'pgfmathresult' uses the argument for 'text opacity'

Question

I'm working on a figure of a graph with nodes. I put a white transparent box behind the node label using fill=white and opacity=0.8 and then I tried using text opacity=1.0 so that the text is still readable. The problem is that the node labels, created in a for-loop, end up having 1.0 instead of the correct number. The graph I get and the graph I want are pictured below:

I haven't found any posts relating text opacity to pgfmathresult. Here is a MWE of the code I have:

\usepackage{xifthen}
\usetikzlibrary{backgrounds}
\begin{document}
    \begin{tikzpicture}
        [
          every label/.style={inner sep=0pt, opacity=0.8, text opacity=1.0, fill=white},
          cnode/.style={draw=black,fill=#1,minimum width=3mm,circle},
        ]
        \node[cnode=red,label=0:{$\hat y_1$}] (s1) at (6,-1) {};
        \node[cnode=red,label=0:{$\hat y_2$}] (s2) at (6,-2) {};
        \node at (6,-3) {$\vdots$};
        \node[cnode=red,label=0:{$\hat y_K$}] (sK) at (6,-4) {};

        \foreach \x in {0,...,4}
        {   
            \pgfmathparse{\x<4 ? \x : "q-1"}
            \ifthenelse{\x>0}
            {
                \node[cnode=gray,label={90:$z_{\pgfmathresult}$}] (h-\x) at (3,{-\x-div(\x,4)+.5}) {};
            }
            {
                \node[cnode=gray,label=90:$1$] (h-0) at (3,0.5) {};
            }
            \begin{scope}[on background layer]
              \draw (h-\x) --  (s1);
            \draw (h-\x) -- (s2);
            \draw (h-\x) --  (sK);
            \end{scope}
        }

        \node at (3,-3.5) {$\vdots$};

        \node[cnode=gray,label=90:$z_{M}$] (h-q) at (3,-5.5) {};
        \begin{scope}[on background layer]
        \draw (h-q) --  (s1);
        \draw (h-q) -- (s2);
        \draw (h-q) --  (sK);
        \end{scope}
    \end{tikzpicture}
\end{document}

Thanks in advanced.

Answer 1

The issue is that \pgfmathresult gets overwritten, so you may want to use \pgfmathsetmacro instead to store the result of the computation in a macro. (I also got rid of xifthen because it is not needed for integer comparisons, for which you can use a simple \ifnum.)

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{backgrounds}
\begin{document}
    \begin{tikzpicture}
        [
          every label/.style={inner sep=0pt, opacity=0.8, text opacity=1.0, fill=white},
          cnode/.style={draw=black,fill=#1,minimum width=3mm,circle},
        ]
        \node[cnode=red,label=0:{$\hat y_1$}] (s1) at (6,-1) {};
        \node[cnode=red,label=0:{$\hat y_2$}] (s2) at (6,-2) {};
        \node at (6,-3) {$\vdots$};
        \node[cnode=red,label=0:{$\hat y_K$}] (sK) at (6,-4) {};

        \foreach \x in {0,...,4}
        {   
            \pgfmathsetmacro{\myindex}{\x<4 ? \x : "q-1"}
            \ifnum\x>0
                \node[cnode=gray,label={90:$z_{\myindex}$}] (h-\x) at (3,{-\x-div(\x,4)+.5}) {};
            \else
                 \node[cnode=gray,label=90:$1$] (h-0) at (3,0.5) {};
            \fi
            \begin{scope}[on background layer]
              \draw (h-\x) --  (s1);
            \draw (h-\x) -- (s2);
            \draw (h-\x) --  (sK);
            \end{scope}
        }

        \node at (3,-3.5) {$\vdots$};

        \node[cnode=gray,label=90:$z_{M}$] (h-q) at (3,-5.5) {};
        \begin{scope}[on background layer]
        \draw (h-q) --  (s1);
        \draw (h-q) -- (s2);
        \draw (h-q) --  (sK);
        \end{scope}
    \end{tikzpicture}
\end{document}

Answer 2

An another approach which not solve your problem with pgfmathparse. With slightly different nodes' positioning calculation with \pgfmathparse for nodes positions is omitted. For positioning are used chains, fit and positioning libraries only:

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{backgrounds,
                chains,
                fit}

\begin{document}
    \begin{tikzpicture}[
node distance = 6mm and 30mm,
  start chain = going below,
every label/.style = {label distance=2pt, inner sep=0.5pt, 
                      fill=white, fill opacity=0.8, text opacity=1},
      cnode/.style = {minimum size=3mm, on chain,
                      append after command = {\pgfextra{% this is used for correct positioning of nodes
                            \node[circle, draw, fill=#1, inner sep=0pt,
                                  fit=(\tikzlastnode)] {};}
                                              }% end of append after command
                     }
                        ]
\node (s1) [cnode=red,label=0:$\hat{y}_1$]  {}; 
\node (s2) [cnode=red,label=0:$\hat{y}_2$]  {};
\node (s3) [inner ysep=0pt, on chain]       {$\vdots$};
\node (s4) [cnode=red,label=0:$\hat{y}_K$]  {}; 
%
\node (h1) [cnode=green,label=$1$,                  
      above left = of  s1]    {};
\foreach \i in {1,2,4}
{
\scoped[on background layer]
\draw (h1) -- (s\i);
}
\foreach \i [count=\ii from 2] in {2,3,4,q-1,M}
{
\ifnum\ii=4 
    \node (h\ii) [inner ysep=0pt,on chain]   {$\vdots$};
\else
    \node (h\ii) [cnode=green,label=$z_{\i}$]    {};
\begin{scope}[on background layer]
    \draw (h\ii) -- (s1);
    \draw (h\ii) -- (s2);
    \draw (h\ii) -- (s4);
            \end{scope}
\fi
}
     \end{tikzpicture}
\end{document}