. (pgf-manual p. 496) Look at this modified code:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}
    \foreach \i in {1,2,...,6}
    {
  \draw[name path global/.expanded=line\i] (-3,\i) -- (3,-\i);
    }
\draw[name intersections={of=line1 and line2}] (intersection-1) circle (5pt);
\end{tikzpicture}

\end{document}

Answer 2

I've found that when passing options to TikZ elements as macros then I need to expand the macro before passing it in:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}
    \foreach \i in {1,2,...,6}
{
  \edef\optname{name path global=line\i}
  \expandafter\draw\expandafter[\optname] (-3,\i) -- (3,-\i);
}
    \draw[name intersections={of=line1 and line2}] (intersection-1) circle (5pt);
\end{tikzpicture}

\end{document}

Now that I've looked up what naming paths does, I notice that the first version of this wouldn't have worked because the path naming occurred within a foreach loop, and that's a local scope. So the path names don't survive to the "outside". This is true even if the \foreach is a one-liner (so no curly braces). To verify this, one can simply do:

\foreach \i in {1}
  \draw[name path=line] (3,1) -- (-3,-1);

and then try to refer to the line path. TikZ will complain.

The solution to this is to use the name path global key instead. This makes the path name available For All Time (well, for the whole picture). Whereupon, it works.

Here's the result of the above: