TikZ: conditional nodes inside \foreach

Question

I want to use a \foreach construct to generate several pairs of nodes, except for one of them I want to do something different. This is what I tried:

\begin{tikzpicture}[dot/.style={fill,circle,inner sep=1.5pt}]
    \foreach \x/\nm/\lbl in {0/na1/$\lnot a_1$, 1/na2/$\lnot a_2$, 2/na3/$\lnot a_3$, 3/dots/$\cdots$, 4/nan/$\lnot a_n$}
    \draw (\x,0) \if\nm{dots}\else node[dot](\nm){}\fi node[left=2pt]{\lbl};
\end{tikzpicture}

This compiles just fine, but the "dots" node still has a dot next to it. How can I do this?

(Also, are there any better ways to have circle nodes which are named? using circle(name) didn't seem to work, so I'm just using node[circle](name).)

Answer 1

When you do

\if\nm{dots}\else node[dot](\nm){}\fi

what happens is that TeX expands \nm and compares the two unexpandable tokens it finds; in the first cycle \nm is na1, so what happens is

\if na1{dots}\else node[dot](\nm){}\fi

and, since n and a are different, the "else" branch is followed. The same happens when \nm is dots: d and o are different.

What you need is a different test:

\begin{tikzpicture}[dot/.style={fill,circle,inner sep=1.5pt}]
  \def\test{dots}
  \foreach \x/\nm/\lbl in {0/na1/$\lnot a_1$, 1/na2/$\lnot a_2$, 2/na3/$\lnot a_3$, 3/dots/$\cdots$, 4/nan/$\lnot a_n$}
  \draw (\x,0) \ifx\nm\test\else node[dot](\nm){}\fi node[left=2pt]{\lbl};
\end{tikzpicture}

so TeX will compare the meaning of \nm with the meaning of \test

Another test that can work and doesn't require defining \test is

\ifnum\pdfstrcmp{\nm}{dots}=0 \else node[dot](\nm){}\fi

It has a limitation: you need pdflatex for it to work. If you plan using the code with XeLaTeX or LuaLaTeX, then say in the preamble

\usepackage{pdftexcmds}
\makeatletter
\let\unistrcmp\pdf@strcmp
\makeatother

and use \unistrcmp instead of \pdfstrcmp.

_{The reason is that XeTeX provides the command under the name \strcmp and LuaTeX doesn't provide it at all. The package defines \pdf@strcmp to be \pdfstrcmp if the engine is pdfTeX, \strcmp if the engine is XeTeX, and defines a Lua function in case the engine is LuaTeX. But the name has @ in it, so we provide an @-free version.}

Answer 2

You could use the xifthen package:

\documentclass[parskip]{scrartcl}
\usepackage[margin=15mm]{geometry}
\usepackage{tikz}
\usepackage{xifthen}

\begin{document}

\begin{tikzpicture}

\foreach \x in {1,...,10}
{ \node[circle,draw] (\x 1) at (0,\x) {\x};
    \node[circle,draw] (\x 2)at (2,\x) {\x};
    \ifthenelse{\NOT 4 = \x \AND \NOT 7 = \x}{\draw (\x 1) -- (\x 2);}{}
}

\end{tikzpicture}

\end{document}

The exaple means if x is not four and also its not 7, then draw a line, and the output:

Answer 3

With functional package, we can rewrite the answer of Tom Bombadil as follows:

\documentclass[parskip]{scrartcl}
\usepackage[margin=15mm]{geometry}
\usepackage{tikz}
\usepackage{functional}
\begin{document}

\begin{tikzpicture}
\IntStepInline{1}{1}{10}{
  \node[circle,draw] (#1 1) at (0,#1) {#1};
  \node[circle,draw] (#1 2) at (2,#1) {#1};
  \BoolVarOrTF{\IntCompare{4}={#1}}{\IntCompare{7}={#1}}{}{
    \draw (#1 1) -- (#1 2);
  }
}
\end{tikzpicture}

\end{document}