Numbering nodes in a for loop

Question

I've just started using TikZ to make basic graphs, and I'm trying to reproduce the Petersen graph. I've managed to make it without using for loops, but this obviously isn't a great method for bigger examples.

Currently, my new code stands at:

\begin{tikzpicture}
\foreach \x in {1,...,5}{
\pgfmathparse{(\x-1)*360/5+90}
\node[draw,circle,inner sep=8pt] (\x) at (\pgfmathresult:120pt) [semithick] {};
}

\foreach \x in {1,...,5}{
\pgfmathparse{(\x-1)*360/5+90}
\node[draw,circle,inner sep=8pt] (5+\x) at (\pgfmathresult:60pt) [semithick] {};
}
\end{tikzpicture}

This works, but isn't complete. However, I've realised that the second set of five nodes aren't numbered as 6, ... , 10 but as 5+1, ... , 5+5, which isn't what I want. I tried using \pgfmathparse{int(5+\x)}\pgfmathresult instead of 5+\x, but this results in an error. Strangely (at least to me), if I put \pgfmathparse{int(5+\x)}\pgfmathresult in the empty braces at the end of the second loop, it'll happily put the numbers 6, ... , 10 in the circles, so I can't see why my method shouldn't work.

What am I doing wrong, and why doesn't it work?

Edit: new code:

\begin{tikzpicture}
\foreach \i[evaluate={\angle=(\i)*360/5+90}] in {0,...,4}
{
    \node[draw,circle,inner sep=8pt] (\i) at (\angle:120pt) [semithick] {};
}
\foreach \i[evaluate={\angle=(\i)*360/5+90}, evaluate={\Name=int(\i+5)}] in {0,...,4}
{
     \node[draw, circle,inner sep=8pt] (\Name) at (\angle:60pt) [semithick] {};
}
\foreach \i in {0,...,4} 
{
    \pgfmathparse{int(mod(\i+1,5))}
    \draw[-] (\i) -- (\pgfmathresult);
}
\foreach \i in {0,...,4} 
{
    \pgfmathparse{int(mod(\i,5)+5)}
    \draw[-] (\i) -- (\pgfmathresult);
}
\foreach \i in {5,...,9} 
{
    \pgfmathparse{int(mod(\i+2,5)+5)}
    \draw[-] (\i) -- (\pgfmathresult);
}
\end{tikzpicture}

Answer 1

The easiest would be to calculate the names as well as the angle as an option to the \foreach loop.

You also might be interested in the handlers .pgfmath and .pgfmath int as well as the strcat function (and its handler).

The strcat function combines textual input (enclosed in a pair of ") and/or mathematical input together as one “string”. We can use this to create names as suggested in percusse’ answer, to build names in the form of n-6, n-7, and so on.

Though, in code 3 I show how you could make your life easier with the chains library and a custom nodes around center style that fully takes care of the positioning.

The lines are drawn in the initial node positioning loop with join on the outer chain and with a single \draw command between the nodes on the chains. The lines between the nodes on the inner chain use a similar loop as in your original code. The final line between the last and the first node on the outer chain is drawn manually.

The macro \numberOfNodes is used so that you do not need to repeat the number 5 (or whatever you want to use) in the code.

References; you can’t (or don’t even need to) use \pgfmath… inside most of the path elements

• How to draw a circle whose radius is a multiple of the distance of two points in TikZ?
• Computing value using pgfmath for use in coordinates?

Code 1

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\foreach \x[evaluate={\angle=(\x-1)*360/5+90}, evaluate={\Name=int(\x+5)}] in {1,...,5}
  \node[draw, circle] (\Name) at (\angle:60pt) {\x};
\end{tikzpicture}
\end{document}

