Problem with loop-generated pkgkeys containing a space

Question

I'm trying to use \pgfkeys in the following setting:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfkeys}

\begin{document}
\begin{tikzpicture}[%
    m1 color/.initial=red,
    m2 color/.initial=green,
    m3 color/.initial=blue,
  ]
  \foreach \i in {1, ..., 3} {%
    \pgfkeys{/tikz/m\i color/.get=\c}
    \node at (0, \i) [color=\c] {\i};
  }
\end{tikzpicture}
\end{document}

And when I compile it, I get the following error:

! Package xcolor Error: Undefined color `\c '.

If I change the code to not use \i to select the key, i.e.:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfkeys}

\begin{document}
\begin{tikzpicture}[%
    m1 color/.initial=red,
    m2 color/.initial=green,
    m3 color/.initial=blue,
  ]
  \foreach \i in {1, ..., 3} {%
    \pgfkeys{/tikz/m1 color/.get=\c}
    \node at (0, \i) [color=\c] {\i};
  }
\end{tikzpicture}
\end{document}

then it compiles, but now I can't iterate over the keys.

I also noticed that this actually works:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfkeys}

\begin{document}
\begin{tikzpicture}[%
    color1/.initial=red,
    color2/.initial=green,
    color3/.initial=blue,
  ]
  \foreach \i in {1, ..., 3} {%
    \pgfkeys{/tikz/color\i/.get=\c}
    \node at (0, \i) [color=\c] {\i};
  }
\end{tikzpicture}
\end{document}

So what's the problem with the first example?

Just for curiosity, I tried changing m1 color to m1_color, and now it works. Hence, it appears to have something to do with the spaces. But what?

Answer 1

Macros remove any spaces behind them, so m\i color will be expanded to m1color not m1 color, etc. You need to add an explict space: m\i\space color:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfkeys}

\begin{document}
\begin{tikzpicture}[%
    m1 color/.initial=red,
    m2 color/.initial=green,
    m3 color/.initial=blue,
  ]
  \foreach \i in {1, ..., 3} {%
    \pgfkeys{/tikz/m\i\space color/.get=\c}
    \node at (0, \i) [color=\c] {\i};
  }
\end{tikzpicture}
\end{document}

Also note that using a macro for a key value is not always supported. In the case of color it is, but for other keys you might want to expand it first by adding /.expand once to the key, like color/.expand once=\c.

Answer 2

It compiles without error if you change

\pgfkeys{/tikz/m\i color/.get=\c}

to

\pgfkeys{/tikz/m\i\space color/.get=\c}

I can't promise it gets the result you intend though.....