TIKZ: foreach not compatible with calc-library?

Question

I'd like to draw a set of lines between some set of points, and on the 1/3 and 2/3 point of each line I'd like to draw a vertical line. In order to do this I first defined the points, and then wanted to use a \foreach loop to draw. However it seems that the foreach loop does not work with calc:

The following code describes what I want to do. The problem is that the expression ($0.333*\x + 0.666*\y$) within the for loop does seem to cause problems. (While e.g. ($0.333*(A) + 0.666*(B)$) works.) Can anyone tell me what I'm doing wrong/what I need to change in order to make this work?

\documentclass{article} 
\usepackage{tikz}
\usepackage{pgfplots} 
\usetikzlibrary{patterns}
\usetikzlibrary{calc}
\usetikzlibrary{arrows.meta}
\usepackage{ifthen}
\begin{document}

\begin{center}
\begin{tikzpicture}
\coordinate (A) at (-3,1);
\coordinate (B) at (3,3);
\coordinate (C) at (0,3);
\foreach \x/\y in {(A)/(B),(B)/(C),(A)/(C)}{
\draw[-] \x -- \y; 
\draw[dashed] ($0.333*\x + 0.666*\y$) -- +(0,1);
\draw[dashed] ($0.666*\x + 0.333*\y$) -- +(0,1);
}
\end{tikzpicture}
\end{center}
\end{document}

score 8 · Accepted Answer · answered Nov 10 '16 at 20:31

The parentheses are needed as syntax characters inside the expression of TikZ library calc. But the coordinate names can be put into macros:

\coordinate (A) at (-3,1);
\coordinate (B) at (3,3);
\coordinate (C) at (0,3);
\foreach \x/\y in {A/B,B/C,A/C}{
  \draw[-] (\x) -- (\y);
  \draw[dashed] ($0.333*(\x) + 0.666*(\y)$) -- +(0,1);
  \draw[dashed] ($0.666*(\x) + 0.333*(\y)$) -- +(0,1);
}

Full example:

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{patterns}
\usetikzlibrary{calc}
\begin{document}

\begin{center}
  \begin{tikzpicture}
    \coordinate (A) at (-3,1);
    \coordinate (B) at (3,3);
    \coordinate (C) at (0,3);
    \foreach \x/\y in {A/B,B/C,A/C}{
      \draw[-] (\x) -- (\y);
      \draw[dashed] ($0.333*(\x) + 0.666*(\y)$) -- +(0,1);
      \draw[dashed] ($0.666*(\x) + 0.333*(\y)$) -- +(0,1);
    }
  \end{tikzpicture}
\end{center}
\end{document}

That is an even easier solution, thank you very much! – flawr Nov 10 '16 at 20:33 — flawr, Nov 10 '16 at 20:33

score 5 · Answer 2 · answered Nov 10 '16 at 20:08

5

You have to expand \x and \y before you can feed them to the calc library. There are two ways, see example.

\documentclass{article}
\pagestyle{empty}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
  \coordinate (A) at (-3,1);
  \coordinate (B) at (3,3);
  \coordinate (C) at (0,3);
  \foreach \x/\y in {(A)/(B),(B)/(C),(A)/(C)}{
    \draw[-] \x -- \y;

    % (1) expand only the point
    \edef\p{($0.333*\x + 0.666*\y$)}
    \draw[dashed] \p -- +(0,1);

    % (2) expand the whole path, protecting everything not to be
    % expanded by \noexpand
    \edef\a{\noexpand\draw[dashed] ($0.666*\x + 0.333*\y$) -- +(0,1);}\a
  }
\end{tikzpicture}

\end{document}

answered Nov 10 '16 at 20:08

Henri Menke

109,596

Thank you for your answer. What does to expand mean here? – flawr Nov 10 '16 at 20:12
@flawr It means that \x and \y are being replaced by their replacement text. You might be interested in these questions: https://tex.stackexchange.com/questions/451 https://tex.stackexchange.com/questions/248741 – Henri Menke Nov 10 '16 at 20:16
Oh now I see, thank you for the references. Just to be sure: Is it correct that in the second example you define \a to be the what is within the {}, where everything within the {} is evaluated except \draw[dashed]? (And afterwards we call \a to execute it?) – flawr Nov 10 '16 at 20:30
@flawr Everything inside the braces is expanded but most of it are letters which are unexpandable by definition. The only thing protected is the single token \draw. – Henri Menke Nov 10 '16 at 20:36

TIKZ: foreach not compatible with calc-library?

2 Answers2