Code 2

\documentclass[tikz]{standalone}
\makeatletter
\pgfkeys{/handlers/.pgfmath/.code=\pgfmathparse{#1}\expandafter\pgfkeys@exp@call\expandafter{\pgfmathresult}}
\pgfkeys{/handlers/.pgfmath int/.code=\pgfmathparse{int(#1)}\expandafter\pgfkeys@exp@call\expandafter{\pgfmathresult}}
\pgfkeys{/handlers/.pgfmath strcat/.code=\pgfmathparse{strcat(#1)}\expandafter\pgfkeys@exp@call\expandafter{\pgfmathresult}}
\pgfmathdeclarefunction{strcat}{...}{%
  \begingroup
    \let\pgfmathresult\pgfutil@empty
    \pgfmathstrcat@@#1\pgfmath@stop}
\def\pgfmathstrcat@@#1{%
  \ifx\pgfmath@stop#1%
    \let\pgfmath@next\pgfmathstrcat@@@
  \else
    \expandafter\def\expandafter\pgfmathresult\expandafter{\pgfmathresult#1}%
    \let\pgfmath@next\pgfmathstrcat@@
  \fi
  \pgfmath@next}
\def\pgfmathstrcat@@@{\pgfmath@smuggleone\pgfmathresult\endgroup}
\makeatother
\begin{document}
\begin{tikzpicture}
\foreach \x[evaluate={\angle=(\x-1)*360/5+90}] in {1,...,5}
  \node[draw, circle, name/.pgfmath strcat={"n-",int(\x+5)}] at (\angle:60pt) {\pgfmathprint{strcat("n-",int(\x+5)}};
\end{tikzpicture}
\end{document}

Code 3

\documentclass[tikz]{standalone}
\usetikzlibrary{chains}
\tikzset{
  nodes around center/.style args={#1:#2:#3:#4}{% #1 = start angle, #2 = number of nodes
                                                % #3 = center,      #4 = distance
    at={(#3)},shift={({(\tikzchaincount-1)*360/(#2)+#1}:{#4})}}}
\begin{document}
\begin{tikzpicture}[
  start chain=inner placed {nodes around center=90:\numberOfNodes:{0,0}:60pt},
  start chain=outer placed {nodes around center=90:\numberOfNodes:{0,0}:120pt}
]
\newcommand*{\numberOfNodes}{5}
\foreach \cnt in {1,...,\numberOfNodes} {
  \node[draw, circle, inner sep=+0pt, minimum size=+20pt, on chain=inner] {};
  \node[draw, circle, inner sep=+0pt, minimum size=+20pt, on chain=outer, join] {};
  \draw (inner-\cnt) -- (outer-\cnt);
}
\foreach \cnt[evaluate={\nextCnt=int(mod(\cnt+2,\numberOfNodes)+1)}] in {1,...,\numberOfNodes}
  \draw (inner-\cnt) -- (inner-\nextCnt);
\draw (outer-end) -- (outer-begin);
\end{tikzpicture}
\end{document}

Output 3

Answer 2

You have to use a macro also for the node name. You can't use again \pgfmathparse because the result is saved in the \pgfmathresult macro and it override the other operation you get. So you have to use \pgfmathtruncatemacro to define your own macro.

The circles are not of the same dimension because the texts inside are not of the same dimensions too but I add them only to show you that the nodes have the correct name.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach \x in {1,...,5}{
\pgfmathparse{(\x-1)*360/5+90}
\node[draw,circle,inner sep=8pt] (\x) at (\pgfmathresult:120pt) [semithick] {};
}

\foreach \x in {1,...,5}{
\pgfmathsetmacro\macroa{(\x-1)*360/5+90}
\pgfmathtruncatemacro\macrob{5+\x}
\node[draw,circle,inner sep=8pt] (\macrob) at (\macroa:60pt) [semithick] {\macrob};
}
\end{tikzpicture}
\end{document}

Answer 3

Don't use standalone numbers as node names, I can't find the nicest example but it's unnecessary trouble for debugging. Also you won't be able to use the angle anchors e.g. 6.45 is not the upper right anchor. It's just a number. Use something with a letter, say, n-3 etc.

And for simple operations use counters or evaluate expressions.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}
\foreach \x[count=\xi from 6] in {1,...,5}{
\pgfmathparse{(\x-1)*360/5+90}
\node[draw,circle] (n-\xi) at (\pgfmathresult:60pt) [semithick] {\xi};
}
\end{tikzpicture}

\end{document}