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I want to plot some dynamical systems with tikz but when I need to iterate on the node names, there is an error because the node name is broken when I need to work on the anchors.

I found multiple posts about accessing nodes in foreach calls such as this one but I could not find any dealing with the anchors.

Is there a specific way to protect the node name in the loop ?

\documentclass[tikz,border = 1mm]{standalone}
\usetikzlibrary{patterns,shapes.geometric,decorations.pathmorphing}
\def\bord{0.1cm}
\def\radius{.2cm}
\tikzset{
spring/.style={thick,decorate,decoration={
zigzag,
segment length = 1mm,
amplitude=0.7mm,
pre length=1mm,
post length=1mm}},
mass/.style={circle,minimum width=2*\radius,anchor=center}
}
\begin{document}
\begin{tikzpicture}[x=1cm,y=1cm,line join=round]
% Bord gauche
\fill [pattern = north east lines] (-.3,-2) rectangle (0,.5);
\draw[thick] (0,-2) -- (0,.5);

% Right edge
\def\borddroit{8}
\fill [fill=black!30!white] (\borddroit,-2) rectangle (\borddroit+.3,.5);
\fill [pattern = north east lines] (\borddroit,-2) rectangle (\borddroit+.3,.5);
\draw[ thick] (\borddroit,-2) -- (\borddroit,.5);

% Masses
\foreach \i in {1,2,...,6}{
    \node[mass,fill=blue] (m\i) at (\i,0) {};
}
\node[mass] (m0) at (0,0) {};
\node[mass,fill=red] (m7) at (7,0) {};

% Springs
% Working plot
\draw[spring] (m0.center) -- (m1.west);

% Working \foreach without anchor
\foreach \i in {0,...,6}{
   \pgfmathsetmacro{\j}{\i+1}
    \draw[spring] (m\i) -- (m\j) node[midway, above] (kuu\i) {$k$};
}


% Non working foreach beacause node name is broken
%\foreach \i in {0,...,6}{
%   \pgfmathsetmacro{\j}{\i+1}
%    \draw[spring] (m\i.east) -- (m\j.west) node[midway, above] (kuu\i) {$k^{\uu}$};
%}

\end{tikzpicture}
\end{document}
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  • 3
    Untested, but use \pgfmathtruncatemacro not \pgfmathsetmacro, the latter makes a decimal value, so you get e.g. 1.0.west, instead of 1.west. – Torbjørn T. Aug 18 '18 at 13:34
  • @TorbjørnT. It look soooo easy when you put it that way, feel free to post an answer so I can accept it ... Is there another way to compute \j say wihtout explicitly calling \pgfmathtruncatemacro ? – BambOo Aug 18 '18 at 13:38
  • 2
    There are multiple duplicates around. You can use \foreach [evaluate={\j=int(\i+1)}] \i in {0,...,6} or \foreach [remember=\j as \i (initially 0)] \j in {1,...,7}. – Torbjørn T. Aug 18 '18 at 13:41
6

  • the lines

    \draw[spring] (m\i) -- (m\j) node[midway, above] (kuu\i) {$k$};
\draw[spring] (m\i.east) -- (m\j.west) node[midway, above] (kuu\i) {$k^{quu}$};

gives the same result (except of nodes contents, which in second case is not defined).

  • instead 

    \foreach \i in {0,...,6}{
   \pgfmathsetmacro{\j}{\i+1}
   \draw[spring] (m\i) -- (m\j) node[midway, above] (kuu\i) {$k$};

    you can write

    \foreach \i [count=\j from 1] in {0,...,6}{
     \draw[spring] (m\i.east) -- (m\j.west) node[midway, above] (kuu\i) {$k^{\uu}$};

    which because \uu is not defined, it doesn't work. replacing it with u gives 

    \documentclass[tikz,border = 1mm]{standalone}
\usetikzlibrary{patterns,shapes.geometric,decorations.pathmorphing}
\def\bord{0.1cm}
\def\radius{.2cm}
\tikzset{
spring/.style={thick,decorate,decoration={
zigzag,
segment length = 1mm,
amplitude=0.7mm,
pre length=1mm,
post length=1mm}},
mass/.style={circle,minimum width=2*\radius,anchor=center}
}
\begin{document}
\begin{tikzpicture}[x=1cm,y=1cm,line join=round]
% Bord gauche
\fill [pattern = north east lines] (-.3,-2) rectangle (0,.5);
\draw[thick] (0,-2) -- (0,.5);

% Right edge
\def\borddroit{8}
\fill [fill=black!30!white] (\borddroit,-2) rectangle (\borddroit+.3,.5);
\fill [pattern = north east lines] (\borddroit,-2) rectangle (\borddroit+.3,.5);
\draw[ thick] (\borddroit,-2) -- (\borddroit,.5);

% Masses
\foreach \i in {1,2,...,6}{
    \node[mass,fill=blue] (m\i) at (\i,0) {};
}
\node[mass] (m0) at (0,0) {};
\node[mass,fill=red] (m7) at (7,0) {};

% Springs
% Working plot
\draw[spring] (m0.center) -- (m1.west);

% Working \foreach without anchor
\foreach \i [count=\j from 1] in {0,...,6}{
    \draw[spring] (m\i.east) -- (m\j.west) node[midway, above] (kuu\i) {$k^{u}$};
}
\end{tikzpicture}
\end{document}

enter image description here

off-topic: above code (which only has corrected two lines in your code) can be written slightly shorter and more concise (with one \foreach loop):

\documentclass[tikz,border = 3mm]{standalone}
\usetikzlibrary{decorations.pathmorphing,
                patterns, positioning,
                quotes,
                shapes.geometric}
\def\radius{.2cm}
\begin{document}
\begin{tikzpicture}[
node distance = 5mm and 10mm,
  bord/.style = {rectangle, fill=#1, %inner sep=0pt,
                 pattern = north east lines,
                 minimum width=5mm, minimum height=25mm,
                 yshift=15mm,
                 node contents={}},
  mass/.style = {circle, minimum width=2*\radius, fill=#1},
spring/.style = {thick,decorate,decoration={zigzag,
                 segment length = 1mm,
                 amplitude=0.7mm,
                 pre length=1mm, post length=1mm}
                 },
                 ]
% Bord gauche
\coordinate (m0) at (\radius,0);
\node (m00) [bord=white, below left=5mm and 0mm of m0];
\draw[thick] (m00.north east) -- (m00.south east);

% Working \foreach without anchor
\foreach \i [count=\j from 1] in {0,...,6}
{
\ifnum\j<7
    \node[mass=blue] (m\j) at (\j,0) {};
\else
    \node[mass=red] (m\j) at (\j,0) {};
\fi
\draw[spring] (m\i) to ["$k^{u}$"] (m\j) ;
}
% Right edge
\node  (m8) [bord=gray!30, below right=of m7.center];
\draw[thick] (m8.north west) -- (m8.south west);
\end{tikzpicture}
\end{document}

result is:

enter image description here

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