How to compute the *key* of a pgfkeyssetvalue command?

Question

I have a list of (angles/value) pairs. I'd like to use TikZ to

1. draw nodes with those values at these angles (easy),
2. then draw an edge from node i to i+1, with the value of node i as a label (not at all easy).

My idea so far: store the values in pgfkeys, with keys the same as the node names. Then, values could be easily accessed. Problem is, I cannot get the pgfkeyssetvalue to evalute the node name first.

Example:

\documentclass[tikz]{standalone}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
  \draw [dotted] (0,0)  circle  (2cm);

  % draw nodes on a circle, remembering their value fails: 
  \foreach \angle/\value [count = \i] in {0/17, 45/22, 80/5, 140/12} {
    \node [circle, draw, fill=white] (p\i) at (\angle:2cm) {\value}; 
    \pgfkeyssetvalue{/nodevalues/p\i}{\value}
  }

  % to illustrate that it works with explicit node names: 
  \pgfkeyssetvalue{/nodevalues/p1}{17}
  \pgfkeyssetvalue{/nodevalues/p2}{22}

  % edges: 
  \foreach  [evaluate = {\j=int(mod(\i, 4)+1)}] \i in {1,...,4}  
      \draw [->] (p\i) to [bend right=45] node[midway] {\pgfkeysvalueof{/nodevalues/p\i}} (p\j) ; 
\end{tikzpicture}

\end{document}

This draws the edges for the manually set keys /nodevalues/p1 and /nodevalues/p2 correctly. But pgfkeyssetkeys does not do what I like. Expected result would be that 5->12 arc is label with 5, and 12->17 with 12.

Answer 1

Here is how I would do it: build up a list and reuse its entries.

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}
  \draw [dotted] (0,0)  circle  (2cm);
  % draw nodes on a circle, remembering their value no longer fails: 
  \foreach \angle/\value [count = \i] in {0/17, 45/22, 80/5, 140/12} {
    \node [circle, draw, fill=white] (p\i) at (\angle:2cm) {\value}; 
    \ifnum\i=1
    \xdef\Lst{\value}
    \else
    \xdef\Lst{\Lst,\value}
    \fi
  }
  % edges: 
  \foreach  [evaluate = {\j=int(mod(\i, 4)+1)}] \i in {1,...,4} 
    \pgfmathsetmacro{\X}{{\Lst}[\i-1]} 
    \typeout{\X}
      \draw [->] (p\i) to [bend right=45] node[midway,auto,swap] 
      {\X} (p\j) ; 
\end{tikzpicture}
\end{document}

EDIT: I think this is a nice question which also deserves an explanation why it fails. Essentially the loop does the keys locally, and you need to make them global. This globalization has been achieved by @percusse in this nice answer. Once one employs it, your original approach can also be made work.

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}
  \draw [dotted] (0,0)  circle  (2cm);

  % draw nodes on a circle, remembering their value no longer fails: 
  \foreach \angle/\value [count = \i] in {0/17, 45/22, 80/5, 140/12} {
    \node [circle, draw, fill=white] (p\i) at (\angle:2cm) {\value}; 
    \begingroup\globaldefs=1\relax
    \edef\temp{\noexpand\pgfkeyssetvalue{/nodevalues/p\i}{\value}}
    \temp
    \endgroup
  }

  % edges: 
  \foreach  [evaluate = {\j=int(mod(\i, 4)+1)}] \i in {1,...,4}  
      \draw [->] (p\i) to [bend right=45] node[midway] {\pgfkeysvalueof{/nodevalues/p\i}} (p\j) ; 
\end{tikzpicture}
\end{document}

Yet another possibility would be to employ Henri Menke's trick to remember node contents, I'd be happy to spell that out if you ask me to.

Answer 2

Just for fun, a single \foreach that does what you want.

\documentclass[tikz,margin=2mm]{standalone}

\usepackage{tikz}

\begin{document}

    \begin{tikzpicture}
        \draw [dotted] (0,0)  circle  (2cm);

        \foreach \angle/\value [count = \i, count=\j from 0, remember=\value as \oldvalue] in {0/17, 45/22, 80/5, 140/12} {
            \node [circle, draw, fill=white] (p\i) at (\angle:2cm) {\value};
            \ifnum\j>0
                \draw[->] (p\j) to [bend right=45] node[midway,auto,swap] {\oldvalue} (p\i);
            \fi
            \ifnum\i=4
                \draw[->] (p\i) to [bend right=45] node[midway,auto,swap] {\value} (p1);
            \fi
        }
    \end{tikzpicture}

\end{document}

Answer 3

It is possible to solve your problem without using pgf keys but only using the native capabilities of \foreach operation.

Here, I used a first loop to draw the nodes and a second to create the arrows between the nodes with the edge operation

\documentclass[tikz]{standalone}

\begin{document}
\begin{tikzpicture}
  \draw [dotted] (0,0)  circle  (2cm);

  % create the node
  \foreach \angle/\value [count=\i,remember=\angle as \lastangle (initially 140),remember=\i as \lasti (initially 4)] in {0/17, 45/22, 80/5, 140/12} {
     \node (\i) at (\angle:2cm) {\value};
  }
  % draw arrow with edge operation
  \foreach \value [count=\i,remember =\i as \lasti (initially 4),remember=\value as \lvalue (initially 12)] in {17,22,5,12}{
  \path[->](\lasti) edge[bend right=45]  node[midway,auto,swap]{\lvalue}(\i);
  }
\end{tikzpicture}

\end{document}